1 Atmosphere, weather and climate

1. The atmosphere is an envelop of gases surrounding the earth.
1. It consists of nitrogen (N₂)(78%), oxygen (O₂)(21%), argon (Ar)(0.9%), carbon dioxide (CO₂)(0.03%) and other gases.
2. The atmosphere is divided into the troposphere, the stratosphere, the mesosphere and the thermosphere.
3. The ozone layer is a concentration of ozone (O₃) in a layer in the stratosphere, esp. in the altitude 20 - 25 km.
1. Ozone absorbs most of the short wave solar radiation.
2. The depletion of ozone layer
1. increases the intensity of ultraviolet (UV) radiation reaching the earth's surface and
2. increases the incidence of skin cancer in humans.
4. The troposphere is of the most direct importance to terrestrial life.
1. Most of the gases and water vapour concentrate in the troposphere.
2. Almost all phenomena of weather and climate take place in the troposphere.
2. Weather and climate
1. The weather is the state or condition of the troposphere in a particular place over a short period of time, e.g. a few hours.
2. The climate is the average weather condition of an area over a long period of time.
3. The elements of weather are temperature, atmospheric pressure, wind, humidity, precipitation, sunshine and visibility.
1. The elements of weather are measured and recorded at a weather station.
2. In Hong Kong, the condition of the weather is measured and recorded by the Hong Kong Observatory.
3. A synoptic chart (or weather chart or weather map) records the main elements of the weather at a particular time.

2 The global energy balance (or the global energy budget)

1. The main source of heat on the earth's surface and in the atmosphere is the radiant energy is emitted from the sun.
2. The average intensity of solar radiation falling just outside the atmosphere is called the solar constant = 1 380 W m^-2 (watt per square metre).
3. Insolation is the interception of solar energy (solar short wave radiation) by an exposed surface.
4. Insolation is absorbed by land and sea surfaces and changed into heat.
5. The lower atmosphere is then warmed by terrestrial long wave radiation.
1. Heat flows from the earth's surface to the atmosphere through conduction, radiation and convection.
2. Water can also carry heat energy into the atmosphere in the form of water vapour (latent heat).
6. For every 100 units of incoming solar radiation,
1. Total 31 units are lost to space by earth-atmosphere system.
1. 6 units are lost to space by scattering.
2. 21 units are reflected back into space by clouds.
3. 4 units are reflected back by the earth's surface.
2. Total 69 units are absorbed by earth-atmosphere system.
1. 21 units are absorbed by molecules, dust, water vapour and clouds.
2. 48 units are absorbed by earth's surface.

3-1 Latitude

1. Temperature decreases from the equator to the poles.
2. In higher latitudes (temperate or polar regions),
1. The sun's rays travel a greater distance through the atmosphere.
1. More solar energy is lost by reflection, scattering and absorption by dust particles and water vapour.
2. Less solar energy reaches the earth's surface.
2. The sun's rays fall at a smaller angle of the mid-day sun.
1. The sun's rays reach the earth's surface more obliquely and spread over a larger area.
2. The insolation is more scattered. The intensity of solar energy is smaller.
3. In lower latitudes (equatorial or tropical regions),
1. The sun's rays travel a shorter distance through the atmosphere.
1. Less solar energy is lost by reflection, scattering and absorption by dust particles and water vapour.
2. More solar energy reaches the earth's surface.
2. The sun's rays fall at a larger angle of the mid-day sun.
1. The sun's rays reach the earth's surface more vertically and spread over a smaller area.
2. The insolation is more concentrated. The intensity of solar energy is higher.

1. The land heats up more quickly than the sea in summer, but cools down more quickly in winter.
2. Onshore winds raise the winter temperature but lower the summer temperature. The annual temperature range is small.
3. Offshore winds lower the winter temperature but raise the summer temperature. The annual temperature range is large.

1. Temperature decreases as altitude increases.
2. The earth radiates the solar energy back to the atmosphere in the form of terrestrial long wave radiation.
3. On higher altitudes,
1. the air is less dense and contains fewer dust particles and water vapour;
2. this allows the heat from the earth's surface escapes more rapidly, lowering the temperature.
4. Near the earth's surface,
1. the air is denser and contains more dust particles and water vapour that absorb heat;
2. this prevents the heat from the earth's surface from escaping.
5. Normally, temperature falls by 6.5^oC for every 1000 m increase in altitude. This is the normal lapse rate.
6. Temperature inversion is the condition in which temperature increases with altitude. It may occur when
1. rapid nocturnal terrestrial radiation cools the earth surface and therefore the air in contact with the ground;
2. in Hong Kong during winter months, when warm westerly winds blow above cold north-easterly winds;
3. warm air rises over cold air, or cold air undercuts warm air at a cold front.

1. The sea heats up and cools down more slowly than the land because water is a poor conductor of heat.
2. In coastal areas,
1. Onshore winds bring moderating effects of the sea. The annual temperature range is smaller.
1. In winter, cool onshore winds lower the temperature of the coastal area.
2. In winter, warm onshore winds raise the temperature of the coastal area.
2. The coastal area with maritime influence has a moderate oceanic climate.
3. In inland areas,
1. Onshore winds blow for some distance, the moderating effects weaken. The annual temperature range is larger.
2. The inland area without maritime influence has an extreme continental climate.

1. Warm ocean currents move from low latitudes to high latitudes carrying heat from equatorial regions to polar regions.
1. Onshore winds are warmed when they pass over a warm ocean current.
2. This raises the temperature of coastal areas in mid-latitudes along the western margins of continents.
3. e.g. the warm North Atlantic Drift on NW coast of Europe.
4. Coastal areas have mild winters and ice-free ports.
2. Cold ocean currents move from high latitudes to low latitudes.
1. Onshore winds are chilled when they pass over a cold ocean current.
2. This lowers the temperature of coastal areas in mid-latitudes along the eastern margins of continents.
3. e.g. the cold Labrador Current on the NE coast of N America.
4. Coastal areas have cold winters and foggy weather.

1. In the N Hemisphere, S-facing slopes are warmer than N-facing slopes because
1. the sun's rays fall at a larger angle of the mid-day sun, and
2. the sun's rays spread over a smaller area, the insolation is more concentrated.
2. In the S Hemisphere, S-facing slopes are cooler than N-facing slopes because
1. the sun's rays fall at a smaller angle of the mid-day sun, and
2. the sun's rays spread over a larger area, the insolation is more scattered.
3. This factor is more marked in high-latitudes than in low-latitudes where the angles of the mid-day sun are high.

1. A vegetated surface heats up and cools down more slowly than a bare surface because heat is lost by transpiration.
2. A snow surface reflects more solar radiation than a rocky surface.

1. In the daytime, clouds shade the sun's rays from reaching the earth's surface and the temperature is lower.
2. At night, clouds reduce the terrestrial radiation from leaving the earth's surface, and the temperature is higher.
3. In equatorial rain forests with dense cloud cover,
1. the daytime temperature rarely exceeds 30^oC, the night-time remains more or less the same,
2. the diurnal range of temperature is small.
4. In tropical deserts, the sky is cloudless,
1. the daytime temperature is very high, the night-time temperature is very low,
2. the diurnal range of temperature is large.

1. At the equator, there is always 12 hours of day and 12 hours of night.
2. As latitude increases, the day gets longer in summer and gets shorter in winter.
3. The nearer to the equator, the smaller the annual temperature range.
4. The nearer to the poles, the larger the annual temperature range.

4-1 The spatial pattern of mean monthly average temperature of the world for July and January

1. In general, temperatures decrease from the equator to the poles in both January and July.
2. In the N Hemisphere, in January (the northern winter),
1. The temperature is generally lower because the sun is overhead at the S Hemisphere.
2. The continents are much cooler than the adjacent oceans because land cools down faster than sea.
3. The isotherms bend southwards over the continents, and northwards over the oceans.
4. There is marked northward bending of isotherms along the west coast of Europe.
5. These coastal areas are warmer due to the effects of warm ocean currents.
3. In the N Hemisphere, in July (the southern summer),
1. The temperature is generally higher because the sun is overhead at the N Hemisphere.
2. The continents are much warmer than the adjacent oceans because land heats up faster than sea.
3. The isotherms bend northwards over the continents, and southwards over the oceans.
4. There is marked southward bending of isotherms along the west coast of N America and NE Asia.
5. These coastal areas are cooler due to the effects of cold ocean currents.
4. In the S Hemisphere,
1. There is continuous mass of ocean water and the land areas are relatively smaller.
2. This produces a smaller temperature difference between land and sea.
3. Therefore the isotherm pattern is less complicated and seasonal contrasts are not great than in the N Hemisphere.

1. The annual range of temperature in general increases from the equator to the poles.
2. The greatest range of temperature is found in the continental interior over Eurasia and North America at about 60^oN.
3. Coastal areas have a small annual range of temperature. Inland areas have a larger annual range of temperature.

5-1 Measuring temperature

1. Temperature is measured in ^oC (degree Celsius).
2. Maximum thermometer
1. A maximum thermometer consists of a glass tube filled with mercury. There is a constriction inside the tube.
2. When temperature rises, the mercury expands.
3. When temperature falls, the constriction prevents the mercury from retreating back to the bulb.
4. Therefore, the upper end of the mercury colummn marks the maximum daily temperature.
5. The thermometer is reset by shaking it gently with the bulb pointing downward.
3. Minimum thermometer
1. A minimum thermometer consists of a glass tube filled with alcohol. There is a glass index inside the tube.
2. When temperature falls, the alcohol contracts. The index is pulled by the surface tension of the meniscus of the alcohol.
3. When temperature rises, the alcohol expands but the glass index stays in the lowest position.
4. Therefore, the end of the index near the meniscus marks the minimum daily temperature.
5. The thermometer is reset by tilting the thermometer with the bulb end upwards.
4. Stevenson Screen
1. The maximum-minimum thermometer is placed horizontally on a frame in the middle of a Stevenson Screen.
2. A Stevenson Screen is a wooden box used to protect the thermometers from direct sunlight and rainfall.
1. The Stevenson Screen is painted white to reflect solar radiation.
2. The roof of the Stevenson Screen is double-boarded to ensure good insulation.
3. The sides of the Stevenson Screen are louvered to allow good air ventilation.
3. The Stevenson Screen is placed in an open and exposed position.
1. The Stevenson Screen is fixed on a stand 1.05 m above ground level to avoid radiation from the earth's surface.
2. The door faces north (in the N Hemisphere) to prevent the sun shining into the Screen when readings are taken.

1. average daily temperature = (the maximum temperature of the day + the minimum temperature of the day) / 2
2. diurnal range of temperature = (the maximum temperature of the day) - (the minimum temperature of the day)
3. monthly average temperature = (total of the average daily temperatures for the month) / (number of days in the month)
4. mean monthly average temperature = average of the monthly average temperature for a particular month over 30 years
5. annual temperature = (total of mean monthly temperatures for one year) / 12
6. mean annual temperature = average of annual temperatures over 30 years
7. annual range of temperature = (highest mean monthly temperature) - (lowest mean monthly temperature)

1. Mean monthly average temperature can be presented on a climatic graph.
2. An isotherm map shows the distribution of temperature over a large area.
1. An isotherm is a line joining all points with the same temperature by interpolation.
2. All temperatures on the isotherms are adjusted to sea level equivalents because temperature falls by 6.5^oC per 1000 m.

1. The atmosphere is an envelop of gases surrounding the earth.
2. Air has weight (because of the pull of gravity) which exerts a pressure on the earth's surface.
3. Atmospheric pressure is the weight of the atmosphere acting on per unit area of the earth's surface.

7-1 Altitude

1. Air pressure drops with increasing height from the ground.
2. In the lower layer of the atmosphere, the pressure is higher because:
1. The air has to support the weight of a relatively thicker air layer above it.
2. The air is denser since it is more compressed.
3. The drop of pressure is greater at the lower atmosphere than that higher up.
1. At about 6 km from the ground, pressure drops to only half of that at sea level.
2. At higher than 15 km from the ground, the drop of pressure is almost negligible.

1. When air is heated, it expands and becomes less dense.
1. The warm air rises. An area of low pressure is formed.
2. The low pressure over the equatorial region is the result of high temperatures.
2. When air is cooled, it contracts and becomes denser.
1. The cool air sinks. An area of high pressure is formed.
2. The high pressure belt over the polar regions is the result of low temperatures.
3. Pressure systems formed in this way are said to be of thermal origin.

1. Over the poles, the air is cold and dense.
1. The cold air sinks. A high pressure system is formed.
2. Air moves from the poles equatorwards on the surface.
3. Air moves from a smaller surface area to a larger surface area.
4. Air expands and becomes thinner.
5. At around 60^oN and S, the air rises, forming a low pressure system.
2. Over the equatorial latitudes, the air is warm and less dense.
1. The warm air rises. A low pressure system is formed.
2. Air moves from the equator polewards in the upper level.
3. Air moves from a larger surface area to a smaller surface area.
4. Air contracts and becomes denser.
5. At around 30^oN and S, the air sinks, forming a high pressure system.
3. Pressure systems formed in this way are said to be of dynamic origin.

1. If the earth were stationary, the high pressure and low pressure belts would be arranged in alternate patterns.
1. Equatorial Low pressure belt (or Doldrums) over the equator
2. Sub-tropical High pressure belts (or Horse Latitudes) around 30^oN and S
3. Sub-polar Low pressure belts around 60^oN and S
4. Polar High pressure systems over the two Poles
2. However, the earth revolves in its orbit around the sun.
1. The above description of world pressure systems is true only when the sun is overhead at the equator.
2. The sun is overhead at the equator only on the vernal equinox (21 March) and the autumn equinox (22 September).
3. On the summer solstice (21 June), the sun is overhead at the Tropic of Cancer (23.5^oN)
1. The Equatorial Low (the belt of greatest heating) shifts 5 - 10^oN of the equator at that time.
2. Other pressure belts also shift 5 - 10^o to the N.
4. On the winter solstice (22 December), the sun is overhead at the Tropic of Capricorn (23.5^oS)
1. The Equatorial Low (the belt of greatest heating) shifts 5 - 10^oS of the equator at that time.
2. Other pressure belts also shift 5 - 10^o to the S.

1. If the earth surface were uniform, a continuous pressure belts would be formed along almost the same latitudes.
2. However, the earth's surface is made up of oceans and land masses which heat and cool differently.
1. In summer, low pressure cells develop over the land and high pressure develop over the sea.
2. In winter, high pressure cells develop over the land and low pressure develop over the sea.
3. In January, the Equatorial Low shifts to the S of the equator. Low pressure cells are formed over the southern continents.
4. In July, the Equatorial Low shifts to the N of the equator and extends further N over the northern continents.
5. The pressure belts are more regular in the S Hemisphere because there is a large mass of ocean.

1. Atmospheric pressure is measured in hectopascal (hPa).
2. At mean sea level in the mid-latitudes, the atmospheric pressure is 1 013 hPa (i.e. 1 atm = 1013 hPa).
3. 1 hPa = 100 Pa (pascals) = 100 N m^-2 (newtons per square metre) = 1 mb (millibar)

1. A simple mercury barometer consists of a vertical glass tube of 1 m long.
1. The tube is filled with mercury and is dipped upside down into a mercury container.
2. The mercury column in the tube is held by the pressure of the air on the mercury surface of the container.
3. The greater the air pressure, the higher the mercury column it can support.
4. A scale attached to the tube gives the air pressure.
2. The average pressure at sea-level is able to support a mercury column of 760 mm (i.e. 760 mm Hg = 1013 hPa).
3. The same principle is used for a mercury barometer.

1. A barograph has an inked nib connected to the barometer mechanism instead of a dial.
2. The barograph consists of a revolving drum operated by clockwork. A recording chart is fixed around the drum.
3. As the pressure changes, the inked nib moves up and down and traces a continuous line graph on the chart.
4. A recording chart takes the recordings of one week.

1. An aneroid barometer consists of a hollow, partial vacuum metal box with a corrugated surface.
1. The upper wall of the chamber is a flexible diaphragm that moves up and down as the outside air pressure varies.
1. When air pressure rises, the surface of the diaphragm is pressed down.
2. When air pressure drops, the surface of the diaphragm bends upward.
2. The movement of the box surface is transferred through a system of levers and springs to a pointer.
3. The scale printed on a disc gives the pressure.
2. An aneroid barometer is not so accurate as the mercury barometer. However, it is light and portable.

1. Air pressure is presented on weather charts by isobars.
2. Isobars are lines joining places with the same pressure drawn at a regular interval of 2 or 4 hPa.
3. Isobar maps show the distribution of air pressure.

1. The change in pressure indicates the weather condition.
2. A rising air pressure indicates improving weather.
3. A falling air pressure indicates unstable or rainy weather.

1. The difference in atmospheric pressure between two places sets up a pressure gradient which causes air to move.
2. Wind is the (horizontal component of) movement of air in relation to the earth surface.
3. Wind blows from an area of high pressure towards an area of low pressure.
4. The wind speed depends on the steepness of the pressure gradient.
5. Pressure gradient is shown by the spacing of isobars.
1. Closely spaced isobars form a steep pressure gradient and indicate a higher wind speed.
2. Widely spaced isobars form a gentle pressure gradient and indicate a slower wind speed.

1. If the earth were stationary, the winds would blow directly from an area of high pressure to an area of low pressure.
2. The earth, however, rotates from W to E.
3. The velocity of the eastward movement is the greatest at the equator (1 600 km h^-1) and decreases towards the poles (0 km h^-1).
1. A body of air over the equator has a eastward movement as the equator, 1 600 km h^-1.
1. As a body of air moves polewards, it is travelling in an eastern direction faster than the earth beneath it.
2. As a body of air moves equatorwards, it is travelling in an eastern direction slower than the earth beneath it.
2. Therefore, the body of air is deflected to the right in the N Hemisphere and to the left in the S Hemisphere.
4. This is called the Coriolis effect (or Coriolis force) that causes the winds
1. to deflect to the right in the N Hemisphere and
2. to deflect to the left in the S Hemisphere.
5. Ferrel's law states that Any object moving horizontally in the N Hemisphere tends to be deflected to the right.
6. Ballot's law states that in the N Hemisphere Stand with your back to the wind and the low pressure will be toward your left.

13 The Planetary Wind Systems

13-1 Trades

1. Trade winds blow
1. from the Sub-tropical Highs (around 30^oN and S)
2. to the Equatorial Low (over the equator).
2. Due to the deflection of wind direction, they are
1. the NE trades in the N Hemisphere and
2. the SE trades in the S Hemisphere.
3. However, as pressure belts shift in summer and winter, following the apparent "movement" of the overhead sun:
1. In the northern summer, SE trades turn to a NE direction as SW trades after crossing the equator.
2. In the northern winter, NE trades turn to a SE direction as NW trades after crossing the equator.

1. The westerlies blow between 40 and 60^oN and S,
1. from the Sub-tropical Highs (around 30^oN and S)
2. to the Sub-polar Lows (around 60^oN and S).
2. Due to the deflection of wind direction, they are
1. the south-westerlies in the N Hemisphere and
2. the north-westerlies in the S Hemisphere.
3. In the N Hemisphere, there are great land masses and high mountains.
1. The south-westerlies vary more in strength and direction.
2. Hence the south-westerlies are also called the south westerly variables.
4. In the S Hemisphere, there is a continuous stretch of ocean water between 40 and 60^oS.
1. The northwesterlies are very strong and persistent.
2. Sailors call these latitudes "roaring forties", "furious fifties" and "screaming sixties".

1. Very cold polar winds blow between 60 and 90^oN and S,
1. from the Polar Highs (over the poles)
2. to the Sub-polar Lows (around 60^oN and S).
2. Due to the deflection of wind direction, they are
1. the NE polar winds in the N Hemisphere and
2. the SE polar winds in the S Hemisphere.

14-1 Monsoons

1. Monsoons are large scale wind systems which the wind direction is completely reversed in summer and in winter.
2. Monsoon climate has clear seasonal changes: wet summer, dry winter.
3. Monsoon winds are most developed in Asia (the Indian Sub-continent, SE Asia, China and Japan).
4. In summer,
1. The land surface is heated up more quickly. The land is warmer than the adjacent seas.
2. A low pressure centre forms over the land. A high pressure centre forms over the seas.
3. On-shore winds are usually moist.
5. In winter,
1. The land surface cools more quickly. The land is cooler than the adjacent seas.
2. A high pressure centre forms over the land. A low pressure centre forms over the seas.
3. Offshore winds are usually dry.

1. In northern winter, the Asian land mass cools rapidly.
1. The interior of the Asian continent forms an intense high pressure area.
2. The N of Australia forms an intense low pressure area (where it is the southern summer).
3. Dry winter NW monsoon winds blow from N China and Japan, NE monsoon winds from S China and SE Asia.
4. After crossing the equator, the winds are deflected to become the NW monsoon winds into N Australia.
2. In northern summer, the Asian land mass is intensely heated.
1. The interior of the Asian continent forms an intense low pressure area.
2. The central part of Australia forms an intense high pressure area (where it is the southern winter).
3. Dry winter SE monsoon winds blow from the Australian high pressure.
1. After crossing the equator, the SE winds are deflected to become the SW monsoon.
2. They continue to blow into Asian low pressure as the wet summer SE monsoon winds.

15 Anticyclones

1. An anticyclone is an area of HIGH pressure which develops when air descends.
2. On weather maps, it is represented by a series of closed isobars circular or oval in shape.
3. Winds blow from the high pressure centre outwards, due to the deflection of wind direction,
1. in a clockwise direction in the N Hemisphere and
2. in an anticlockwise direction in the S Hemisphere.
4. An anticyclone covers a wide area, 1 000 - 2 000 m across.
5. An anticyclone moves very slowly or is stationary for several weeks.
6. An anticyclone usually gives fine weather.
1. Air is descending. Condensation is unlikely to take place. Cloud and rain do not develop.
2. At night, the cloudless sky allows rapid radiation. Dew and frost may be formed.
3. Winds are light because the pressure gradient is gentle.
4. In summer, the days are warm, dry and sunny.
5. In winter, the days are cold, dry and stable weather dominates.

1. A cyclone is an area of LOW pressure which develops when air rises.
2. On weather maps, it is represented by a series of closed isobars circular or oval in shape.
3. Winds blow into the low pressure centre, due to the deflection of wind direction,
1. in an anticlockwise direction in the N Hemisphere and
2. in a clockwise direction in the S Hemisphere.
4. A cyclone is usually smaller than an anticyclone in extent.
5. A cyclone moves faster is rarely stationary.
6. A cyclone usually gives unsettled weather.
1. Air is rising. Condensation takes place. Clouds and rain develops. Thunderstorms are common.
2. Winds are strong because the pressure gradient is steep.
7. There are two types of cyclones,
1. in tropical latitudes, tropical cyclone (cyclone) and
2. in temperate latitudes, temperate depression (depression).

1. A tropical cyclone is a low pressure area formed over the tropical oceans in latitudes 5-15^oN and S.
1. Tropical cyclones will not develop within 5^oN and S where the Coriolis effect is the weakest.
2. Tropical cyclones only develop over seas where plentiful supply of moisture provides energy for the cyclone.
2. Tropical cyclones are also known as
1. typhoons in the western Pacific and the China Seas,
2. hurricanes in North America,
3. willy-willies in northern Australia.
3. Life cycle of tropical cyclone
1. A tropical cyclone develops over the tropical ocean where hot and humid air rises rapidly when
1. the sea surface is intensely heated up, or
2. northerly and southerly trade winds converge in the inter-tropical convergence zone.
2. An area of low pressure develops.
3. Winds blow into the low pressure centre, due to the deflection of wind direction,
1. in an anticlockwise direction in the N Hemisphere and
2. in a clockwise direction in the S Hemisphere.
4. The air gets warm and up-thrusts rapidly in a spiral path.
1. The air rises and cools. Water vapour condenses.
2. Towering cumulonimbus clouds form. Latent heat of condensation is released.
5. The energy released causes further heating and uplifting.
1. This draws in more surrounding warm moist air which forms more clouds and releases more latent heat.
2. This strengthens the cyclonic circulation.
3. Huge cumulonimbus brings torrential rain.
6. Once the tropical cyclone moves inland, it weakens and dies out quickly because
1. it loses its source of energy (warm, moist air which provides latent heat), and
2. mountain barriers reduce the speed of the winds.
4. Structure of tropical cyclones
1. The small centre of the tropical cyclone is the "eye", characterised by cool and descending air and calm condition.
2. The uplifted air flows outward from the centre at higher levels. Moist air from the surrounding sea surface draws in.
3. A vortex, with spiral ascending currents of strong gale, surrounds the eye. Nimbocumulus extends to great height.
5. Movements of tropical cyclones
1. Tropical cyclones generally move westward following the path of the trade winds.
2. Tropical cyclones may be deflected to the right in the N Hemisphere by the Coriolis effect.
6. High wind velocity accompanied by heavy rainfall may cause great damage and destruction to buildings and vegetation.

1. A temperate depression is a low pressure area formed around 60^oN and S.
2. Life cycle of a temperate depression
1. Stage 1
1. Cold, dry polar air moves W. Warm, moist tropical air moves E.
2. The two air masses meet. A stationary front forms.
2. Stage 2
1. A small bulge or wave develops in the front due to friction. As the bulge develops, pressure at its centre falls.
2. Warm air pushes into the wave. Cold air pushes into the warm air at the rear of the wave.
3. Winds blow into the low pressure centre, due to the deflection of wind direction,
1. in an anticlockwise direction in the N Hemisphere and
2. in a clockwise direction in the S Hemisphere.
3. Stage 3
1. The wave continues to develop.
2. The warm air, being lighter, rises over the cold air. At the front of the wave, a warm front develops.
3. The cold air undercuts the warm air. At the rear of the wave, a cold front develops.
4. The warm air between the two fronts is called a warm sector.
4. Stage 4
1. The cold front advances faster than the warm front and finally overtakes the warm front.
2. The two fronts merge together, an occluded front develops.
5. Stage 5
1. The warm air in the warm sector is completely uplifted and is lost in the upper air.
2. The cyclone begins to die out.
3. Changes in weather caused by a temperate depression
1. During the passage of a warm front
1. The temperature rises.
2. The air pressure drops.
3. Clouds develops, and rain is getting heavier.
2. In the warm sector
1. The temperature is high.
2. The air pressure is low.
3. The humidity is high, rain stops.
3. During the passage of a cold front
1. The temperature drops.
2. The air pressure rises.
3. Cumulonimbus brings heavy rain and thunderstorms.
1. The sloping boundary of the cold front is steeper than that of the warm front.
2. The precipitation along the cold front is much heavier and shorter in duration than that along the warm front.
4. After the passage of the depression
1. The temperature is low.
2. The pressure is high.
3. The sky is clear.

17-1 Air masses and fronts

1. An air mass is a large body of air which has a fairly uniform temperature and humidity.
1. An air mass develops over an extensive surface which is physically uniform, e.g. oceans, deserts.
2. The characteristics of an air mass are derived from the region over which it formed.
3. When the air mass moves away, even over a considerable distance, its characteristics retain.
4. As the air mass passes over an area, it modifies the weather of the area.
2. Air masses are classified according to their place of origin.
1. Continents or oceans
1. continental c - dry air mass formed over the continents
2. maritime m - moist air mass formed over the oceans
2. Latitudes
1. Arctic A - Arctic ocean and fringing land
2. Antarctic AA - Antarctica
3. Polar P - 50-60^oN or S (continents and oceans)
4. Tropical T - 20-35^oN or S (continents and oceans)
5. Equatorial E - near the equator (oceans)
3. In spring, the maritime tropical (mT) air mass moves over Hong Kong. It is warm and moist.
1. The surface of Hong Kong is cooler than the warm air.
2. The bottom of the air is cooled to form advection fog.

1. A front is the surface of contact between two unlike air masses develop when the two unlike air masses meet.
2. Polar front is formed by the meeting of a cold polar air mass and a warm tropical air mass.

18-1 Sea breezes and land breezes

1. Sea breezes
1. During the day, the land is heated more rapidly than the sea. The land is warmer than the sea.
2. A high pressure area develops locally over the sea.
3. A gentle sea breeze blows from the sea to the land.
2. Land breeze
1. At night, the land is cooled more rapidly than the sea. The land is cooler than the sea.
2. A high pressure area develops locally over the land.
3. A gentle land breeze blows from the land to the sea.
3. Land and sea breezes moderate the temperature of the coastal areas.

1. Anabatic winds
1. During the day, the mountain slopes are intensely heated.
2. Air is heated, expands and rises.
3. A low pressure area develops locally on higher slopes.
4. A warm anabatic wind (or valley wind) blows from the valley floor up to the mountain slopes.
2. Katabatic winds
1. At night, the mountain slopes lose heat by radiation rapidly.
2. Air is cooled, contracts and descends.
3. A high pressure area develops locally on higher slopes.
4. A cold katabatic wind (or mountain wind) blows from the mountain slopes down to the valley floor.
3. The cold katabatic wind may lead to temperature inversion.

1. Fohn winds, chinook winds etc. are warm, dry winds which descends on the leeward side of a mountain.
1. Fohn wind is on the northern slopes of the Alps.
2. Chinook wind is on the eastern slopes of the Rockies.
2. When air meets a mountain, it is forced to rise up the windward slope of the mountain.
3. Unsaturated air cools at the dry adiabatic lapse rate (DALR) 10^oC per 1000 m.
4. Air becomes saturated. Water vapour condenses into water droplets.
5. Clouds and heavy rain are formed on the windward slope.
6. Saturated air cools at the saturated adiabatic lapse rate (SALR) 5^oC per 1000 m.
7. After the air crosses over the mountain, the air has lost most of its moisture.
8. Dry air descends the leeward slope under the force of gravity.
9. The air is compressed and warmed adiabatically at the dry adiabatic lapse rate (DALR) 10^oC per 1000 m.

1. Wind vane
1. The wind direction is the compass point from which it blows, e.g. a north wind is one which blows from N to S.
2. A wind vane is used to measure wind direction.
1. A wind vane consists of a pointer which rotates freely on a vertical shaft.
2. A framework with the four directions (N, E, S and W) is fixed on the vertical shaft.
3. When wind blows, the pointer rotates and points into the wind.
3. A wind vane should be mounted at least 10 m above the ground surface and should not be obstructed by buildings.
2. Anemometer
1. The wind speed is measured in m s^-1 or km h ^-1.
2. An anemometer is used to measure wind speed.
1. An anemometer consists of three cups fixed to three metal arms on a rotating spindle.
2. When wind blows, the cups rotate. The speed of revolution is recorded by a meter.
3. The wind speed can be estimated by visual observation using the Beaufort Scale.
3. Presenting wind direction and wind speed on weather charts
1. On weather charts, the wind direction and wind are recorded by an arrow.
2. The arrow shaft shows the wind direction which points towards the weather station.
3. The arrow feathers (fleche) or pennents (solid triangle) shows the wind speed.
1. a half feature = 2.5 m s^-1
2. a full feather = 5 m s^-1
3. a pennent = 25 m s^-1
4. A circle represents calm or light variable wind.
4. Wind rose
1. A wind rose shows the frequency of occurrence of winds at a certain place over a period time, e.g. a month.
2. A wind rose consists of a small circle from which bars radiate.
3. The direction of the bar shows the direction of wind in compass point.
4. The length of the bar indicates the frequency of wind from that direction.
5. The number of calm days or the percentage of calm days is given in the circle.

1. Humidity is the amount of moisture in the air, which comes from evaporation of water from seas, lakes and ground.
2. Absolute humidity is the actual weight of water vapour in the air, measured in g m^-3 (g of water vapour per m³ of air).
3. The maximum amount of water vapour mass of air can hold is its capacity. Warm air can hold more moisture than cold air.
4. Relative humidity is the percentage of the absolute humidity to the capacity, e.g.
1. Air temperature at 30^oC has a capacity of about 30 g of water vapour per m³ of air.
2. Suppose the absolute humidity of a mass of air at 30^oC is 25 g m^-3.
1. The air can still hold more moisture.
2. The relative humidity is 25 / 30 = 83%.
3. Suppose the mass of air is chilled to 26.5^oC.
1. The absolute humidity of 25 g m^-3 is sufficient to make the air saturated.
2. The relative humidity is 100%. This temperature is called the dew point.
4. Water vapour changes into water droplets in condensation when there is
1. any additional moisture, or
2. further cooling of the air below the dew point.

1. Humidity is measured a hygrometer.
2. One type of hygrometer is the wet and dry bulb thermometer.
1. It consists of a wet-bulb thermometer and a dry-bulb thermometer which are kept in a Stevenson Screen.
1. The dry bulb thermometer is an ordinary thermometer which reads normal air temperature.
2. The wet bulb thermometer has its bulb wrapped by a piece of wet cloth, e.g. muslin, dipped in water in a container.
2. When water evaporates from the wet cloth, heat is lost. The temperature is lowered.
3. The temperature difference is used to find the relative humidity using the hygrometric tables, e.g.
1. If the dry bulb is 26^oC and the wet bulb is 22^oC, the wet bulb depression (temperature difference) is 4^oC.
2. The hydrometric table shows that the relative humidity is 69%.
3. If the air is drier, the sky is clear or cloudless.
1. Water evaporates faster. There is a greater heat loss.
2. The wet bulb depression is greater. The relative humidity is lower.
4. If the air is wetter, it may be foggy or rainy.
1. Water evaporates slower. There is a smaller heat loss.
2. The wet bulb depression is smaller. The relative humidity is higher.
5. If the air is saturated, the air is very wet.
1. Water does not evaporate. There is no heat lost.
2. The wet bulb and dry bulb thermometers read the same. The relative humidity is 100%.

22-1 Fog

1. Characteristics of fog
1. Fog is a dense mass of water droplets near the land or sea surfaces.
2. The occurrence of fog is associated with low visibility, high relative humidity, lack of sunshine and gentle or no wind.
3. Fog may be dispersed by evaporation when temperature rises during the day, or when there is strong wind.
4. There are two types of fog: radiation fog and advection fog.
5. Advection fog is common in Hong Kong in spring when
1. the warm moist southerly wind moves over the land or sea surface which is still cold in early spring, or
2. the warm moist southerly wind meets the cool dry northerly wind.
2. Formation of radiation fog
1. At night, the calm (no wind) and clear (no cloud) sky causes rapid terrestrial radiation and rapid cooling on the ground.
2. As there is no wind, the moist air is in contact with the cold ground long enough to be chilled to the dew point.
3. Water vapour condenses on hygroscopic nuclei (or condensation nuclei, e.g. dust and smoke) in the air.
3. Formation of advection fog
1. A warm moist air passes over a cooler land or sea surface.
2. The lower layer of the warm moist air in contact with the cooler surface is chilled to the dew point.
3. Water vapour condenses on hygroscopic nuclei (or condensation nuclei, e.g. dust and smoke) in the air.

1. Characteristics of dew
1. Dew is the deposit of water droplets on the ground surface.
2. Dew drops disappear through evaporation shortly after sunrise.
3. In Hong Kong, dew is common in cool nights in spring and autumn.
2. Formation of dew
1. At night, the calm (no wind) and clear (no cloud) sky causes rapid terrestrial radiation and rapid cooling on the ground.
2. The moist air near the ground is cooled below the dew point which is above 0^oC.
3. Water vapour condenses into water droplets as dew on the ground surface, e.g. on leaf surfaces.

1. Characteristics of frost
1. Frost is the deposit of ice crystals on the ground surface.
2. Frost in early spring may damage crops.
3. In Hong Kong, frost occurs occasionally on higher grounds in late winter with the arrival of strong cold northerly winds.
2. Formation of frost
1. At night, the calm (no wind) and clear (no cloud) sky causes rapid terrestrial radiation and rapid cooling on the ground.
2. The moist air near the ground is cooled below the dew point which is below 0^oC.
3. Water vapour condenses into ice crystals as frost on the ground surface, e.g. on leaf surfaces.

1. Cloud is a visible mass of water droplets or ice particles suspended in the upper level air.
2. Formation of clouds
1. When air rises, the air is cooled.
2. When the air reaches the condensation level, the air is cooled to the dew point.
1. The height of condensation level depends on the temperature and humidity of the rising air.
2. The condensation level will be lower if the air is wetter and warmer.
3. The air saturates and the relative humidity is 100%.
4. Water vapour condenses into water droplets which suspend in the air.
5. The water droplets gather to form clouds.
3. Types of clouds classified according to forms
1. Cumuliform clouds are vertical tower clouds.
1. Cumuliform clouds are formed when the air rises by strong convection currents.
2. Cumuliform clouds are common in hot summers.
2. Stratiform clouds are sheet clouds in horizontal layers.
1. Stratiform clouds are formed when the air rises over mountains, or over the cold air along a warm front.
2. Stratiform clouds are common in cold winters.
4. Types of clouds classified according to height
1. High clouds are clouds between 6 000 and 12 000 m.
1. High clouds include cirrus (Ci), cirrocumulus (Cc) and cirrostratus (Cs).
2. High clouds are thin, white and fleecy.
3. High clouds indicate fine weather.
2. Medium clouds are clouds between 2 000 and 6 000 m.
1. Altocumulus (Ac) are patches of globular masses.
2. Altostratus (As) are greyish and produce snow or rain.
3. Medium clouds indicate fair weather.
3. Low clouds are clouds below 2 000 m.
1. Cumulus (Cu) and stratocumulus (Sc) are white and indicate fair weather.
2. Stratus (St) and nimbostratus (Ns) are greyish and indicate bad weather.
4. Cumulonimbus (Cb) are formed by strong rising convection currents on hot summer days.
1. Cumulonimbus are high vertical clouds at 500 m at the base and extend to 9 000 m to 12 000 m.
2. Cumulonimbus often look like a flat-topped anvil since the air at the upper level often spreads out.
3. Cumulonimbus indicate unstable weather with torrential rain, hail, gust, thunder and lightning.

23-1 Rain

1. Rain is a form of precipitation consisting of tiny water droplets which coalesce into larger droplets of 1 to 5 mm in diameter.
2. Water vapour condenses into water droplets if the dew point at the condensation level is above 0^oC.

1. Snow is a form of precipitation consisting of tiny ice crystals which coalesce into larger snowflakes.
2. Water vapour sublimes into ice crystals if the dew point at the condensation level is below 0^oC.
3. If the ground temperature is below 0^oC, the snowflakes fall onto the ground as snow.

1. Characteristics of hail
1. Hail is a form of precipitation consisting of small ice pellets called hailstones of 5 to 50 mm in diameter.
2. A hailstone has alternate concentric layers of clear ice and opaque ice.
2. Formation of hail
1. Strong rising convection current produces cumulonimbus.
2. Water droplets are carried upwards by violent up-draughts to higher level which is below 0^oC.
3. The water droplets freeze as clear ice crystals.
4. Strong up-draughts carry the ice crystals to an even higher level.
5. Super-cooled water droplets freeze on the ice crystals. The ice pellets accret an addition "shell" of ice.
6. The ice pellets rise and fall many times until they grow heavy enough. Then they fall on the ground as hailstones.

24-1 Convectional rain

1. Occurrence of convectional rain
1. Convectional rain occurs in hot, wet equatorial and tropical regions.
2. In higher latitudes, convectional rain occurs in continental interiors in hot summers.
2. Formation of convectional rain
1. The land is intensely heated. Surface air in contact with it gets hot by conduction.
2. Hot air rises and expands as strong convection currents.
3. When air rises, the air is cooled. When the air reaches the condensation level, the air is cooled to the dew point.
4. The air saturates and the relative humidity is 100%.
5. Water vapour condenses into water droplets which suspend in the air. The water droplets gather to form clouds.
6. Raindrops are formed by coalescence of water droplets.
3. Characteristics of convectional rain
1. Convectional rain is torrential rain of short duration, often in the afternoon.
2. Convectional rain is often associated with cumulonimbus and thunderstorms.
1. Strong rising convection current produces cumulonimbus.
2. Water droplets are carried up and down by the convection current.
3. Negative charges are collected at the bottom and positive charges at the top of the cloud.
4. The ground surface has positive charges.
5. As the charges grow, a strong electric potential difference develops.
6. Discharge takes place as lightning within the clouds or between the cloud and the ground.
7. The air is heated up suddenly by the lightning and expands explosively causing thunder.

1. Occurrence of relief rain
1. Relief rain occurs in coastal areas where the trend of mountain runs parallel to the coast.
2. The windward slope is the rain-bearing side.
2. Formation of relief rain
1. Warm moist onshore wind meets coastal mountain.
2. Air rises along the windward slope and expands.
3. When air rises, the air is cooled. When the air reaches the condensation level, the air is cooled to the dew point.
4. The air saturates and the relative humidity is 100%.
5. Water vapour condenses into water droplets which suspend in the air. The water droplets gather to form clouds.
6. Raindrops are formed by coalescence of water droplets.

1. Cyclonic rain occurs in temperate latitudes but it may extend to lower latitudes in winter.
2. Formation of cyclonic rain
1. Warm, moist tropical air mass meets a dry, cold polar air mass. A warm polar front forms between the two air masses.
1. The cold polar air is denser and heavier. It remains close to the ground.
2. The warm tropical air is less dense and lighter. It rises over the polar air and expands.
2. When air rises, the air is cooled. When the air reaches the condensation level, the air is cooled to the dew point.
3. The air saturates and the relative humidity is 100%.
4. Water vapour condenses into water droplets which suspend in the air. The water droplets gather to form clouds.

1. Temperature and pressure
1. Areas with high temperatures have abundant rainfall.
1. Warm air forms low pressure.
2. Air rises, expands and is cooled. Water vapour condenses into water droplets to form cloud and rain.
2. Areas with low temperatures have little rainfall.
1. Cold air forms high pressure.
2. Air sinks, compresses and is warmed. This discourages condensation and the formation of cloud and rain.
2. Winds
1. On-shore winds are usually wet and bring rain.
2. Off-shore winds are usually dry and do not bring rain.
3. Distance from the sea
1. Coastal areas are wet because onshore wet winds bring rain.
2. Inland areas are dry because onshore winds reaching there have already lost their moisture and do not bring rain.
4. Ocean currents
1. Onshore winds are wet when they pass over a warm ocean current.
2. Onshore winds are dry when they pass over a cold ocean current.
5. Relief
1. When onshore winds are forced to climb over a mountain along the coast, the windward slope is the rain-bearing side.
2. The coastal mountain acts as a climatic barrier, the leeward slope is the rainshadow area.

1. Regions with rainfall all year
1. Equatorial climate
1. The equatorial region has heavy rainfall throughout the year.
2. The Equatorial Low pressure belt is formed.
3. Throughout the year, rain is brought by
1. convection uplift due to strong heating, and
2. rain is brought by convergence of trade winds.
2. Warm temperate east margin climate
1. The east coast in warm temperate region has rainfall throughout the year with a summer maximum.
2. In summer, rain is brought by onshore monsoon winds.
3. In winter, rain is brought by temperate cyclones.
3. Cool temperate west margin climate
1. The west coast in cool temperate region has rainfall throughout the year with a winter maximum.
2. Throughout the year, rain is brought by onshore westerlies.
3. In winter, rain is brought by temperate cyclones.
2. Regions with rainfall mainly in summer
1. Tropical monsoon climate
1. The east coast in tropical region has a wet summer and a dry winter.
2. In summer, rain is brought by onshore monsoon winds.
3. In winter, offshore monsoon winds are dry.
2. Tropical continental climate
1. The interior and west coasts in tropical regions has a wet summer and a dry winter.
2. In summer, rain is brought by convection uplift due to strong heating.
3. In winter, winds are out-blowing and dry because the Subtropical High pressure belts extend over these region.
3. Cool temperate east margin climate
1. The east coast in cool temperate region has a wet summer and a dry winter.
2. In summer, rain is brought by convection uplift due to strong heating, and onshore monsoon winds.
3. In winter, offshore monsoon winds are dry.
3. Regions with rainfall mainly in winter
1. Warm temperate west margin climate (or Mediterranean climate)
1. The west coast of warm temperate region has a dry summer and a wet winter.
2. In summer, offshore trade winds are dry.
3. In winter, rain is brought by onshore westerlies and depressions because the westerlies shift southwards.
4. Regions with scanty rainfall all year
1. Tropical desert climate
1. The tropical desert is dry throughout the year.
2. Offshore trade winds are out-blowing and dry because of the Subtropical High pressure belt.
2. Temperate continental climate
1. The continental interior of temperate region is dry throughout the year.
2. It is dry because of continentality and rainshadow effect.
3. Arctic climate
1. The polar region is dry throughout the year.
2. Polar winds are out-blowing and dry because of the Polar High pressure belt.

1. Instrument for measuring rainfall
1. Rainfall is recorded by a rain gauge.
2. A rain gauge consists of metal cylinder containing a glass bottle.
1. There is a metal funnel on top of the cylinder; its diameter may be 128 mm, 204 mm or 254 mm.
2. When it rains, rain water drops a into the glass bottle through the metal funnel.
3. The water is measured daily by a measuring cylinder. Rainfall is measured in mm.
4. The measuring cylinder is tapered so that a small amount of rainfall can be measured.
5. If the amount of rain water is too small to be measured, it is recorded as a "‘trace".
3. Installation of the rain gauge
1. The rain gauge is placed in an open space away from trees, walls and buildings.
2. The outer case is half sunk into the ground to prevent it from being knocked over.
3. The funnel rim is 30 cm above the ground surface to prevent the rain from splashing into the gauge from the ground.
2. Rainfall recordings
1. monthly rainfall = total daily rainfall of the month
2. mean monthly rainfall = average rainfall of a particular month for a long period of time, e.g. 30 years
3. annual rainfall = total monthly rainfall of the year
4. mean annual rainfall = average annual rainfall for a long period of time, e.g. 30 years
3. Presentation of rainfall recordings
1. Mean monthly rainfall can be presented on a climatic graph.
2. A rainfall map shows the distribution of rainfall over a large area.
3. An isohyets is a line joining all points with the same rainfall by interpolation, usually drawn at regular intervals, e.g. 25 mm.

28 Climate of Hong Kong

28-1 Temperature

1. Hong Kong lies just within the tropics, on latitude 22^o20' N.
2. Hong Kong has a subtropical climate with a hot summer and a cool winter.
3. Mean annual temperature is 23.0^oC. (1961-1990)
1. The hottest month is July. The mean monthly temperature is 28.8^oC.
2. The coolest month is January. The mean monthly temperature is 15.8^oC.
4. Hong Kong is located on the SE coast of Asia.
5. The maritime influence has a moderating effect on temperatures.
1. The mean annual range of temperature is small, 28.8^oC - 15.8^oC = 13^oC.
2. The mean diurnal range of temperature is small, below 6^oC.

1. The highest atmospheric pressure is 1020.2 hPa in December and January.
2. The lowest atmospheric pressure is 1005.1 hPa in August.
3. Pressure systems affecting Hong Kong
1. High pressure systems affecting Hong Kong include anticyclone and ridge of high pressure.
2. Low pressure systems affecting Hong Kong include tropical cyclone and trough of low pressure.
4. Monsoon system
1. The prevailing wind in winter is NE.
2. The prevailing wind in July is SW.
5. Tropical cyclone (typhoon)
1. About 5 to 6 tropical cyclones affect Hong Kong each year.
1. Tropical cyclones affecting Hong Kong are formed over W Pacific Ocean and S China Sea in summer.
2. The tropical cyclones generally follow a parabolic track and move W or WNW and later turn NE.
2. Different names are used for tropical cyclones by Hong Kong Observatory according to their intensity.
1. Tropical depression (T.D.) wind speed < 63. km h^-1
2. Tropical storm (T.S.) wind speed 63 - 87 km h^-1
3. Severe tropical storm (S.T.S.) wind speed 88 - 117 km h^-1
4. Typhoon (T.) wind speed > = 118 km h^-1
3. Tropical Cyclone Warning Signals
1. 1 Stand By

A tropical cyclone is centred within 800 km of Hong Kong and may later affect Hong Kong.
2. 3 Strong Wind

Strong wind with a speed of 41-62 km h^-1 and gusts which may exceed 100 km h^-1.
3. 8_NW, 8_SW, 8_NE and 8_SE Gale or Storm

Gale or storm with a speed of 63-117 km h^-1 from the quarter indicated and gusts which may exceed 180 km h^-1.
4. 9 Increasing Gale or Storm

Gale or Storm with a speed of 88-117 km h^-1 expected to increase significantly in strength.
5. 10 Hurricane

Hurricane force wind with a speed > = 118 km h^-1 and gusts which may exceed 220 km h^-1.

1. Mean annual rainfall is 2214.3 mm.
2. The seasonal distribution of rainfall in Hong Kong
1. About 80% of the rain falls between May and September.
2. The monsoon system characterises a summer maximum.
3. There is no month which is completely without rain.
1. The wettest month is August. The mean monthly rainfall is 391.4 mm. Rain falls about four days out of seven.
2. The driest month is January. The mean monthly rainfall is 23.4 mm. Rain falls about six days in the month.
3. The spatial distribution of rainfall in Hong Kong
1. The wettest parts are on the windward slopes of Tai Mo Shan, Lantau Peak, Ma On Shan and Pat Sin Range.
2. The NW parts of Hong Kong are rainshadow areas of these mountains which run generally from NE to SW.
3. Yuen Long is the driest areas. (see Agricultural Activities - Farming constraints in Hong Kong.)
4. Rainfall brought by tropical cyclones greatly affects the overall rainfall of Hong Kong.
1. A quarter of the mean annual rainfall in Hong Kong comes from tropical cyclones, especially in late summer and autumn.
2. However, rainfall associated with tropical cyclone and their contribution to the annual total vary greatly from year to year.
5. Rainstorm Warning Signals
1. A Rainstorm Amber Warning Signals indicates over 30 mm rainfall has been recorded in one hours.
2. A Rainstorm Red Warning Signal indicates 50 mm rainfall has been recorded in one hour.
3. A Rainstorm Black Warning Signals indicates over 100 mm rainfall has been recorded in two hours.

In Hong Kong four well-marked seasons can be identified, but they are of unequal duration.

1. Spring, from March to April, is damp and foggy.
2. Summer, from May to September, is hot and wet.
3. Autumn, from October to November, is dry and sunny.
4. Winter, from November to February, is cold and dry.

1. Spring is a transitional season.
2. The anticyclone in central Asia begins to weaken.
1. The cold dry NE offshore monsoon is gradually replaced by the warm moist SE onshore monsoon.
2. The pressure gradient is gentle. Winds are light. Air is calm.
3. Temperature begins to rise. The temperature is mild when compared to winter.
4. Air pressure begins to fall.
5. Relative humidity begins to rise.
6. Advection fog is formed.
1. Northerly winds weaken. Warm moist air moves onshore.
2. The coastal water near Hong Kong remains relatively cool because the sea is heated up more slowly.
3. The warm moist onshore air passes over the cooler sea surface and is chilled and condenses into fog.
1. Fog results in low visibility, disturbs air and water transport, and causes traffic accidents.
2. High relative humidity provides moisture to crops but causes discomfort to man.
4. Low stratus cloud and drizzle are formed.
7. Cold front brings unstable changeable weather. (see 32)
1. With the passage of a cold front, temperature falls and air pressure rises. Torrential frontal rain falls.
2. Fog dissipates. The relative humidity falls.
3. Winds become stronger and change in direction from southerly to northerly.
4. When the cold front has passed, the weather becomes fine.

1. In northern summer, the sun is overhead N of equator. The Asian land mass is intensely heated.
1. The interior of Asia forms an intense low pressure area.
2. The central part of Australia forms an intense high pressure area (where it is the southern winter).
3. Wind blows from high pressure to low pressure.
4. Dry winter SE monsoon winds blow from the high pressure in the central part of Australia.
1. After crossing the equator, the SE monsoon winds are deflected to become the SW monsoon winds.
2. They continue to blow into Asian low pressure as SE monsoon winds.
5. By the Coriolis effect, the wind blowing into low pressure in the interior of Asia is anticlockwise.
2. Temperature is high.
3. Air pressure is low.
4. Warm moist onshore SW monsoon winds bring torrential rain to Hong Kong.
5. Relative humidity is high.
6. Trough of low pressure (see 33) and tropical cyclones (see 34) may affect Hong Kong.

1. Autumn is another short transitional season. It is the most pleasant season of the year.
2. The low pressure system over the interior of China gradually disappears and is replaced by a high pressure system.
3. Temperature begins to fall.
4. Air pressure begins to rise.
5. Prevailing winds change to a easterly direction.
6. Relative humidity begins to fall.
1. The weather is dry with abundant sunshine.
2. Rainfall stop suddenly when the NE monsoon blow at the end of October.

1. In northern winter, the sun is overhead S of equator. The N of Australia is intensely heated.
1. The interior of Asia forms an intense high pressure area.
2. The N of Australia forms an intense low pressure area (where it is the southern summer).
3. Wind blows from high pressure to low pressure.
4. Dry winter NW monsoon winds blow from N China and Japan, NE monsoon winds from S China and SE Asia.
5. After crossing the equator, the monsoon winds are deflected to become the NW monsoon winds.
2. Temperature is low.
3. Air pressure is high.
4. Cold dry offshore NE monsoon winds bring little rain to Hong Kong.
5. Relative humidity is low. The weather is fine with little cloud and rain.
1. There may be some occasional showers or drizzles when the cold front is moving S.
2. On exceptional cold night ground frost may occur.
6. Anticyclone (see 30) and ridge of high pressure (see 31) may affect Hong Kong.

30 Anticyclone

1. An anticyclone is presented on a weather chart by more or less circular isobars, with the highest pressure at the centre.
2. In winter, anticyclones develop over W China and is strong enough to influence Hong Kong.
1. The weather is cold, fine and settled.
2. Rapid night cooling may suddenly lower the temperature to freezing point on higher ground and frost is formed.
3. Prevailing winds come mainly from the N or NE.
4. N and NE facing slopes are much cooler than S-facing slopes.
3. Occasionally, in summer, anticyclones may form over the Pacific Ocean and extends W to affect Hong Kong.
1. The weather is hot and sunny.
2. This type of weather may last from a few days to one or two weeks.
4. Figure 21.3 (in the textbook) shows an anticyclone.
1. The centre of the anticyclone was over E China.
2. The pressure at the centre was 1033 hPa.
3. In E and N China, temperatures were low.
4. Many stations near the centre had calm conditions.
5. Winds in S China were northerlies, since the air was out-blowing from the high pressure centre in a clockwise direction.
5. At 0200 hours on 25 February 1993 (Figure 21.3), the weather conditions in Hong Kong were
1. Pressure: 1022 hPa
2. Temperature: 15^oC
3. Wind speed: 2.5 m s^-1
4. Wind direction: ESE
5. Precipitation: Nil

1. A ridge of high pressure is a narrow area of heavy air extending from an anticyclone.
2. The weather in the ridge is very fine but this lasts only a short time.
3. Figure 21.4 (in the textbook) shows a ridge of high pressure over central China.
1. Low pressure was found on both sides of the ridge of high pressure.
2. The weather associated with the ridge was fine.
3. Winds were light, calm or below 2.5 m s^-1.
4. At 0200 hours on 11 February 1993 (Figure 21.4), the general weather conditions in Hong Kong were
1. Pressure: 1021 hPa
2. Temperature: 16^oC
3. Wind speed: 2.5 m s^-1
4. Wind direction: SE
5. Precipitation: Nil

1. The cold front is the boundary of this cold polar air and warm tropical air.
1. Sometimes it is associated with temperate depression.
2. The cold heavy air undercuts the warmer air.
3. The warm air is forced to rise, forming cloud and rain.
2. Figure 21.11 (in the textbook) show the passage of a cold front from N to S across Hong Kong.
3. It brings the following weather changes:
1. drop in temperature,
2. rise in air pressure,
3. change in wind direction,
4. increase in wind speed, and
5. heavy rain and thunderstorm.
4. At 0200 hours on 24 February 1994 (Figure 21.11(a)), the general weather conditions in Hong Kong were
1. Pressure: 1011 hPa
2. Temperature: 21^oC
3. Wind speed: 0 m s^-1
4. Wind direction: -
5. Precipitation: Nil
5. At 0200 hours on 25 February 1994 (Figure 21.11(b)), the general weather conditions in Hong Kong were
1. Pressure: 1019 hPa
2. Temperature: 14^oC
3. Wind speed: 2.5 m s^-1
4. Wind direction: NE
5. Precipitation: Nil. (The cold front had passed. Cold air is dry.)

1. A trough is an elongated area of low pressure extending from a depression.
1. An axis along the middle of the trough marks the line of lowest pressure.
2. On weather maps, the axis is shown by a black line.
3. In summer, a low pressure trough is formed with its axis extends approximately E-W over Hong Kong.
2. Figure 22.7 (in the textbook) shows a trough of low pressure.
1. Winds blow inwards to the axis of the trough.
2. There is sudden change in wind direction.
3. There is bad weather with thunderstorms, heavy rain and strong winds.

1. Tropical cyclones or typhoons hit Hong Kong during summer and autumn.
2. Before Typhoon Helen Arrives (Figure 21.7 in the textbook)
1. The air was generally still.
2. The temperature and humidity were high.
3. High clouds were formed.
4. Air pressure began to drop.
5. At 0200 hours on 10 August 1995 (Figure 21.7), the general weather conditions in Hong Kong were
1. Pressure: 1010 hPa
2. Temperature: 27^oC
3. Wind speed: 0 m s^-1
4. Wind direction: -
5. Precipitation: Nil
6. The Standby Signal No.1 was hoisted at 1600 hours on 9 August 1995.
3. Typhoon Coming near Hong Kong
1. The front vortex approached Hong Kong.
2. Pressure dropped steadily.
3. Winds were gusty.
4. Showers occurred more frequently.
5. At 0200 hours on 11 August 1995 (Figure 21.8), the general weather conditions in Hong Kong were
1. Pressure: 1008 hPa
2. Temperature: 27^oC
3. Wind speed: 2.5 m s^-1
4. Wind direction: SE
5. Precipitation: Nil
6. The Strong Wind Signal No. 3 was hoisted at 0545 hours on 11 August 1995.
7. The North-easterly Gale or Storm Signal No.8 NE was hoisted at 2230 hours on 11 August 1995.
1. Pressure dropped steadily.
2. Winds intensified into gales and turned north-easterly.
3. Dense cumulonimbus brought heavy downpours.
4. Strong gusts accompanied by torrential rain can cause severe damage to Hong Kong.
8. The No. 8 NE was replaced by No, 8 NW Gale or Storm Signal at 0430 hours on 12 August 1995.
9. At 0200 hours on 12 August 1995 (Figure 21.9), the general weather conditions in Hong Kong were
1. Pressure: 998 hPa
2. Temperature: 25^oC
3. Wind speed: 5 m s^-1
4. Wind direction: NE
5. Precipitation: Shower
4. Typhoon Centre over Hong Kong
1. Pressure dropped to the lowest, 993 hPa.
2. Wind suddenly reduced to gentle or moderate breeze.
3. The rain stopped and the sky became clear. Such a calm period may last for one to a few hours.
5. Typhoon Leaving Hong Kong
1. The rear vortex brings more violent winds. There are dense clouds and torrential rain again.
2. Winds direction are opposite to that brought by the front vortex.
3. Flooding and landslides may bring more damage.
6. Typhoon Moving away from Hong Kong
1. The pressure gently rises again.
2. Heavy rainfall may still continue for several hours.
3. Gusty winds are replaced by light breezes.
4. There is normal weather soon after the cyclone has completely passed.

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