Chapter 1(I): WEATHER STUDIES

Weather & Climate

Composition & Structure of Atmosphere

ELEMENTS OF WEATHER

• Temperature

• Humidity

• Clouds

• Precipitation/ Rainfall

• Pressure

• Winds

STUDYING WEATHER

• Weather Instruments

• Stevenson Screen

• Synoptic Chart

QUIZ

WEATHER & CLIMATE

Weather

The state of the atmosphere at a particular place at a specific time or over a short time.

Climate

The average weather conditions over a period of at least 30 years.

INTRODUCTION ON ATMOSPHERE
Structure

COMPOSITION & STRUCTURE OF ATMOSPHERE

Layers of Earth's Atmosphere : 4 layers

The Atmosphere & Earth-Space Interface

- shows layers of the Earth's atmosphere with emphasis on ionized layers.

ELEMENTS OF WEATHER          [Back to Top]

I. TEMPERATURE

   Temperature is the degree of hotness or coldness of the atmosphere.

   It is measured in °C or °F.

Factors Affecting Temperature:

• Latitude

• Altitude

• Distance From The Sea

• Ocean Currents

• Humidity & Cloud Cover

• Aspect (Microclimatic factor)

• Types of Land Surfaces

Instrument Used For Measuring Temperature

• Maximum & Minimum Thermometer

Maximum thermometer (mercury)

Minimum thermometer (alcohol)

• Six's Thermometer

Calculation of Temperature

1. Diurnal (daily) range of temperature:

2. Average (mean) daily temperature:

3. Mean monthly temperature:

4. Mean annual temperature:

II. HUMIDITY

Humidity is the amount of water vapour in the atmosphere. It can be measured in terms of relative humidity, in %. 

Terms & Definitions

• Saturation point - the point where atmosphere has reached its maximum capacity to hold water (can no longer hold additional water vapour). The air is saturated (moist).

• Dew point temperature - the temperature at which saturation takes place.

• Absolute (specific) Humidity - the actual amount of water vapour in a given volume of air (g/m³) at a given temperature.

• Relative Humidity - the ratio of the actual amount of water vapour in the air compared with the maximum amount of water vapour that a unit of air can hold at that temperature.

Instrument Used For Measuring Relative Humidity

• Hygrometer (wet & dry bulb thermometer)

III. CLOUDS

Clouds are formed when air above the earth's surface is cooled below its dew point.

FOUR main groups of clouds, based on their height, form and appearance:

IV. PRECIPITATION (RAIN)

Precipitation refers to the moisture that falls from the clouds to the ground.

Types of Rainfall:

   Instrument Used For Measuring Rainfall

• Rain gauge

• Tipping bucket

   Calculation Of Rainfall

1. Total daily rainfall

2. Total monthly rainfall:

3. Total annual rainfall:

V. AIR PRESSURE

Air pressure is the weight of air exerted on any exposed surface. It is expressed in millibars (mb).

Factors Affecting Air Pressure:

• Temperature

• Altitude

Instrument Used For Measuring Pressure

Ø Mercury Barometer How a mercury barometer works   Taking pressure readings
Aneroid Barometer How an aneroid barometer works
Aneroid Barograph   How an aneroid barograph works

Read more on air pressure: http://www.usatoday.com/weather/wbarocx.htm

VI. WINDS

Wind is air in motion.

Air moves from high pressure to low pressure areas. These movements give rise to winds.

Factors Affecting Wind:

• The differences in air pressure

• Friction

• Earth's Rotation (Cariolis Effect)
  Animation: Cariolis effect

Cariolis Force

Types of Wind/Wind Systems:

1. Local Winds (small scale)

• Sea Breeze (Day)

• Land Breeze (Night)

2. Regional Winds (large scale)

• Northeast Monsoon Winds (December & January)

• Southwest Monsoon Winds (June& July)

3. Planetary Winds (global scale)

• Trade winds

• Westerlies & Easterlies

	Planetary Pressure System - give rise to planetary winds because winds blow from high pressure to low pressure area to replace the air that has risen. Equatorial low pressure belt (doldrums) Polar high-pressure belts Sub-tropical high-pressure belts (horse latitudes) Sub-polar low-pressure belts

 Wind Measurement:

• Wind vane - measures wind direction

• Anemometer - measures wind speed

• Wind rose

• Beaufort Scale

• Wind sock

STUDYING WEATHER AT METEOROLOGICAL STATION        

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A Meteorological station uses weather instruments to collect and record information on the weather elements so that information about the weather is available and weather forecasts can be made.  

1. WEATHER INSTRUMENTS

• List of Weather Instruments used at meteorological station.

• Siting of Weather Instruments

2. STEVENSON SCREEN

Stevenson Screen

• a wooden box about 120cm above ground.

• contains a Hygrometer and Six's Thermometer.

• painted white to reflect heat.

• four louvred sides to allow free flow of air.

• double layered roof to prevent direct heat of sun from reaching inside of screen.

• located over an open grass field and not on concrete ground, as concrete ground gets heated faster & may affect temperature readings.
   

3. WEATHER FORECASTING

 Synoptic Chart  - shows the weather conditions at different weather stations. The different weather elements are represented by different symbols.

Weather Symbols