GEOGRAPHY 101 Front PageMOISTURE, CONDENSATION & PRECIPITATION
MATERIALS NEEDED FOR CLASS:
Daily Lesson Plans. H.K.C--Take roll and announcements.TEXT: Essential of Physical Geography: 6th Edition, Robert E. Gabler, Robert J. Sager, Daniel L. Wise, and James Peterson. Saunders College Publishing, Harcourt Brace College Publishers 1999. Rand McNally GOODE's WORLD ATLASGENERAL OUTLINE:
VII. Moisture, Condensation, and Precipitation
A. To outline the Hydrologic Cycle.
B. To point-out the Water in the Atmosphere.
C. To identify Saturation and the Dew Point
D. To describe Humidity and the differences between Absolute, Specific, and
Relative Humidity.
E. To portray Condensation.
F. To list the different types of Fogs and other Minor Forms of Condensation.
G. To outline the formation of Clouds
H. To list the different forms of Clouds.
I. To list the different forms of Precipitation.
J. To map the Global distribution of Precipitation.
K. To point-out the Variability of Precipitation.THE MATERIAL IN THIS OUTLINE COVERS PAGES 150-181 BUT THERE IS
MATERIAL NOT IN YOUR BOOK--KNOW IT--KNOW IT--KNOW IT
ANTICIPATORY SET What are some of the unusual qualities of water? What is Adiabatic Heating and Cooling? What is Water Vapor and the Hydrologic Cycle? What are the different forms of Precipitation and what are the conditions causing Precipitation? What is the connection between water and the Atmosphere? How is Precipitation distributed over the planet? What are the sources of Atmospheric Moisture?. How does Condensation work and what are the different forms? How are Clouds formed?
BEHAVIORAL OBJECTIVES To identify water as a universal solvent. To classify Related Humidity Concepts To compare and contrast Stability and Instability and conditional instability.. To identify capillary action of water. To identify horizontal distribution of relative humidity. To identify the different types of Clouds--Cirru s, Stratus, Cumulus or cumiliform, and nimbus. To relate the Movement of Water between the surface and the air. To identify transpiration and evapotranspiration To recall the different forms of Precipitation Rain, Snow, Sleet, Hail, and Glaze. To recall runoff and soil moisture To portray the three factors in Rate of Evaporation To identify Convective Precipitation To recount the Movement on and Beneath Earth's Surface To describe Potential Evapotranspiration To identify the Orographic Precipitation, and the rain shadow effect. To define percolation and identify interstices. To outline the Process of Condensation and the conditions needed for condensation. To describe Frontal Precipitation To identify the six storage areas for water. To recall hygroscopic particles or condensation nuclei. To define Convergent Precipitation To recall Residence times To recount the different types of Fogs--Radiation Fog or Surface Inversion Fog, and Advection Fog. To trace the Global Pattern or Distribution of Precipitation To relate the Water Budget and the Heat Budget To depict minor forms of Condensation like Dew, Frost, and Rime. To recognize Variability of Precipitation. To describe saturation capacity and the dew point. To depict the role of Clouds. To identify the hair hygrometer To relate Adiabatic cooling and heating- To identify Humidity To identify the dry adiabatic lapse rate, Precipitation To describe Absolute Humidity and Specific Humidity condensation level, and the saturated adiabatic lapse rate. To recognize Relative Humidity. It will help if you on the next point to think of Buoyancy.
INSTRUCTIONAL INPUT
Content Methods: Lecture and Classroom discussionI THE NATURE OF WATER: COMMON PLACE BUT
UNIQUEAQUA VITAE--WATER OF LIFE
A. Water is essential for all life on this planet. Water is the most important element on the planet and the most common. Without it, life as we know it could not have evolved. Some organisms can thrive without air but nothing can live without water. Water is needed for photosynthesis, cell growth, protein formation, soil formation, and the absorption of nutrients by plants and animals. It is also necessary for fly fishing and the basic ingredient for Coke and beer. (I thought I might be getting too serious, so I put some other important things that contain water.) (What is the name of life-form which can survive without air?)
B. Most of the water found on the planet is in liquid form and blankets 70 percent of the entire planet, mostly in the Earth's vast oceans. It is a little bit of a misnomer to call this planet Earth meaning land when most of it is water. Even if your chemistry is as bad as mine, you know that water is a compound of two elements hydrogen and oxygen. Water is composed of three atoms and the formula is H2O. (Take a look at Figure 7.1 on p.154 for an inventory of the World's Waters--Know where most of the water is stored and where most of the fresh water is stored.)
UNIVERSAL SOLVENT
C. Water is a powerful solvent which means it can dissolve other things. It is called a "universal solvent," in short it is a mild acid. Sometimes this action is fast, other times it is slow. This results in water containing trace elements other than hydrogen and oxygen. One of the trace elements in rain water is carbon dioxide combined with water it becomes a weak form of carbonic acid. Water carries dissolved materials and some very small solid particles in suspension. Water is almost always impure because it is the "universal solvent." Pure water only exists when it is distilled. Pure water, something that does not occur in nature, is colorless, tasteless, and has no smell. Water not only carries dissolved materials but particles as well. Sometimes, depending on stream flow these can be quite large and these sediments can built up floodplains and river deltas.
SURFACE TENSION---CAPILLARY ACTION
D. Water is not always pulled downward by gravity because of high surface tension. In short, water molecules stick together. Added to surface tension is the wetting ability of water which causes water to rise upward. Water's upward movement against the flow of gravity is called capillary action. It is significant where water is confined in limited spaces like the porous areas in the soil or narrow tubes. When water is confined like this it can climb several inches or even several feet. Capillary action makes it possible for water to be drawn upward through rock, soil, roots and stems, of plants and capillary action also moves the blood in our bodies. Otherwise, many of our cells would die. On a lighter note it is capillary action that makes possible the rapid wetting and retention of liquids by absorbent paper and fabrics. If you are into chemistry, the reason for this is the bonding power of the hydrogen. (This will become more important when I discuss surface tension and condensation nuclei concerning condensation.) A bookish definition might be, Capillary action is the ability of water to pull itself upward through small openings against gravity.
WATER EXPANDS WHEN IT FREEZES AND
HAS A UNUSUALLY HIGH BOILING POINTE. Water does not behave like most other forms of matter which contract as they get colder. However, water does the same thing until it reaches 39š F and then it expands as it cools from 39š F until it reaches the freezing point of 32š F, 0š C. Ice (solidified water ) is less dense than water and floats in liquid water, just take a look at icebergs or the ice in your drink. On the other end of the scale at sea level the boiling point of water is 212š F, 100š C, which is an unusually high boiling point.
F. The hydrosphere is the most widespread, and common of the four "spheres" of the planet's physical environment, remember the chart from the first day of class. If you recall, all the liquid water in the oceans, lakes, rivers, swamps, and rain are only part of the hydrosphere. There is solid water locked in ice and snow. There is water stored in plants and animals. The term hydrosphere describes one of Earth's "spheres" and it includes all of the water on the planet. About 73 percent of the Earth's surface is blanketed by water, most of it in the oceans. Water circulates between all four sphere, but the total amount of water remains the same--constant. (How much water do Human beings contain--Some more than others?)
II HYDROLOGIC CYCLE
HYDROLOGIC CYCLE--SHORT VERSION
A. The circulation of water from one part of the general Earth system to another is known as the hydrologic cycle. This is a fine book definition but the key idea is movement. Almost all of the moisture which comes into the atmosphere occurs from evaporation from the world's oceans and most of this Precipitation falls back into the oceans. A typical water molecule evaporated into the atmosphere remains there a maximum of a few days--less than two weeks. (As could be expected there are some exceptions. On a test I'll be specific on whether I want a short time period or a very long time period.) During this short time it can move substantial distances through convection vertically or horizontally by the wind currents (advection) and can reach the land masses. The Precipitation falling on interior continental areas ultimately comes from moisture evaporated from the ocean. (Take a look at Figure 7.2 on pp 156--157. A hard look not a glance remember pictures are better than words. Did you look at the big arrows?)
RUNOFF AND SOIL MOISTURE
B. Water which falls on the land can become runoff. Runoff is liquid water which becomes part of a stream or river and flows back to the sea. Runoff is not the entire picture because most water, which falls on the continents collects as temporary pools and can evaporate again. Sometimes the water which permeates into the ground can be temporarily stored as soil moisture. Much of the soil moisture evaporates or transpires back into the atmosphere.Depending on soils, geological conditions, vegetation, and other factors the water could percolate faster or slower or not at all into underground water system.
PERCOLATION--NOT IN YOUR BOOK
C. Sometimes water soaks through the soil and permeates into the ground in a vertical process called percolation. At times the water that soaks into the ground percolates through open spaces between sand, gravel, and the voids between solid rocks called interstices. Through percolation water can become part of underground water system and can appear later as springs, which sometimes join with streams.(What might happen to the water while it is underground. It can be a healthy answer?)
D. Sometimes Precipitation takes the form of snow and falls on the permanent ice sheets of Greenland or Antarctica or high mountain glaciers. Additional water is needed and stored by plants and animals. The water is stored underground and in the great ice sheets. Of the earth's liquid water inventory, most of the fresh water is stored in the form of glacial ice. (What city in Colorado gets its water from a glacier?)
SIX STORAGE AREAS
E. The six water storage areas can be pictured as a series of six transient storage units connected by different exchange points. (I've used the term storage areas a few times in passing.) This table is the complete list of water storage areas:
1. The Atmosphere 2. The Oceans 3. Fresh water on the surface 4. Plants and animals 5. Underground 6. Glacial Ice. Most of the water in the world is in storage. For the most part, the inventory of water in storage is generally constant, except for the Ice Ages which locked not only surface water but atmospheric water as well. In terms of the total global supply of water, not very much is in circulation between surface waters and Precipitation. Only a small percentage is in play at any given time, less then one percent. This one percent is vital for life on Earth. For the most part the hydrologic cycle is evaporation, condensation, and Precipitation and is a closed system because there is no gain or loss of water. This is not to say that the system is passive. It is a dynamic system and sometimes there will be more water in some places and less water in other places at any given time. Sometimes these balances take centuries to find equilibrium.
RESIDENCE TIMES
F. Like other systems on the Earth the hydrologic cycle is a closed system, however there is great difference in recycling time of water. An individual molecule can be stored in the oceans, deep lakes, or as glacial ice for thousands of years. Sometimes water can be trapped for hundreds of thousands of years beneath the Earth's surface. No matter what length of time water is stored, the hydrologic cycle refers to the non-stop circulation of Earth's water supply. (What is the natural spring water which Yuppies drink and how old is it?)
III WATER IN THE ATMOSPHERE
A. The invisible form of water, water vapor in the atmosphere is the most common. Water vapor (just to review) is a tasteless, odorless, and a transparent gas but it is also energy-rich because of latent heat of condensation. Most of it is found in the same place most of the air can be found, and it is the troposphere. In fact most of the water vapor is found at altitudes lower than 5,280 feet and only insignificant amounts of water exist above four miles, 25,0000 feet. Although altitudes vary from text to text the important fact remains that water vapor is the most significant factor in the atmosphere.
WATER BUDGET
B. Water vapor stockpiles energy and jump starts the atmosphere into action. Water vapor is the source of all condensation, precipitation and evaporation. It also plays a role as a thermostat for the planet keeping temperatures moderate. As I've mentioned before, additions in one part the the hydrosphere are subtracted from another part. Perhaps you have figured out I am talking about the water budget.
C. As you know moisture is evaporated from the surface of the Earth, mostly from the world's oceans, by the radiation from the Sun and falls back either as a solid or liquid somewhere on the planet. The water the skies gives up can take several forms. Rain is the most common and widespread form of precipitation, but there is also snow, hail, sleet, and two major forms of condensation, fog and dew. Just as the atmosphere gives up water it gains water in the form of evaporation. (I'll talk about each form of Precipitation so stay tuned for more weather reports in another section of the notes.)
FUEL FOR STORMS
D. I've discussed the energy budget in the Chapter and pointed out the heat released during condensation. On a global scale this heat is very small but in human terms it is very large. This is the latent heat of condensation which fuels hurricanes, tornadoes, and thunderstorms.
THE REASON FOR BAD HAIR DAYS--THIS IS NOT JUST A BAD JOKE
E. Water vapor cannot be seen because it is imperceptible by sight, taste, or smell, but it can be felt. In Colorado this does not happen very often, but in states in the Midwest and the East Coast the muggy air can make a person feel tired and miserable even if they are fresh out of a shower. The flip-side is when the air dries out the skin and every one has a bad hair day. The condition is high humidity the other is low humidity. Perhaps you might think I'm being a little silly by using good and bad hair days with humidity. One measure of relative humidity is the hair hygrometer which uses human hair as a measuring device. As hair absorbs or loses water in the air its length changes and is one indictor of relative humidity. (I'll talk more about relative humidity in the notes.)
F. The water in the atmosphere varies greatly from place to place and from season to season. In any given parcel of air the air can only hold so much water vapor. As the temperature of a parcel of air increases, it ability to hold water vapor increases. The determining factor is temperature and becomes a good guideline. Warmer air can hold greater amounts of water vapor (higher possible humidity.) By using the guideline, air at the poles holds less water vapor than the hot air in the tropics. Remember this is only a guideline because other factors influence humidity. (Take a look at Figure 7.3 on p. 158. This is one of those figures you need to look at several times.)
IV SATURATION AND DEW POINT
A. When a parcel of air at a given temperature holds all of the water vapor that it possibly can, it is said to be in a state of saturation and has reached it capacity. As I've mentioned, temperature is a major key. Assuming uniform temperature in a given volume of air is sustained, but if more water vapor is added the air will become saturated (full) and cannot hold more water vapor. When the saturation is exceeded some of the water vapor must become liquid. The example in the book of water condensing on the mirrors and walls during a shower makes the point.
WHAT THE MATH STUFF IS TRYING TO SAY.
B. The rule is, the warmer the temperature of the air, the more water vapor it can contain before becoming saturated. (There are some good math examples in your text book if you care to read them.) They boil down to this, the capacity of air to hold water increases as temperature increases. If you did not get it reread paragraph F in the last section.
DEW POINT
C. When temperatures cool enough the air becomes saturated and liquid water is formed in a process of condensation. (I'll talk more about this in another section.) The critical temperature at which saturation is reached is called the Dew point. Some times an example is better than technical words. Take a look at a can of Coke and see the little beads of water which form on the exterior. The air nearest to the can is colder than the surrounding air and has become saturated. Remember that dew point and condensation are linked.
D. When cool air goes below its dew-point temperature it causes condensation, which must come before precipitation. Since the capacity of air varies because of temperature certain places in the world, like the tropics, have much higher dew points then places at the poles. It does not take a meteorologist to figure out that most of the the wettest areas around the world are located in the tropics. In the most of the world, it is summer with higher temperatures which have more potential for more rain fall. (Yes, I know about snow in Colorado but remember there are other factors like mountains which can come into play.)
V HUMIDITY
A. The amount of water vapor in the air at any one time and place is humidity. This can be measured in three ways and each can be useful. The first is an elementary measurement, which expressed by the quantity of water vapor in given amount of air and is called absolute humidity. This measure of humidity has it limits because the volume of air can change, but the amount of water is still the same. Another problem is temperature, cold air carries less water vapor then warm air. As a general rule, absolute humidity is highest near the equator and over the oceans and decreases over the continents and the poles.
B. Specific Humidity is the amount of water vapor in a specific volume of air. Specific humidity measures just the water vapor, the amount of air is not measured. It is very good for investigating the properties and movements of air masses. The first two measures of humidity are indicators of potential precipitation and are used more by meteorologists than others. When examining the changes of water vapor in moving air masses either vertically or horizontally specific humidity is the choice of meteorologists.
RELATIVE HUMIDITY
C. The measurement of humidity which concerns most people is relative humidity. It is this measure which is used the most by the media and is the best-known measure of humidity. It, like many things in science, is a formula. It is the amount of water vapor in the air compared to the amount required for saturation or maximum humidity at that temperature. In other words it tell us how close the air is to saturation. You can read the example in the book, mine is the same idea, but I use Fahrenheit temperatures. If the temperature is 70š and contains 80 grams of water vapor when saturated, but this volume of air is only is holding (actual content) 40 grams. Then formula would look like this (40 grams/80 grams) X 100= 50% relative humidity. Good farming areas have a relative humidity of about 65 percent. The three variables are: the actual amount of water vapor in the air, the holding capacity, and the saturation of the air, which is determined by the temperature. Relative humidity changes during the day because of temperature with the highest relative humidity just before dawn. (Take a look at Figure 7.4 on p. 160 to see how relative humidity changes during the day.)
SENSIBLE TEMPERATURE
D. Sometimes a person will feel hotter or colder than the temperature indicates. This is called sensible temperature. The other factors involved are wind and humidity. The technical reasons are evaporation and convection of body heat. On humid days, not so much in Colorado, but back East or down South, a person can feel hotter than it actually is. (Your book uses examples from humid Atlanta and dry Tucson, they work.) This is because there is little evaporative cooling when the air is saturated. The National Weather Service combines relative humidity and temperature to warn people about the dangers of hot weather. It is called the heat index. The heat index quantifies (measures) how hot the air feels to one's skin. Sometimes is pays to be cautious with outdoor activities during the summer. (In Colorado we have the opposite problem other than high temperatures and humidity. What is it called that combines temperature and wind during the winter?)
VI SOURCES OF ATMOSPHERIC MOISTURE
A. The atmosphere acquires the majority of its water vapor from evaporation. The process of evaporation has many different sources. The most important source is from surface water and the lion's share is from the world's oceans but there are many other sources as well. Water evaporates from wet ground surfaces and soils, the moisture from plants, man-made surfaces, and from falling precipitation. (What is the name of precipitation which evaporates before it reaches the ground?)
TRANSPIRATION--PLANTS
B. Smaller quantities but significant amounts of evaporation come from the land itself and there are two sources. First, the soil and second, the plants. Water evaporating from the soil is not very important in the evaporation picture. The second and most important of land evaporation is through the leaves of plants and is called transpiration. The process by which plants give up moisture to the air is transpiration. This is a substantial source of land evaporation. Your text discusses the amounts of transpiration from oak trees and corn fields. These are large numbers, but the major source of transpiration for the planet is the tropical rain forest.
EVAPOTRANSPIRATION THE COMBINED PROCESS
C. Taken together, evaporation and transpiration are the combined process is evapotranspiration. This accounts for nearly all of the water vapor in the atmosphere and evapotranspiration is the total amount of water leaving the Earth's surface.
D. If you are thinking ahead you might make the connection between evapotranspiration and precipitation and whether the land will be wet or dry. Anyone who has traveled just a little bit or has seen pictures of different places knows that certain places are wetter or dryer than other places. All one has to do is get on top of the Library (the big building were all the books are keep) and look to the North, you will see prairie as far as the horizon, then look to the South, of course there will be a city with grass, trees, and houses, beyond that the prairie again. The city is a man-made environment that is well watered. Two factors make the difference, the rate of evapotranspiration and precipitation.
VII RATE OF EVAPORATION AND
POTENTIAL EVAPOTRANSPIRATIONA. There are several factors involved in the rates of evaporation. The first and most obvious is how much water is in the area. (Take a look at Table 7.1 on p. 161.) As could be expected there is more evaporation over the oceans than over the land masses. There is an exception to this rule and I've mentioned it before, that in the tropical rain forest located between 0š and 10š N and S is where the vegetation furnish significant amounts of water to the atmosphere by transpiration.
TEMPERATURE
B. The second factor in the rate of evaporation is temperature of both air and water. When the air temperature increases the capacity of air to hold water vapor increases and creates more space in the atmosphere. (I've said this at least three times in different words. I wonder if I have a reason?) As surface air temperatures increase so do water temperatures. If you recall, it is on the molecular level where the water molecules brake away from the liquid and become water vapor. This process can occur at any temperature but a higher temperature causes the process to accelerate. Usually the term evaporation is used for normal temperatures, while words like boiling or vaporization are used for higher temperatures.
RELATIVE HUMIDITY
C. The third factor in the rate of evaporation is relative humidity. When the air is dryer the relative humidity is lower and greater will be the rate of evaporation. In the days before home dryers you had to make sure the air was not humid before you hanged up your cloths outside to dry. This was not usually a problem in Colorado, but could be in other parts of the country. I'm afraid my age is showing again.
WIND
D. Once in a while the air is very still over a body of water, I've only seen this a few times fishing, the water is just like a mirror. If the conditions are right, no wind and no changes in temperature, the air over the lake can be saturated, and the exchange between water in the liquid state and the gaseous state is about the same meaning that there is no net evaporation. These conditions do not last very long. Once the wind picks up, or there is movement in the water, the water vapor over the lake begins to move. These actions scatter the water vapor and now the air is no longer saturated and evaporation can resume. The factors which affect the rate of evaporation are: the temperature of the air and water, relative humidity and wind speed and the availability of water itself.
POTENTIAL EVAPOTRANSPIRATION
E. If you remember the field trip to the top of the library I pointed out the dry prairies and the lush green city. This much can be figured out just by looking and leads to the idea of potential evapotranspiration. (Take a look at Figure 7.5 on p. 162) This is again a creative assumption that the ground is always soaked for all possible evapotranspiration to occur. Potential evapotranspiration is a theoretical term used to estimate the maximum loss of moisture through evaporation and transpiration.
F. There are different formulas (models) to figure potential evapotranspiration. They are based on temperature, vegetation, types of soil, and latitude. In places where there is a lot of precipitation the percentage of ground water is higher than the potential evapotranspiration rate, resulting in a water surplus. However, some places in the world do not have a surplus of ground water and the potential evapotranspiration is greater than the precipitation rate. When this happens there is little or no water in the soil, resulting in dry soil and brown vegetation. Does this look or sound familiar? (Does Pueblo and half of Colorado have a potential evapotranspiration that is greater than the precipitation rate and must it import most of its water from some place else?)
VIII CONDENSATION
CONDITIONS FOR CONDENSATION
A. The flip-side of evaporation is condensation. Water vapor, the gases stage of water, turns into liquid water in a process called condensation. There is more to the process than just magic words or just seeing it happen. Certain conditions must meet before condensation can occur. The first condition is that the air must be saturated. Saturation can exist when more water vapor is added, but the second condition that must be met is that the air must be cooled to a temperature below the dew point. Your book uses the soda can in discussing condensation I used in discussing the the dew point. They are the same thing. There is a third condition for condensation--Condensation Nuclei. That one is so important that I'll talk about separately.
SURFACE TENSION--NOT IN YOUR BOOK
B. A barrier to condensation is the surface tension of water, think of a drop of pure water having a skin like an orange or a wall like a balloon, granted it is a very thin wall, but it is enough. A surface is needed to form liquid water, without a surface the air becomes supersaturated and the cooling process just goes on. This only happens in the atmosphere, remember the film and thunder clouds. The supersaturated condition does not happen very often, because there are numerous objects on the surface of the planet for water to condense on like windshields, grass, trees, the list could be endless.
CONDENSATION NUCLEI
C. Even in the atmosphere there are more than enough surfaces for condensation to occur. They are hygroscopic particles or condensation nuclei which means water vapor condenses around them. If you like the analogy they are seeds which water droplets can grow. This is the principle behind cloud seeding with silver iodine. These surfaces are in the forms of dust, smoke, salt, pollen, and other submicroscopic particles like bacteria. These particles are concentrated over cities, seacoasts, and volcanoes. (It seems I've said this before.) The most widespread particles and the best cloud condensation nuclei which begin to grow at relative humidities of about 70% and condense at 92% relative humidity are sea salt particles. (Why are salt particles the best particles to grow clouds? All of the information is in this paragraph if you remember some vocabulary words and make the connection.) Since industrialization and especially the automobile more and more condensation nuclei contains harmful chemicals which can cause environmental damage.
IX FOGS
A. In the big picture fog is not a very important form of condensation and only plays a small part in the water budget and hydrologic cycle. However, there are some desert coastlines where the fog condenses and is the only form of water available to plant and animal life. (There are several examples of this phenomena where might one be located?)
FOG AND PEOPLE
B. From a human view point it is important fogs can be important. Fogs are just low lying clouds and can make any type of movement difficult or even impossible because of visibility problems. This has been a major problem for sailors for centuries. Fog is a nightmare not to mention very dangerous for air line travel. Fog is also a problem for surface transportation and can lead to many chain-reactions pile-ups on the roads. (What measures were taken to protect ships from breaking up on the rocks near sea coasts because of fog?)
RADIATION FOG
C. The first type of fog is radiation fog, or surface-inversion fog. (It would not hurt to review the chapter "Atmosphere--Heating and Temperature the Section Surface Inversions on p. 18.) Radiation fog is likely to form during the occurrence of a ground (surface) inversion. The Earth gives up warmth through outward radiation, generally during cold clear nights with calm winds in middle latitudes. The air nearest to the ground cools conductively from the ground and an inversion forms.
D. If the cold air is below the dew point condensation (fog) will form. If you recall certain valleys in Colorado can have air drainage and given inversion conditions the whole valley can fill up with fog like a pond or lake. The surrounding mountain tops can be in bright sunlight while the whole valley can be socked-in. This condition will continue until the fog "burns off" from the ground up as the heat of the sun warms the valley. (What valley in Colorado mentioned in your notes"Atmosphere--Heating and Temperature the Section Surface Inversions on p. 18 could this condition occur?)
ADVECTION FOG
E. Advection fogs are most common along seacoasts and near major of inland bodies water. They are formed when warm air is cooled below the dew point by passing over cold surfaces, like snow-covered ground, cold ocean currents, or a lake. Advection fogs occur most often when a warm moist marine air mass moves over a cold land mass. They are not limited to just marine environments (the way marine is used here means any large body of water) and is more general than radiation fog and last longer. The example is warm air rising from the Gulf of Mexico and flows poleward over the upper Mississippi valley. Advection fogs also happen when warm continental air moves over cool large lakes. Any city along the American Great Lakes can have advection fog.
SEA FOG
F. Advection fogs are also formed when warm moist marine air flows over cold ocean currents which have risen up to the surface. This type of advection fog is called sea fog. They do not last very long and can happen any time of the year. An example of this is the famous fogs of San Francisco. Not far off from San Francisco are fog banks, caused by cold California current upwelling to the surface. These fogs are carried by the warm westerly winds to the Northern Californian coast. (Take a look at Figure 7.7 on p. 165.)
X OTHER MINOR FORMS OF CONDENSATION
DEW, WHITE FROST, AND RIME
A. Cool air causes the surrounding water vapor to condense in the form of Dew, liquid water. Dew clings on things like grass, pavement, or other surfaces. The cooler the air gets the more liquid water is deposited. Dew is usually formed on still clear nights, after objects on surfaces lose their heat. If the wind is blowing the cool air cannot stay into contact with things on the surface long enough to form Dew. Dew is more likely on dark and metallic objects because because they radiate heat better.
B. If the air temperature is below freezing white frost forms rather than water droplets. Frost, a type of condensation, is a form of sublimation rather than just frozen dew. Most of the time this happens at night after the land has lost its heat and under cloudless skies. Sometimes the frost is very difficult to get off your windshield especially when you are in a hurry to get to school and cannot find your scraper.
C. If you remember the film about precipitation sometimes liquid water does not freeze at 32š F and is called supercooled water. Most of the time this supercooled water is in thunderclouds, but sometimes these clouds come into contact with a surface. More often then not the surface is an airplane wing , but sometimes these supercooled clouds drop and form rime on tree branches. This happens under very still wind conditions and low air pressure.
XI CLOUDS AND ADIABATIC HEATING AND COOLING
A. Clouds are more than just cute fluffy things that float in the sky. They are the most widespread from of condensation and are a mass of water vapor in the form of condensed water droplets or ice crystals. Clouds are also indicators for coming weather and could be considered high altitude fogs. No matter what the time, about fifty percent of the Earth's surface is cloud covered. Clouds are the origin of all precipitation, but not all clouds precipitate.
B. Clouds also affect the sunlight on the Earth. They acquire heat from the sun overhead and terrestrial radiation from the Earth itself. They can absorb, reflect, scatter, or reradiate energy, they are a major modifier in the heat budget. However, they are still cute fluffy things in the sky, and sometimes look like dragons.
ADIABATIC COOLING
C. The process, which leads to cloud formation is very distinct from other forms of condensation. The primary way large amounts of condensation are brought about in nature is by making air rise. As I told you before, the temperature of air changes depending on whether it is rising or falling. The upward movement is due to a change in air pressure. As air rises, it also expands and less air pressure is placed on it. The process is called adiabatic cooling--cooling by air expansion in rising air.
D. Adiabatic cooling is the exclusive means which large air masses can be cooled to the dew point. It is the most important component for the formation of clouds and the creation of rain. As an unsaturated air mass rises it cools at the steady rate called the dry adiabatic lapse rate which is fixed at 5.6š F per 1000 feet. You do not need to know how to do the formula on this one, but remember it is fixed rate. The term dry adiabatic lapse rate is relative because the air does contain moisture, but not saturated.
ADIABATIC WARMING
E. When air falls it is compressed because there is more air above it and more air pressure on it. Remember the analogy of the blanket. The reverse of the process is when air falls and compression increases and temperatures rise. This is called adiabatic warming--compression in descending air. In both parts of the process no heat is lost.
JUST KNOW THAT THERE IS TWO AND THEY ARE DIFFERENT.
LIFTING CONDENSATION LEVELF. The altitude where the air masses begins to cool with the formation of clouds and condensation commences is called lifting condensation level. This can be plainly seen in the base of flat bottom clouds. It means that rainfall is possible. The latent heat is freed, assuming the air still ascends, but the latent heat slows the process. The reduced rate is called the wet adiabatic lapse rate, its lapse rate is 3.2š per 1000 feet, which is different than the dry adiabatic lapse rate. Rising air will cool at one of these rates. Descending air which is being compressed always uses the fixed dry adiabatic rate. (If you want to get real technical you can look at Figure 7.8)
XII STABILITY AND INSTABILITY
BUOYANCY--THINK OF HOT AIR BALLOONS
A. Both condensation and precipitation are the consequences of the vertical movement of air. The vertical movement of air is critical to weather and climate. Of course, the vertical movement of air depends on the buoyancy of air. The tendency for an object to rise in gas or liquid until it reaches its own level is called buoyancy. If you want a non scientific word think floating. The concept of air buoyancy is important in understanding air masses. Picture an air mass as an invisible balloon, which rises to a surrounding air of equal density. If the invisible balloon is warmer and less dense it will rise, if it is cooler and more dense it will sink. Warm air is more buoyant then cool air. This is the principle of a hot air balloon, if a hot air balloon loses altitude just turn on the burner.
UNSTABLE AIR RISES--WARMER AIR
B. Unstable air exists when there is no outside forces acting other than the buoyant air itself. Unstable air is buoyant (it floats up because it is warmer) when an air mass is heated beyond the temperature of surrounding air. If you recall, different surfaces radiated heat differently, and when one surface becomes warmer than the surrounding area updrafts are created. This usually happens on warm afternoons in summer. These thermals will rise until they meet air with a corresponding temperature and density or equilibrium level As the air climbs, it will be cooled adiabatically, clouds are likely to be formed under these conditions.
STABLE AIR SINKS OR STAYS MOST OF THE TIME--COOLER AIR
C. An air mass is said to be stable when it is colder and denser than surrounding air and its vertical movement is checked. Stable air either sinks or stays where it is unless it is forced by the wind to rise over a topographic barrier. It will only rise as long as the barrier exists, then it will fall again because stable air is nonbuoyant. This frequency occurs during a temperature inversion when there is high stability and when cold air is below warm air. If you recall during inversions there is little chance for Precipitation.
XIII CLOUD FORMS
TAXONOMY OF CLOUDS
A. Clouds can seem white, shades of gray, or even black. Their colors differ because of their density or the direction of the sun. Clouds have many different shapes and sizes and some can only be found in certain locations. Also certain clouds can only be found at certain altitudes. Scientists love to classify things, this process is called taxonomy. Clouds are no different and are based on two components, form or shape and altitude. The established international standard has three forms for clouds: Cirriform, Statiform, and Cumuliform. There are many subtypes and overlaps but all I want you to know are the major types and characteristics and the word nimbus. The three major groups of clouds are cirrus clouds, stratus clouds, and cumulus clouds. These clouds are not static, but can change form and shapes. (Take a look at Figure 7.10 on p. 168 for altitudes and how they look.)
KNOW THE SHAPES, WHAT THEY LOOK LIKE, AND WHERE THEY ARE FOUND
CIRRUS
B. Cirrus clouds are filmy or feathery, wispy white clouds which usually contain only ice crystals rather than droplets of water and are found between 20,000 to 36,000 feet. They are fair weather clouds and are irregularly scattered and are white patches in a clear blue sky.
STRATUS
C. Stratus clouds are spread out, sheet like and shapeless gray in color and lie in layers with uniform thickness. They are typical winter cloud formations in the middle latitudes during winter and are low altitude clouds and reach altitudes of only 20,000 feet. There are several variations of stratus based on altitudes. Many times they hang around for days and precipitation will be steady and unceasing.
CUMULUS OR CUMULIFORM
D. Cumulus clouds are massive and roundedwith a flat base with a cauliflower domed shape top, and can grow to great heights and are vertical formations. They are also described as heaped or piled formations. Their altitude ranges from 1,600 to 40,000 feet above sea level. (Different texts vary a little on the altitudes of clouds but they are all close.) Cumulus clouds build up into towering formations and indicate unstable air. From the base of these clouds condensation is beginning.
CUMULONIMBUS--THE THUNDER CLOUD
E. Cumulus clouds are good examples of the fickle nature of clouds, they generally are fair weather clouds, but can change form on hot, sultry, summer afternoons, and take the form of an anvil head as well as a flat base and indicate showers or squalls. Remember what I told you about lifting condensation level. In meteorology these clouds are called cumulonimbus clouds. In common language it is the "thunderhead," this cloud has flat top or is anvil shaped. It becomes darker as condensation expands and blocks the sun.
Sometimes the pictures in this book are just outstanding and very beautiful so, (Take a look at Figure 7.11 on p. 170 just because they are beautiful if nothing else.)
KNOW WHAT NIMB MEANS--PRECIPITATION
F. In more technical terms any cloud which has the term nimb in it are the clouds which bring precipitation, examples are Cumulonimbus (thunderhead) and Nimbostratus. Nimbostratus clouds created steady rain or snow, and low ceilings, poor visibility, and icing conditions. Cumulonimbus clouds, the thunderhead, an overgrown cumulus, is vertical in nature and can have winds over 100 miles per hour. They can drop large amounts or rain and put on brilliant displays of lighting and thunder, and many times they also bring hail. The term nimbus, when used in describing clouds, means precipitation.
XIV PRECIPITATION
A. As a rule clouds do not give up precipitation, but all precipitation comes from clouds. Condensation is not enough for raindrops, they are just too buoyant and even the normal turbulence of the atmosphere would keep them in the sky. Even in a still sky, it would take days for rain to fall. It is believed that two conditions are essential for precipitation. First the formation of ice crystal and collision and coalescence, (the merging,) of water droplets. There are several forms of precipitation and form it takes depends on temperature and air turbulence. Be familiar with each one.
FORMS OF PRECIPITATION-- KNOW THEM
RAIN
B. The most common form of precipitation is rain. Some clouds or parts of them reach far enough in the atmosphere to encounter temperatures below freezing. When these conditions exist both ice crystal and supercooled water droplets coexist in the same cloud. Each contend for the remaining water vapor, the ice crystal have the edge and collect most of the water vapor and increase in size until they are oversized enough to fall. These droplets are very small and it takes million of these droplets to form a single rain drop. During the fall, they can melt and pick up more water vapor, if this is the condition it rains. It is responsible for most of the precipitation outside of the tropics.
C. All liquid water is not the same, but falls in three different categories. "Rain" itself, which lasts a comparatively long time. "Showers" which are short-lived, but have large drops, and "Drizzle" which can last a long time, but has very fine drops or mist. Drizzle is very light that the wind currents toss the water so much that drizzles rarely fall vertically.
SNOW
D. Snow is the second most common form of Precipitation. It is the generic name for solid Precipitation in the form of ice crystals, small pellets, or flakes. It is brought about directly from water vapor into ice crystals without an intermediate liquid water stage, the process is called sublimation. (Remember I told you it was important.) Snow forms around very small ice crystals, which usually form in six-sided symmetrical shapes. It is said no two snow flakes are ever alike. Snow is much dryer than rain. Generally it takes six inches of moist snow to equal or 30 inches of dry snow to equal one inch of rain fall.
SLEET
E. Sleet, in the United States is frozen rain. In some other English speaking countries, it is the mixture of rain and snow. I do not know why, but the English in the United States is different than English in other places. I think I've only seen it a very times and is uncommon in Colorado but is more common back East.
HAIL
F. Hail is the less common form of Precipitation of the last three described. Usually it takes place in the summer and is the result of a cumulonimbus clouds, the thunder head. Hail is the most complex of all types of Precipitation is hail. It can be small and pea like, or large blobs of ice. If you cut a hailstone in half, you will find crude concentric layers of clear and cloudy ice. The cloudy part is comprised of thousands of air bubbles and diminutive crystals of ice. Hail can cut wheat or corn fields to ribbons is just a matter of seconds and cause a great deal of damage to cars and buildings. Roofers love them because they increase business. You may have to replace a roof every ten years in Pueblo because of hail.
G. The formation of hail is complicated because it is formed in strong vertical updrafts and downdrafts of cumulonimbus clouds. The cloud must have a temperature above 32š in its lower parts, but have a temperature of 0š in the upper part. Updrafts take the water droplets to the freezing zone, and small pellets are formed. These pellets fall gathering moisture in the fall, but another updraft takes them aloft again. This process can go on several times, before they hit the ground.
H. Glaze, or ice storm in Colorado, is rain which turns to ice when it come into contact with a solid object. It is caused by subfreezing air near the surface. This type of storm is a mess, when it takes you just five minutes to get place to place it then takes you a half hour, and tree limbs and power lines break.
XV CONDITIONS CAUSING PRECIPITATION
A. Adiabatic cooling and the role or rising air has been a major part of this chapter. There are three kinds of atmospheric precipitation, and are moved by outside forces. Many times more than one process is involved at the same time. The three ways which air is forced to rise are: convectional precipitation, Cyclonic or frontal precipitation, and orographic precipitation. I'll go into details on each one. (Take a look at Figure 7.12 a, b, and c on p. 172 remember pictures are sometimes better than words. I said really look it is for all three types of precipitation)
CONVECTIVE PRECIPITATION--NEXT TWO POINTS.
B. As mentioned before in the text and notes, there is a different heating rate with different surfaces. Sometimes a surface becomes warmer than the surrounding air, the heated surface warms the air above it by convection. The warmed air mass rises in a regular convective process. This is a spontaneous uplift and is more prominent when there is unstable air on a warm summer day. The air pressure drops as air rises and cools to the dew point by adiabatic cooling. Condensation starts and a cumulus cloud can form.
C. Most of the time that is the end of the process, but given the right humidity, temperature, and stability A cloud type is formed by strong convection currents is the cumulonimbus the thunderhead. Recall the film or any of last summer's thunderstorms and you will know what happens. The precipitation from this type of event is called convective precipitation. This type of precipitation most common in the tropics, but occurs in the midlatitudes in the summer.
FRONTAL PRECIPITATION
D. When different air masses meet they are like oil and water and do not mix. They create a zone of discontinuity (contact) between them called a front. In this type of system the cooler air is at the bottom, while the more buoyant warm moist air rises above the cooler air. Large scale precipitation can be the outcome, this is called a frontal precipitation.
The Midwest's winter precipitation is chiefly attributable to the interaction of contrasting air masses. I've briefly talked about it before and have used the laser on the map many times.This topic will be discussed at great length in the next chapter. This type of system is not very important in the tropics, but is very important in the midlatitudes where polar air masses meet warmer tropical air masses.
OROGRAPHIC PRECIPITATION=MOUNTAINS
E. Mountains and other orographic barriers obstruct the path of horizontal air flow and deflects the air upslope. As the air rises orographic precipitation can occur on the windward side of the mountain. Things are different on the leeward side because adiabatic cooling is substituted by adiabatic warming.
RAIN SHADOW EFFECT--OROGRAPHIC
F. In short, the windward side gets much more moisture than the leeward side. The dry leeward side can extend just beyond the mountain slope, this is called the rain shadow effect. (I've talked about this one before so don't miss it on the test.) (Take a look at Figure 7.14 on p. 174 the pictures are excellent.) Orographic precipitation happens almost any where, any time of the year, or as long as the mountain is there. It is also steady because of the nature of the wind patterns. Examples can be seen in many places, the one in your book is a good one concerning Hawaii (p. 178) Another is the Pacific Northwest where the windward sides of mountains are very wet, but on the other side, the leeward side is very dry.
XVI DISTRIBUTION OF PRECIPITATION
A. There are several alternatives in portraying the precipitation an area acquires. There is the average annual precipitation and might be all right for an overview but it ignores any seasonal variations. The total number of raindays could be counted. Precipitation could be figured by dividing the number of rain days in a year by the total number of days in a year, would yield the probability or rainfall. This sort of number could be useful to farmers and the tourist business. Perhaps the best way to get the best information about the most important factors is to look at a climograph. (Take a look at Figure 7.15 on p. 174)
B. Like every thing else on Earth atmospheric moisture has a spatial pattern and the big picture is connected to latitude, but there are may variables to the equation. There are two components which factor for the possibility of precipitation and total precipitation. First is the degree of adiabatic cooling or lifting for whatever reason. The second depends on the internal characteristics of the air, hot or cold, wet or dry. (Take a look at Figure 7.17 on p.179)
DISTRIBUTION WITHIN LATITUDINAL ZONES
PLACES WITH A GREAT DEAL OF PRECIPITATIONC. The most eye-catching characteristic of the planet's precipitation is the tropics. They have the wettest areas. As I've mentioned, the trade winds carry vast quantities of water. When these winds are blown against mountains they dump their loads. Examples are Central America and northeastern South America. Other wet areas in the tropics like the West Coast of Africa are caused by the diversion of the normal trade wind pattern.
There are other zones of heavy rain fall. They are located on the west coasts of North and South America, I've mentioned these before, they lie between 40š and 60š. These areas of heavy rain fall have a combination of factors, the westerlies, many storms, and mountains which run north to south. In both North and South America the mountains create rain shadow effects on the leeward sides of mountains. These rain shadows create deserts.
PLACES WITH LITTLE PRECIPITATION
D. Dry conditions are the most significant feature on the western sides of the continents in subtropical latitudes around 25š or 30š. Many of the great deserts of the world coincide with the Subtropical Highs. To locate them on the map look at North Africa, Arabia, North America and Australia, there are exceptions to the rules.
E. The interior lowlands of the United States are not as dry as might be expected because of the conflict between south moving polar air and north moving air from the Gulf.
GUIDED PRACTICE INDEPENDENT PRACTICE Questions during the lecture. Readings at home Culmination List the different forms of Precipitation List some of the unusual qualities of water? and what are the conditions causing Trace the Hydrologic Cycle? Precipitation? List the sources of Atmospheric Moisture?. How is Precipitation distributed over the planet? Know the different forms of Condensation. How are Clouds formed and what are the basic types? Describe Adiabatic Heating and Cooling?