1. Introduction to the Atmosphere

The atmosphere is a cloud of gas and suspended solids extending from the Earth's surface out many thousands of miles, becoming increasingly thinner with distance but always held by the Earth's gravitational pull.

The atmosphere surrounds the Earth and holds the air we breathe; it protects us from outer space; and holds moisture (clouds), gases, and tiny particles. In short, the atmosphere is the protective bubble in which we live.

This protective bubble consists of several gases (listed in the table below) with the top four making up 99.998% of all gases. Of the dry composition of the atmosphere nitrogen, by far, is the most common. Nitrogen dilutes oxygen and prevents rapid burning at the Earth's surface. Living things need it to make proteins.

Oxygen is used by all living things and is essential for respiration. It is also necessary for combustion or burning.

Argon is used in light bulbs, in double-pane windows, and used to preserve the original Declaration of Independence and the Constitution. Plants use carbon dioxide to make oxygen. Carbon dioxide also acts as a blanket that prevents the escape of heat into outer space.

These percentages of atmospheric gases are for a completely dry atmosphere. The atmosphere is rarely, if ever, dry. Water vapor (water in a 'gas' state) is nearly always present up to about 4% of the total volume. 

In the Earth's desert regions (30°N/S) when dry winds are blowing, the water vapor contribution to the composition of the atmosphere will be near zero.

Water vapor contribution climbs to near 3% on extremely hot/humid days. The upper limit, approaching 4%, is found in tropical climates. The table (below) shows the changes in atmospheric composition with the inclusion of different amounts of water vapor.

2. Layers of the Atmosphere

The envelope of gas surrounding the Earth changes from the ground up. Five distinct layers have been identified using...

   thermal characteristics (temperature changes),

   chemical composition,

   movement, and

   density.

Each of the layers is bounded by "pauses" where the greatest changes in thermal characteristics, chemical composition, movement, and density occur.

Exosphere

This is the outermost layer of the atmosphere. It extends from the top of the thermosphere to 6,200 miles (10,000 km) above the earth. In this layer, atoms and molecules escape into space and satellites orbit the earth. At the bottom of the exosphere is the thermopause located around 375 miles (600 km) above the earth.

Thermosphere

Between about 53 miles (85 km) and 375 miles (600 km) lies the thermosphere. This layer is known as the upper atmosphere. While still extremely thin, the gases of the thermosphere become increasingly more dense as one descends toward the earth.

As such, incoming high energy ultraviolet and x-ray radiation from the sun begins to be absorbed by the molecules in this layer and causes a large temperature increase.

Because of this absorption, the temperature increases with height. From as low as -184°F (-120°C) at the bottom of this layer, temperatures can reach as high as 3,600°F (2,000°C) near the top.

However, despite the high temperature, this layer of the atmosphere would still feel very cold to our skin due to the very thin atmosphere. The high temperature indicates the amount of the energy absorbed by the molecules but with so few in this layer, the total number of molecules is not enough to heat our skin.

Mesosphere

This layer extends from around 31 miles (50 km) above the Earth's surface to 53 miles (85 km). The gases, including the oxygen molecules, continue to become more dense as one descends. As such, temperatures increase as one descends rising to about 5°F (-15°C) near the bottom of this layer.

The gases in the mesosphere are now thick enough to slow down meteors hurtling into the atmosphere, where they burn up, leaving fiery trails in the night sky. Both the stratosphere (next layer down) and the mesosphere are considered the middle atmosphere. The transition boundary which separates the mesosphere from the stratosphere is called the stratopause.

Stratosphere

The Stratosphere extends around 31 miles (50 km) down to anywhere from 4 to 12 miles (6 to 20 km) above the Earth's surface. This layer holds 19 percent of the atmosphere's gases but very little water vapor.

In this region the temperature increases with height. Heat is produced in the process of the formation of Ozone and this heat is responsible for temperature increases from an average -60°F (-51°C) at tropopause to a maximum of about 5°F (-15°C) at the top of the stratosphere.

This increase in temperature with height means warmer air is located above cooler air. This prevents "convection" as there is no upward vertical movement of the gases. As such the location of the bottom of this layer is readily seen by the 'anvil-shaped' tops of cumulonimbus clouds.

Troposphere

Known as the lower atmosphere almost all weather occurs in this region. The troposphere begins at the Earth's surface and extends from 4 to 12 miles (6 to 20 km) high.

The height of the troposphere varies from the equator to the poles. At the equator it is around 11-12 miles (18-20 km) high, at 50°N and 50°S, 5½ miles and at the poles just under four miles high.

As the density of the gases in this layer decrease with height, the air becomes thinner. Therefore, the temperature in the troposphere also decreases with height in response. As one climbs higher, the temperature drops from an average around 62°F (17°C) to -60°F (-51°C) at the tropopause.

Average temperature profile for the lower layers of the atmosphere

3. Air Pressure

3.1 Introduction to Air Pressure

The atoms and molecules that make up the various layers in the atmosphere are constantly moving in random directions. Despite their tiny size, when they strike a surface they exert a force on that surface in what we observe as pressure.

Each molecule is too small to feel and only exerts a tiny bit of force. However, when we sum the total forces from the large number of molecules that strike a surface each moment, then the total observed pressure can be considerable.

Air pressure can be increased (or decreased) one of two ways. First, simply adding molecules to any particular container will increase the pressure. A larger number of molecules in any particular container will increase the number of collisions with the container's boundary which is observed as an increase in pressure.

The number of molecules in the atmosphere decreases with height.

A good example of this is adding (or subtracting) air in an automobile tire. By adding air, the number of molecules increase as well as the total number of the collisions with the tire's inner boundary. The increased number of collisions forces the tire's pressure increase to expand in size.

The second way of increasing (or decreasing) is by the addition (or subtraction) of heat. Adding heat to any particular container can transfer energy to air molecules. The molecules therefore move with increased velocity striking the container's boundary with greater force and is observed as an increase in pressure.

Since molecules move in all directions, they can even exert air pressure upwards as they smash into object from underneath. In the atmosphere, air pressure can be exerted in all directions.

In the International Space Station, the density of the air is maintained so that it is similar to the density at the earth's surface. Therefore, the air pressure is the same in the space station as the earth's surface (14.7 pounds per square inch).

Back on Earth, as elevation increases, the number of molecules decreases and the density of air therefore is less, meaning a decrease in air pressure. In fact, while the atmosphere extends more than 15 miles (24 km) up, one half of the air molecules in the atmosphere are contained within the first 18,000 feet (5.6 km).

Because of this decrease in pressure with height, it makes it very hard to compare the air pressure at ground level from one location to another, especially when the elevations of each site differ. Therefore, to give meaning to the pressure values observed at each station, we convert the station air pressures reading to a value with a common denominator.

The difference in pressure as height increases

The common denominator we use is the sea-level elevation. At observation stations around the world the air pressure reading, regardless of the observation station elevation, is converted to a value that would be observed if that instrument were located at sea level.

The two most common units in the United States to measure the pressure are "Inches of Mercury" and "Millibars". Inches of mercury refers to the height of a column of mercury measured in hundredths of inches. At sea level, standard air pressure is 29.92 inches of mercury.

Millibars comes from the original term for pressure "bar". Bar is from the Greek "báros" meaning weight. A millibar is 1/1000th of a bar and is approximately equal to 1000 dynes (one dyne is the amount of force it takes to accelerate an object with a mass of one gram at the rate of one centimeter per second squared). Millibar values used in meteorology range from about 100 to 1050. At sea level, standard air pressure in millibars is 1013.2. Weather maps showing the pressure at the surface are drawn using millibars.

Although the changes are usually too slow to observe directly, air pressure is almost always changing. This change in pressure is caused by changes in air density, and air density is related to temperature.

Warm air is less dense than cooler air because the gas molecules in warm air have a greater velocity and are farther apart than in cooler air. So, while the average altitude of the 500 millibar level is around 18,000 feet (5,600 meters) the actual elevation will be higher in warm air than in cold air.

The most basic change in pressure is the twice daily rise and fall in due to the heating from the sun. Each day, around 4 a.m./p.m. the pressure is at its lowest and near its peak around 10 a.m./p.m. The magnitude of the daily cycle is greatest near the equator decreasing toward the poles.

On top of the daily fluctuations are the larger pressure changes as a result of the migrating weather systems. These weather systems are identified by the blue H's and red L's seen on weather maps.

Fast Facts

The scientific unit of pressure is the Pascal (Pa) named after Blaise Pascal (1623-1662). One pascal equals 0.01 millibar or 0.00001 bar. Meteorology has used the millibar for air pressure since 1929.

When the change to scientific unit occurred in the 1960's many meteorologists preferred to keep using the magnitude they are used to and use a prefix "hecto" (h), meaning 100.

Therefore, 1 hectopascal (hPa) equals 100 Pa which equals 1 millibar. 100,000 Pa equals 1000 hPa which equals 1000 millibars.

The end result is although the units we refer to in meteorology may be different, their numerical value remains the same. The standard pressure at sea-level is 1013.25 in both millibars (mb) and hectopascal (hPa).

3.2 The Highs and Lows of Air Pressure

Standing on the ground and looking up, you are looking through the atmosphere. It might not look like anything is there, especially if there are no clouds in the sky. But what you don’t see is air – lots of it. We live at the bottom of the atmosphere and the weight of all the air above us is called air pressure. A tower of air that is 1 inch square and goes from the bottom of the atmosphere is 14.7 pounds. That means air exerts 14.7 pounds per square inch (psi) of pressure at the Earth’s surface. High in the atmosphere, air pressure decreases. With less air molecules above, there is less pressure from the weight of air above. 

Pressure varies from day-to-day at the Earth’s surface - the bottom of the atmosphere. This is, in part, because the Earth is not equally heated by the Sun. Areas where air is warmed often have lower pressure because the warm air rises and are called low pressure systems. Places where air pressure is high are called high pressure systems.

Air near the surface flows down and away in a high pressure system (left) and air flows up and together at a low pressure system (right). 

Credit: NESTA

A low pressure system has lower pressure at its center than the areas around it. Winds blow towards the low pressure, and the air rises in the atmosphere where they meet. As the air rises, the water vapor within it condenses forming clouds and often precipitation too. Because of Earth’s spin and the Coriolis Effect, winds of a low pressure system swirl counterclockwise north of the equator and clockwise south of the equator. This is called cyclonic flow. On weather maps a low pressure system is labeled with red L.

A high pressure system has higher pressure at its center than the areas around it. Wind blows away from high pressure. Winds of a high pressure system swirl in the opposite direction as a low pressure system - clockwise north of the equator and counterclockwise south of the equator. This is called anticyclonic flow. Air from higher in the atmosphere sinks down to fill the space left as air blew outward. On a weather map the location of a high pressure system is labeled with a blue H. 

How do we know what the pressure is? How do we know how it changes over time? Today, electronic sensors are used to measure air pressure in weather stations. The sensors are able to make continuous measurements of pressure over time. In the past, barometers were used that measured how much air pushed on a fluid such as mercury. Historically, measurements of air pressure were described as “inches of mercury.” Today, meteorologists use millibars (mb) to describe air pressure. 

3.3 Pressure Belts of the Earth

The distribution of atmospheric pressure across the latitudes is termed global horizontal distribution of pressure. Its main feature is its zonal character known as pressure belts. 

Pressure Belts of Earth

On the earth’s surface, there are seven pressure belts. They are the Equatorial Low, the two Subtropical highs, the two Subpolar lows, and the two Polar highs. Except the Equatorial low. The others form matching pairs in the Northern and Southern Hemispheres. There is a pattern of alternate high and low pressure belts over the earth. This is due to the spherical shape of the earth—different parts of the earth are heated unequally. The Equatorial region receives great amount of heat throughout the year. Warm air being light, the air at the Equator rises, creating a low pressure. At the poles the cold heavy air causes high pressure to be created/formed. It is also due to the rotation of the earth. In the Subpolar region around latitudes 60° to 65° North and South of the Equator, the rotation of the earth pushes up the bulk of the air towards the Equator, creating a low pressure belt in this region.

(i) Equatorial Low Pressure Belts

This low pressure belt extends from 0 to 5° North and South of Equator. Due to the vertical rays of the sun here, there is intense heating. The air therefore, expands and rises as convection current causing a low pressure to develop here. This low pressure belt is also called as doldrums, because it is a zone of total calm without any breeze.

(ii) Subtropical High Pressure Belts

At about 30°North and South of Equator lies the area where the ascending equatorial air currents descend. This area is thus an area of high pressure. It is also called as the Horse latitude. Winds always blow from high pressure to low pressure. So the winds from subtropical region blow towards Equator as Trade winds and another wind blows towards Sub-Polar Low-Pressure as Westerlies.

(iii) Circum-Polar Low Pressure Belts

These belts located between 60° and 70° in each hemisphere are known as Circum-Polar Low Pressure Belts. In the Subtropical region the descending air gets divided into two parts. One part blows towards the Equatorial Low Pressure Belt. The other part blows towards the Circum-Polar Low Pressure Belt. This zone is marked by ascent of warm Subtropical air over cold polar air blowing from poles. Due to earth’s rotation, the winds surrounding the Polar region blow towards the Equator. Centrifugal forces operating in this region create the low pressure belt appropriately called Circumpolar Low Pressure Belt. This region is marked by violent storms in winter.

(iv) Polar High Pressure Areas

At the North and South Poles, between 70° to 90° North and South, the temperatures are always extremely low. The cold descending air gives rise to high pressures over the Poles. These areas of Polar high pressure are known as the Polar Highs. These regions are characterized by permanent Ice Caps.

SHIFTING OF PRESSURE BELTS

If the earth had not been inclined towards the sun, the pressure belts, as described above, would have been as they are. But it is not so, because the earth is inclined 23 1/2° towards the sun. On account of this inclination, differences in heating of the continents, oceans and pressure conditions in January and July vary greatly. January represents winter season and July, summer season in the Northern Hemisphere. Opposite conditions prevail in the Southern Hemisphere. When the sun is overhead on the Tropic of Cancer (21 June) the pressure belts shift 5° northward and when it shines vertically overhead on Tropic of Capricorn (22 December), they shift 5° southward from their original position. The shifting of the pressure belts cause seasonal changes in the climate, especially between latitudes 30° and 40° in both hemispheres. In this region the Mediterranean type of climate is experienced because of shifting of permanent belts southwards and northwards with the overhead position of the sun. During winters Westerlies prevail and cause rain. During summers dry Trade Winds blow offshore and are unable to give rainfall in these regions. When the sun shines vertically over the Equator on 21st March and 23rd September (the Equinoxes), the pressure belts remain balanced in both the hemispheres.

4. Air masses

4.1 Introduction to Air masses

Air masses are large bodies of air that create distinctive weather conditions across the globe.

An air mass is defined as a body or 'mass' of air with uniform weather conditions, such as similar temperature and humidity. Air masses may cover several million square kilometres and extend high up into the atmosphere.

They are primarily defined by the area in which they originate, this is called the 'source region'. The characteristics of an air mass can then be modified as they travel across the globe. Where two air masses of different temperatures meet, a boundary forms which is termed a 'front'.

In the following pages in this section, the formation and behaviour of air masses will be looked at more detail. 

4.2 Air mass source regions

The source region of an air mass defines its main characteristics.

The temperature of an air mass will depend largely on its point of origin and its subsequent journey over the land or sea. This might lead to warming or cooling by the prolonged contact with a warm or cool surface.

A warm air mass is produced by prolonged contact with a warm surface, and conversely a cold air mass is produced by prolonged contact with a cold surface. The heat transfer processes that warms or cools the air takes place slowly. It may take a week or more to warm up the air by 10 ℃ right through the atmosphere, and in order for these changes to take place a large mass of air must stagnate over a region. Parts of the earth's surface where the air can stagnate and gradually gain properties of the underlying surface are called source regions.

The main source regions are the high pressure belts in the sub tropics (giving rise to tropical air masses) and around the poles (the source for polar air masses).

Warm source regions (tropical air masses):

   Sahara Desert - warm and dry

   Tropical Oceans - warm and moist

Cold source regions (polar air masses):

   Arctic Ocean - cold and moist

   Siberia - cold and dry

   Northern Canada - cold and dry

   Southern Ocean - cold and moist

4.3 Air mass modification

Air masses can become modified as they move away from their source region.

In its source region an air mass gains properties which are characteristic of the underlying surface. It may be cold or warm and it may be dry or moist.

The stability of the air can also be derived. In general, the more unstable an air mass is the more likely you are to experience unsettled weather such as wind or rain.

   Tropical air is unstable because it has been heated from below.

   Polar air is stable because it has been cooled from below.

As air moves from its source region the air is modified due to variations in the nature of the underlying surface. Two processes, acting either independently or together, may modify an air mass.

   An air mass moving over the sea is said to have a maritime track. This air mass will typically increase its moisture content, particularly in its lowest layers, by evaporation of water from the sea surface (as seen in the above diagram). On the other hand, an air mass moving over the land (with a continental track) will remain relatively dry.

   A cold air mass flowing away from its source region over a warmer surface will be warmed from below making the air more unstable in its lowest layers. A warm air mass flowing over a colder surface is cooled from below and becomes stable in its lowest layers. In its source region an air mass gains properties which are characteristic of the underlying surface. It may be cold or warm and it may be dry or moist.

4.4 Types of Air Mass

An air mass is a very large body of air that has a similar temperature and humidity in any horizontal direction. It can cover hundreds of thousands of square miles. According to the Bergeron Climatic Classification System, air masses form when a surface source region (continental or maritime) combines with a latitude source region (tropical, polar, arctic or Antarctic). Each type of air mass produces different weather and can affect the earth’s climate for days or months.

Continental Polar

The continental polar air mass forms over a large, subpolar land area. It is cold and stable and has low humidity. This type of air mass creates very cold winter weather without precipitation or clouds. 

Continental Arctic

The continental Arctic air mass develops only in the winter over large areas of snow and ice. It is extremely cold and dry due to frigid conditions near the polar circle, caused in part by polar nights -- periods of 24-hour darkness. This air mass can produce record-breaking cold temperatures in Canada and the United States.

Continental Antarctic

The continental Antarctic air mass forms solely over Antarctica. It is stable, extremely cold and extremely dry. It has colder temperatures than any other air mass during any season. Travel over the ocean modifies this air mass. By the time it reaches land in the Southern Hemisphere, it usually changes classification to maritime polar.

Continental Tropical

Continental tropical air is produced over the world’s deserts, including the Sahara, Arabian and Australian deserts. The southwestern desert in the U.S. is also a source during the summer. The air mass is hot and has extremely low humidity. It affects summer weather and is capable of causing drought if it lingers over a region. Heat waves that result in human and animal deaths can be caused by this air mass.

Maritime Polar

The maritime polar air mass forms over cold, polar oceans. It is cool and moist and can create mild weather in coastal areas depending on the time of year. In the winter, it produces warmer weather when the surface temperature of the ocean is higher than the land temperature. In the summer, it brings cooler weather when the ocean is colder than the continent.

Maritime Tropical

The principal type of maritime air is maritime tropical. This very warm and humid air mass develops over tropical and subtropical seas and oceans. It creates rainy conditions east of the Rocky Mountains in the winter, particularly in the southeastern United States.

5. Weather fronts

Weather fronts mark the boundary or transition zone between two air masses and have an important impact upon the weather.

Weather fronts mark the boundary between two air masses, which often have contrasting properties. For example, one air mass may be cold and dry and the other air mass may be relatively warm and moist. These differences produce a reaction in a zone known as a front.

What is a weather front?

A weather front is the boundary between two air masses. It can be thought of as the front line in a battle where the warm air represents one side and its 'enemy', the cold air, the other side.

Across a front there can be large variations in temperature, as warm air comes into contact with cooler air. The difference in temperature can indicate the 'strength' of a front, e.g. if very cold air comes into contact with warm tropical air the front can be 'strong' or 'intense'. If, however, there is little difference in temperature between the two air masses the front may be 'weak'.

Three of the most common weather fronts are explained below.

Cold fronts

A cold front is symbolized on a weather map as a line with triangles. The triangles can be thought of as icicles. Cold fronts are often coloured blue.

The presence of a cold front means that cold air is advancing and pushing underneath warmer air. This is because the cold air is 'heavier', or more dense, than the warm air. Cold air is thus replacing warm air at the surface. The tips of the 'icicles' indicate the direction of movement of the cold air.

Warm fronts

A warm front is symbolized on a weather map as a line with semicircles. The semicircles can be thought of as half suns. Warm fronts are often coloured red.

The presence of a warm front means that warm air is advancing and rising up over cold air. This is because warm air is 'lighter' or less dense, than cold air. Warm air is replacing cooler air at the surface. The edges of the 'suns' indicate the direction of movement of the warm air.

Occluded fronts

An occluded front is symbolized on a weather map as a line with both semicircles and triangles. They are often coloured purple.

These are slightly more complex than cold or warm fronts. The word 'occluded' means 'hidden' and an occlusion occurs when the cold front 'catches up' with the warm front. The warm air is then lifted up from the surface, and therefore 'hidden'. An occlusion can be thought of as having the characteristics of both warm and cold fronts.

6. Global Atmospheric Circulations

Because more solar energy hits the equator, the air warms and forms a low pressure zone. At the top of the troposphere, half moves toward the North Pole and half toward the South Pole. As it moves along the top of the troposphere it cools. The cool air is dense and when it reaches a high pressure zone it sinks to the ground. The air is sucked back toward the low pressure at the equator. This describes the convection cells north and south of the equator.

If the Earth did not rotate, there would be one convection cell in the northern hemisphere and one in the southern with the rising air at the equator and the sinking air at each pole. But because the planet does rotate, the situation is more complicated. The planet’s rotation means that the Coriolis Effect must be taken into account. 

Let’s look at atmospheric circulation in the Northern Hemisphere as a result of the Coriolis Effect. Air rises at the equator, but as it moves toward the pole at the top of the troposphere, it deflects to the right. (Remember that it just appears to deflect to the right because the ground beneath it moves.) At about 30oN latitude, the air from the equator meets air flowing toward the equator from the higher latitudes. This air is cool because it has come from higher latitudes. Both batches of air descend, creating a high pressure zone. Once on the ground, the air returns to the equator. This convection cell is called the Hadley Cell and is found between 0 degrees and 30 degrees N.

There are two more convection cells in the Northern Hemisphere. The Ferrell cell is between 30oN and 50o to 60oN. This cell shares its southern, descending side with the Hadley cell to its south. Its northern rising limb is shared with the Polar cell located between 50 degrees N to 60 degrees N and the North Pole, where cold air descends.

There are three mirror image circulation cells in the Southern Hemisphere. In that hemisphere, the Coriolis Effect makes objects appear to deflect to the left. Ultimately, because there are three large-scale convection cells in the Northern Hemisphere and are repeated in the Southern Hemisphere, the model to understand these patterns is called the three-cell model.

Global Wind Patterns

Global winds blow in belts encircling the planet. The global wind belts are enormous and the winds are relatively steady. These winds are the result of air movement at the bottom of the major atmospheric circulation cells, where the air moves horizontally from high to low pressure. Technology today allows anyone to see global wind patterns in real-time, such as Earth Wind Map. Take a look at the Earth Wind Map and determine what patterns you can see occurring in the atmosphere in real-time. Are low pressure systems rotating counter-clockwise in the Northern Hemisphere? Are high pressure systems rotating clockwise in the Northern Hemisphere? Can you see the global wind patterns over the Atlantic and Pacific Oceans? Also notice how the winds flow faster over water than over continents because of land friction.

Let’s look at the global wind belts in the Northern Hemisphere:

In the Hadley cell air should move north to south, but it is deflected to the right by Coriolis. So the air blows from northeast to the southwest. This belt is the trade winds, so called because at the time of sailing ships they were good for trade.

In the Ferrel cell air should move south to north, but the winds actually blow from the southwest. This belt is the westerly winds or westerlies. Why do you think a flight across the United States from San Francisco to New York City takes less time than the reverse trip?

Finally, in the Polar cell, the winds travel from the northeast and are called the polar easterlies. The wind belts are named for the directions from which the winds come. The westerly winds, for example, blow from west to east. These names hold for the winds in the wind belts of the Southern Hemisphere as well.

Global Winds and Precipitation

Besides their effect on the global wind belts, the high and low pressure areas created by the six atmospheric circulation cells determine in a general way the amount of precipitation a region receives. In low pressure regions, where air is rising, rain is common. In high pressure areas, the sinking air causes evaporation and the region is usually dry. 

7. ATMOSPHERIC WATER VAPOR

Water vapor varies by volume in the atmosphere from a trace to about 4%. Therefore, on average, only about 2 to 3% of the molecules in the air are water vapor molecules. The amount of water vapor in the air is small in extremely arid areas and in locations where the temperatures are very low (i.e. Polar Regions, very cold weather). The volume of water vapor is about 4% in the very warm and humid tropical air. 

Why can't the amount of water vapor in the air be greater than 4%? The answer is because temperature sets a limit to how much water vapor can be in the air. Even in tropical air, once the volume of water vapor in the atmosphere approaches 4% it will begin to condense out of the air. The condensing of water vapor prevents the percentage of water vapor in the air from increasing. If temperatures were much warmer, there would be a potential to have more than 4% water vapor in the atmosphere. Think about the steam trapped in a tea kettle. The very warm temperatures and higher pressures allow for a large amount of water vapor to exist in the air within the tea kettle. Just from watching the steam leave the tea kettle, one can get an idea of the water vapor density within that kettle. The amount of water vapor within the air in the kettle is greater than 4%. If the earth's oceans were placed on the planet Venus, the ocean water would boil into the atmosphere and produce a very dense steam (current surface temperatures on Venus are 900 degrees Fahrenheit with an average sea level pressure of 92,000 millibars (92 times that of Earth)). Under this amount of enormous heat and pressure (hot enough to melt lead), water vapor would well exceed 4% of the atmosphere by volume. As a note, Venus does not have any significant amounts of water vapor; the atmosphere of Venus is 96% carbon dioxide and 3.5% nitrogen. In summary, temperature determines the maximum amount of water vapor that can exist in the air. The higher the temperature, the greater the potential percentages of water vapor in the air.

8. The Transfer of Heat Energy

The heat source for our planet is the sun. Energy from the sun is transferred through space and through the earth's atmosphere to the earth's surface. Since this energy warms the earth's surface and atmosphere, some of it is or becomes heat energy. There are three ways heat is transferred into and through the atmosphere:

    radiation

    conduction

    convection

Radiation

If you have stood in front of a fireplace or near a campfire, you have felt the heat transfer known as radiation. The side of your body nearest the fire warms, while your other side remains unaffected by the heat. Although you are surrounded by air, the air has nothing to do with this transfer of heat. Heat lamps, that keep food warm, work in the same way. Radiation is the transfer of heat energy through space by electromagnetic radiation.

Most of the electromagnetic radiation that comes to the earth from the sun is invisible. Only a small portion comes as visible light. Light is made of waves of different frequencies. The frequency is the number of instances that a repeated event occurs, over a set time. In electromagnetic radiation, its frequency is the number of electromagnetic waves moving past a point each second.

Our brains interpret these different frequencies into colors, including red, orange, yellow, green, blue, indigo, and violet. When the eye views all these different colors at the same time, it is interpreted as white. Waves from the sun which we cannot see are infrared, which have lower frequencies than red, and ultraviolet, which have higher frequencies than violet light. It is infrared radiation that produce the warm feeling on our bodies.

Most of the solar radiation is absorbed by the atmosphere and much of what reaches the earth's surface is radiated back into the atmosphere to become heat energy. Dark colored objects, such as asphalt, absorb radiant energy faster that light colored objects. However, they also radiate their energy faster than lighter colored objects.

Conduction

Conduction is the transfer of heat energy from one substance to another or within a substance. Have you ever left a metal spoon in a pot of soup being heated on a stove? After a short time the handle of the spoon will become hot.

This is due to transfer of heat energy from molecule to molecule or from atom to atom. Also, when objects are welded together, the metal becomes hot (the orange-red glow) by the transfer of heat from an arc.

This is called conduction and is a very effective method of heat transfer in metals. However, air conducts heat poorly.

Convection

Convection is the transfer of heat energy in a fluid. This type of heating is most commonly seen in the kitchen when you see liquid boiling.

Air in the atmosphere acts as a fluid. The sun's radiation strikes the ground, thus warming the rocks. As the rock's temperature rises due to conduction, heat energy is released into the atmosphere, forming a bubble of air which is warmer than the surrounding air. This bubble of air rises into the atmosphere. As it rises, the bubble cools with the heat contained in the bubble moving into the atmosphere.

As the hot air mass rises, the air is replaced by the surrounding cooler, more dense air, what we feel as wind. These movements of air masses can be small in a certain region, such as local cumulus clouds, or large cycles in the troposphere, covering large sections of the earth. Convection currents are responsible for many weather patterns in the troposphere.

9. The Hydrologic Cycle

The hydrologic cycle involves the continuous circulation of water in the Earth-Atmosphere system. At its core, the water cycle is the motion of the water from the ground to the atmosphere and back again. Of the many processes involved in the hydrologic cycle, the most important are...

    evaporation

    transpiration

    condensation

    precipitation

    runoff

The basic hydrologic (water) cycle

Evaporation

Evaporation is the change of state in a substance from a liquid to a gas. In meteorology, the substance we are concerned about the most is water.

For evaporation to take place, energy is required. The energy can come from any source: the sun, the atmosphere, the earth, or objects on the earth such as humans.

Everyone has experienced evaporation personally. When the body heats up due to the air temperature or through exercise, the body sweats, secreting water onto the skin.

The purpose is to cause the body to use its heat to evaporate the liquid, thereby removing heat and cooling the body. It is the same effect that can be seen when you step out of a shower or swimming pool. The coolness you feel is from the removing of bodily heat to evaporate the water on your skin.

Transpiration

Transpiration is the evaporation of water from plants through stomata. Stomata are small openings found on the underside of leaves that are connected to vascular plant tissues. In most plants, transpiration is a passive process largely controlled by the humidity of the atmosphere and the moisture content of the soil. Of the transpired water passing through a plant only 1% is used in the growth process of the plant. The remaining 99% is passed into the atmosphere.

Condensation

Condensation is the process whereby water vapor in the atmosphere is changed into a liquid state. In the atmosphere condensation may appear as clouds or dew. Condensation is the process whereby water appears on the side of an uninsulated cold drink can or bottle.

Condensation is not a matter of one particular temperature but of a difference between two temperatures; the air temperature and the dewpoint temperature. At its basic meaning, the dew point is the temperature where dew can form.

Actually, it is the temperature that, if the air is cool to that level, the air becomes saturated. Any additional cooling causes water vapor to condense. Foggy conditions often occur when air temperature and dew point are equal.

Condensation is the opposite of evaporation. Since water vapor has a higher energy level than that of liquid water, when condensation occurs, the excess energy in the form of heat energy is released. This release of heat aids in the formation of hurricanes.

Precipitation

Precipitation is the result when the tiny condensation particles grow too large, through collision and coalescence, for the rising air to support, and thus fall to the earth. Precipitation can be in the form of rain, hail, snow or sleet.

Precipitation is the primary way we receive fresh water on earth. On average, the world receives about 38½" (980 mm) each year over both the oceans and land masses. 

Runoff

Runoff occurs when there is excessive precipitation and the ground is saturated (cannot absorb any more water). Rivers and lakes are results of runoff. There is some evaporation from runoff into the atmosphere but for the most part water in rivers and lakes returns to the oceans.

If runoff water flows into the lake only (with no outlet for water to flow out of the lake), then evaporation is the only means for water to return to the atmosphere. As water evaporates, impurities or salts are left behind. The result is the lake becomes salty as in the case of the Great Salt Lake in Utah or Dead Sea in Israel.

Evaporation of this runoff into the atmosphere begins the hydrologic cycle over again. Some of the water percolates into the soil and into the ground water only to be drawn into plants again for transpiration to take place.