Chapter 22

The Atmosphere of the Earth

The Atmosphere

• The atmosphere is a relatively thin shell of gases that surrounds the solid Earth.

• There are rapidly moving particles with billions of collisions every second.

• The force of gravity attracts the particles towards the surface of the earth, so the atmosphere thins rapidly with increasing distance above the surface.

• Eventually it merges with the very diffuse medium of outer space.

Air Density

• Since air density is defined by the number of molecules in a unit of volume, the density of the atmosphere decreases rapidly with increasing altitude.

At greater altitudes the same volume contains fewer molecules of the gases that make upThe air. That means that the density of air decreases with increasing altitude.

The earth’s atmosphere thins rapidly with increasing altitude and is much closer to the earth than most people realize.

Composition of the Atmosphere• Composition by Volume:78% N2, 21% O2, approx. 1% Ar.• The molecules are well mixed. N2 doesn’t react

easily with rocks, so it has accumulated in the atmosphere.

• Some bacteria in soil remove N2 from the atmosphere and by lightning. N2 is part of the nitrogen cycle, since plants need N2 to produce amino acids and nucleic acids.

• The nitrogen compounds are then carried through the food chain. Eventually the N2 returns to the atmosphere through the decay of plant and animal matter.

• Overall the amount of N2 in the atmosphere remains essentially constant over time.

Earth’s atmosphere has a unique composition of gases when compared to that of Other planets in the solar system.

Oxygen in the Atmosphere

• Oxygen is removed from the atmosphere by:

1. Living organisms use it to oxidize food to carbon dioxide and water.

2. Rocks consume it as weathering occurs. Metals and other elements in rocks combine with oxygen to form oxides.

• Plants release oxygen as a result of photosynthesis and the amount released balances the amount removed by the organisms and by weathering.

• Oxygen is also maintained in a state of constant composition.

Other components of the Atmosphere• The argon in the atmosphere is chemically inert

and doesn’t react. • Water vapor is also present in the atmosphere. • Water vapor is invisible and different from fog

and clouds.• Fog and clouds are tiny droplets of liquid water.• The amount of water vapor varies from a fraction

of a percent (cold, dry air) to about 4% (warm, humid air) and is essential to maintaining life on the earth.

• Water enters the atmosphere by evaporation, mostly from the ocean, and leaves the atmosphere as rain or snow. This is called the hydrologic cycle.

The Atmosphere

Water vapor – fixed but highly variable (0-4%)Supplied by evaporationRemoved by precipitation

Other components of the Atmosphere• The remaining .03% of the atmosphere is

mostly carbon dioxide (CO2) and traces of the inert gases neon, helium, krypton, and xenon, along with less than 5 parts per million of free hydrogen, methane, and nitrous oxide.

• The CO2 content varies locally near cities from the combustion of fossil fuels and from the respiration and decay of organisms and materials produced by organisms.

Carbon Dioxide in the Atmosphere• The overall atmospheric concentration of CO2 is regulated by:1. the removal from the atmosphere through photosynthesis

process of green plants.

2. the massive exchanges of CO2 between the ocean and the atmosphere.

3. chemical reactions between the atmosphere and rocks of the surface, primarily limestone.

• The ocean contains about 50 times more CO2 than the atmosphere in the form of carbonate ions and as dissolved CO2

gas. It serves as a buffer by releasing CO2 if the atmospheric concentration decreases and absorbing CO2 if the atmospheric concentration increases.

• Limestone rocks contain an amount of carbon dioxide that is equal to about 20 times the mass of all of Earth’s present atmosphere

Carbon Dioxide in the Atmosphere• Limestone is CaCO3, so you can say that

each formula unit of CaCO3 contains one molecule of CO2.

• There is a yearly increase of about 1 part per million of CO2 in the atmosphere over the last several decades.

• The increase is believed to be a result of the destruction of tropical rainforests along with increased fossil fuel combustion.

Aerosols

• The atmosphere also contains particles of dust, smoke, salt crystals (from mist created by ocean surf and waves) and tiny solid or liquid particles called aerosols.

• Aerosol particles become suspended and are dispersed among the molecules of the atmosphere gases.

• Aerosols are produced by combustion, often resulting in air pollution, and by volcanoes and forest fires.

The Atmosphere

Atmosphere contains dust, smoke and salt crystals (aerosols).

Atmospheric Pressure• At the earth’s surface the atmosphere

exerts a force of about 14.7 lbs/sq. in.. As you go to higher altitudes above sea level the pressure rapidly decreases with increasing altitude.

• The decrease in pressure is due to having less weight of the atmosphere above an object at higher altitudes.

• A barometer is used to measure pressure. The mercury barometer was invented in 1643 by an Italian named Torricelli.

Barometers• The original barometer was a glass tube with

one end closed and filled with mercury (Hg). • The tube was placed, open end down, in a bowl

of mercury.• As the atmospheric pressure increases and

decreases, the height of the supported mercury column moves up and down.

• Pressure is indicated according to the height of the mercury column.

• The standard atmospheric pressure is the atmospheric pressure at sea level and it is 76.00 cm or 29.92 in of mercury. This is also called one atmosphere of pressure.

Warming the Atmosphere• Radiation from the Sun must pass through the

atmosphere before reaching the earth’s surface.• The atmosphere filters, absorbs, and reflects

incoming solar radiation. On the average, about 30% of the total radiation is reflected back into space, most of that occurring from clouds.

• The amount reflected at any one time depends on the extent of cloud cover, the amount of dust in the atmosphere, and the extent of snow and vegetation on the surface.

• Only about ½ of the incoming solar radiation reaches the earth’s surface after deducting the amounts reflected and absorbed by air and clouds and scattered.

Absorbed light• The incoming light that does reach the earth’s surface is

absorbed by rocks, soil, water, or anything else on the ground.

• These materials then emit the absorbed energy as infrared radiation or heat.

• Carbon dioxide and water vapor molecules absorb this infrared radiation and their temperature increases. They then go on to reemit this infrared radiation, or heat, in all directions and it gets reabsorbed and reemitted by other materials. Eventually some ends up going back to outer space.

• The result is that the infrared radiation lingers for a longer length of time before is released to outer space. The temperature near the surface then increases.

Greenhouse Effect• The greenhouse effect is the process of

heating the atmosphere in the lower parts by the absorption of solar radiation and reemission of infrared radiation.

• This is somewhat analogous to what happens in a greenhouse, since the glass allows the solar energy to enter but does not allow the heat to leave.

• Carbon Dioxide and water vapor molecules do not trap the infrared radiation, they just delay the process of releasing the heat back to outer space, resulting in increased temperatures. The more carbon dioxide present the higher the temperatures will tend to be.

Structure of the Atmosphere• The greenhouse effect causes the

atmosphere to be heated from the ground up.

• The higher altitude parts of the atmosphere lose radiation to space more readily than the lower altitude parts.

• The lowest part of the atmosphere is warmer and the temperature decreases with increasing altitude at first. The temperature decreases on average 3.5oF per 1000 ft.

The Troposphere• The temperature decreases with altitude until an average

altitude of about 11 km. • The temperature then begins to remain more or less

constant with increasing altitude. • The lower part of the atmosphere until the point when

the temperature stops decreasing with altitude is called the troposphere.

• Almost all weather occurs in the troposphere. The word is derive from the Greek meaning “turning layer”.

• The air at the bottom is continually warmed by the ground and the ocean, like a stove, and the air at the bottom is most dense, so it absorbs heat the most.

• The warm ground-level air rises and the colder air sinks to take its place causing weather (turbulence, clouds, winds, rains, etc.).

The Tropopause

• The upper boundary of the troposphere is called the tropopause and it varies with latitude and with the seasons. It is generally one and a half times higher than average near the equator and about half the average height over the poles.

• It is also higher in the summer than in the winter at any given altitude.

• The average temperature at the tropopause is about -60oC or -80oF.

The altitude of the tropopause is a function of latitude. It is higher at the equatorial areas and lower at the poles, since the air is warmer due to more heat emanating from the earth at the equator. It is at 16 km at the equator, 10 km at the poles.

Poles

Equator

The Stratosphere• The stratosphere is the layer above the tropopause.• The word is from the Greek for “stratified layer”.• The temperature increases with height in this layer.• The stratosphere contains little moisture, dust or

turbulence, making it a desirable altitude to fly.• Most ozone is present in the upper stratosphere and it

absorbs UV radiation from the Sun and converts it to heat. Towards the bottom less UV radiation is left, so less heat is produced and the temperatures are lower.

• Not much turbulence. Jet aircraft mostly cruise through here.

• The temperature increases gradually to a height of about 48 km (30 mi), where it reaches a maximum of about 10oC (about 50oF).

• The upper layer of the stratosphere is called the stratopause.

In the mid-1980s, scientists identified a massive hole in the Earth's ozone layer over Antarctica, confirming a prediction made a decade earlier that the rampant use of refrigerants and aerosols called chlorofluorocarbons would destroy the protective layer of the atmosphere. In 1990, the world came together in Canada and agreed to tackle the problem once and for all, before skin cancers and other ill effects of unfettered ultraviolet light became a permanent part of spending time outdoors. A strengthening of 1987's Montreal Protocol was the result -- and it worked. The level of CFCs detected in the lower atmosphere peaked in 1995, and in the Antarctic stratosphere in 2001. But the story isn't over. Though industrialized nations stopped producing most CFCs in 1996, and developing nations stopped in 2006, the ozone hole reached its greatest ever recorded area -- the size of North America -- just last year. That's because CFCs last in the atmosphere for about 40 years, so while the United Nations expects incremental improvement every year, the ozone layer isn't expected to fully recover until 2036. For more information about the state of the ozone layer, visit ozonewatch.gsfc.nasa.gov.

Fig. 1.9

The stratopause

• The upper layer of the stratosphere is called the stratopause.

• At the stratopause the temperature is approximately 0oC, but the pressure is 1/1000 of the surface pressure.

The Mesosphere and the Thermosphere• The mesosphere, from the Greek “middle layer” is

above the stratopause. The temperature in this layer decreases again with altitude. There is little ozone present to absorb solar radiation. The minimum temperature reached in the atmosphere occurs here, approximately -100oC (-148oF)

• The layer above the mesosphere is called the thermosphere, from the Greek for “warm layer”. The temperature goes back to increasing as you get higher due to absorption of solar radiation. A thermometer would not actually record these high temperatures, however, since there are so few particles present that there is not much heat transferred to a thermometer.

The Ionosphere and the Exosphere

• The thermosphere and upper mesosphere are sometimes called the ionosphere.

• There are free electrons and free ions at this altitude which are responsible for reflecting radio waves around the earth and for the northern lights.

• The exosphere is the outermost layer where the molecules merge with the diffuse vacuum of space.

• The molecules of this layer that have sufficient kinetic energy are able to escape and move off into space.

•The ionosphere is part of the thermosphere.

•In the ionosphere some molecules of gas are broken apart by radiation to form electrically charged gaseous ions.

•The aurora is caused by electrons streaming in from the Sun combining with ionized gases to form neutral atoms and giving off light rays in the process.

•This layer can scatter or reflect AM radio waves.

•Above 100 km the atmosphere is so thin that orbiting satellites can pass through with very little friction.

Ionosphere

The Winds

• There is uneven heating of the Earth’s surface due to the different abilities of materials to absorb heat due to their specific heat capacities.

• As a local region of air becomes heated the increased kinetic energy of the molecules expands the mass of air, reducing its density.

• Less dense air is buoyant and is pushed upward by nearby cooler, more dense air. This results in 3 motions for air:

1. The upward movement of air over a region of greater heating.

2. The sinking movement of air over a cooler region.3. A horizontal air movement between the cooler and

warmer regions.

The Winds

• Wind is a horizontal movement of air. The direction of a wind is defined as the direction from which it blows.

• Clouds form over areas where the air is moving upwards.

• The clear air between the clouds is over areas where the air is moving downward.

• Air can be observed to move from a field of cool grass toward an adjacent asphalt parking lot on a calm, sunlit day and can be observed with soap bubbles or smoke.

Local Wind Patterns• The upward and downward movement of air

leads to:1. The upward movement produces a lifting effect

on the surface that results in an area of lower atmospheric pressure.

2. The downward movement produces a piling up effect on the surface that results in an area of higher atmospheric pressure.

• On the surface, air is seen to move from the “piled up” area of higher pressure horizontally to the “lifted” area of lower pressure. The movement of air and the pressure differences occur together. This is called convective movement of air.

Cool air pushes the less dense, warm air upward, reducing the surface pressure. As theUplifted air cools and becomes more dense, it sinks, increasing the surface pressure.

Local Wind Patterns

• A local wind pattern may result from the temperature differences between a body of water and adjacent landmasses.

• A cool, refreshing, gentle breeze blows from the water toward the land during the summer.

• During the day the temperature of the land increases more rapidly than the water temperature due to differences in specific heat capacity. The air over the land is therefore heated more, expands, and becomes less dense.

• Cool, dense air from over the water moves inland under the air over the land, buoying it up. This is called a sea breeze.

• During the night the land surface cools more rapidly than the water and the air moves from the land to the sea.

Global Wind Patterns

• Temperature imbalances are what drive the global circulation of the atmosphere.

• The earth receives more direct solar radiation in the equatorial region than it does at higher latitudes.

• The temperatures of the lower troposphere are generally higher in the equatorial region, decreasing with latitude toward both the poles.

• Hot air rises in the belt around the equator, known as the intertropical convergence zone.

On a global, yearly basis, the equatorial region of the earth receives more directincoming solar radiation than the higher latitudes. As a result, average temperaturesare higher in the equatorial region and decrease with latitude toward the poles. Thissets the stage for worldwide patterns of prevailing winds, high and low areas of atmospheric pressure, and climatic patterns.

Part of the generalized global circulation pattern of the earth’s atmosphere.

Water and the Atmosphere• Water exists on Earth as solid, in the form of ice, snow, or hail, as

liquid, and as water vapor, the invisible form of water in the gaseous state.

• Over 98% of water on the earth exists in the liquid state, mostly in the ocean. Only a small, variable amount of water vapor is in the atmosphere at any given time.

• The small amount of water vapor present in the atmosphere is responsible for:

1. contributing to the greenhouse effect, which helps make the earth a warmer planet.

2. serving as one of the principal agents in the weathering and erosion of the land, which creates soils and sculptures the landscape.

3. maintaining life, since almost all plants and animals cannot survive without water.

• It is the ongoing cycling of water vapor into and out of the atmosphere that makes all this possible.

Evaporation and Condensation

Evaporation and condensation are occurring at all times.If the number of liquid molecules leaving the liquidstate exceeds the number returning, the water is evaporating. If the number of molecules returning to the liquid state exceeds the number leaving, the water vapor is condensing. If both rates are equal,the air is saturated, and the humidity is 100%.

Evaporation and Condensation

• If the temperature is increased more water vapor must be added to the air to maintain the saturated condition.

• Warm air can hold more water vapor than cooler air.

• Warm air on a typical summer day can hold five times as much water vapor as cold air on a cold winter day.

Humidity

• The amount of water vapor in the air is referred to as humidity.• Damp, moist air is said to have a high humidity.• Dry air is said to have low humidity.• A measure of the amount of water vapor in the air at a particular

time is called the absolute humidity. • The relationship between the actual absolute humidity at a particular

temperature and the maximum absolute humidity that can occur at that temperature is called the relative humidity (RH).

• A humidity of 100% (saturated air) means that the air has all the water vapor it can hold.

• Evaporation is a cooling process since the molecules with higher kinetic energy and therefore higher temperature are the ones that evaporate. If the air is saturated then no further evaporation can occur and this is why it feels hotter when the humidity is high.

Humidity

Humidity – water vapor in the air.

Absolute humidity – actual amount of water vapor

Relative humidity – relationship between actual water vapor and how much air can hold.

100 x humidity maximum

humidity absolute RH

The Condensation Process• During the condensation process molecules on the

surface of an object form as dew or in the air as the droplets of water making up fog and clouds.

• Water molecules can also join together to produce solid water in the form of frost or snow.

• Before condensation can occur the air must be saturated. If cooling then occurs then the capacity of the air to hold water vapor decreases and condensation occurs.

• This is why you can see your exhaled breath when it is cold, since the high moisture content of your exhaled breath is condensed into tiny water droplets by cold air and why you see a white trail behind an airplane, since water is one of the products of fuel combustion by airplane engines.

• The temperature at which condensation begins is called the dew point temperature, since it is when dew forms.

Water and the atmosphere

Dew point temperature – temperature at which dew (or frost) will form.

How are clouds and rain formed?

Fog and Clouds

• Fog and clouds are both accumulations of tiny droplets of water that have been condensed from the air.

• Fog is sometimes described as a cloud that forms near the surface.

• These water droplets are very small and a very slight upward movement of the air will keep them from falling. If they do fall they usually evaporate immediately.

• Fog and clouds form because air containing water vapor has been cooled to the dew point. There must be a condensation nucleus present, which can be a particle of salt or dust, in order for the water droplets to gather and gain size. Since salt attracts water, it is particularly effective.

Water and the atmosphereFor condensation, air must be saturated and condensation nuclei present.

Water and the atmosphereParticles of salt, dust, smoke, soot or other aerosols act as condensation nuclei.

Water and the atmosphere

As condensation begins, process continues to attract more moisture adding to its size.

Water and the atmosphere

Other factors are involved to increase the cloud size to large raindrops within a short time frame.

Review Exercises

• P. 537-538 Applying the Concepts:

#2, 3, 5, 6, 7, 8, 9, 10, 11, 12, 16, 17, 19, 20, 21

New Book:

p. 594-597 # 1, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 14, 15, 16, 17, 18, 19, 20, 22, 23, 24, 25, 27, 28, 29, 30, 31, 32, 33, 34, 35, 36, 37, 38, 39, 40, 41, 42, 43, 44, 45, 47, 48

Summary• The density of the atmosphere decreases rapidly with

increasing altitude.• Composition of the atmosphere by volume: 78% N2, 21% O2, 1% Ar. Also CO2 and H2O vapor.• N2 is relatively inert and involved in the nitrogen cycle

with plants. O2 is used by rocks for weathering and by animals for breathing and released by plants during photosynthesis. Ar is inert.

• Water enters the atmosphere by evaporation, mostly from the ocean, and leaves the atmosphere as rain or snow. This is called the hydrologic cycle.

• CO2 is used for photosynthesis by green plants. There are massive exchanges of CO2 between the ocean and the atmosphere. There are chemical reactions involving CO2 between the atmosphere and rocks of the surface, primarily limestone.

• There are also aerosols in the atmosphere:, dust, smoke, and salt crystals which result in pollution.

• Mercury barometer, atmospheric pressure.• The atmosphere filters, absorbs, and reflects incoming

solar radiation. 30% of the total radiation is reflected back into space, most of that occurring from clouds. The amount reflected at any one time depends on the extent of cloud cover, the amount of dust in the atmosphere, and the extent of snow and vegetation on the surface. Only about ½ of the incoming solar radiation reaches the earth’s surface after deducting the amounts reflected and absorbed by air and clouds and scattered.

• The result of absorption of infrared radiation by CO2 and H2O vapor is that the infrared radiation lingers for a longer length of time before is released to outer space. The temperature near the surface then increases. Greenhouse effect.

• Layers of the atmosphere: Troposphere (T decreases upwards, where weather occurs), stratosphere (T increases upwards due to ozone layer, very calm), mesosphere (temperature decreases with altitude), and thermosphere (temperature increases with altitude but not felt because of very few particles).

• The tropopause (its height varies, higher at the equator) is the layer between the troposphere and the stratosphere.

• The thermosphere and upper mesosphere are the ionosphere-radio waves travel here due to free ions. The exosphere is the outermost layer.

• Freon and other refrigerants can destroy the ozone layer, which normally protects us vs. harmful UV radiation by absorbing it.

• Because warm air is less dense than cool air warm air rises, cool air sinks, and air moves horizontally (wind).

• Convective movement of air from area of high pressure (air moving downwards) to area of low pressure (air moving upwards).

• During the day sea breeze is movement of air from cooler air above sea to warmer air above land. At night the air moves from the land to the sea, since air above the sea is warmer at night.

• Hot air rises in the belt around the equator, known as the intertropical convergence zone. This leads to wind patterns.

• Most water on earth is liquid. It maintains life on earth. The small amount of water vapor present in the atmosphere is responsible for contributing to the greenhouse effect, which helps make the earth a warmer planet, weathers and erodes the land, which creates soils and sculptures the landscape, and maintains life, since almost all plants and animals cannot survive without water.

• Warm air on a typical summer day can hold five times as much water vapor as cold air on a cold winter day. Air with maximum humidity (water vapor content) is saturated.

• Formula for relative humidity. • For condensation to occur air must be saturated and the

temperature must be low (dew point temperature). Forms dew and clouds. If colder temperatures forms frost and snow.

• For clouds and fog to form there must be condensation nuclei (dust or salt particles).