Chapter OneChapter One

Composition andComposition andStructure of the AtmosphereStructure of the Atmosphere

The atmosphere is a mixture of gas molecules,microscopically small suspended particles of

solid and liquid, and falling precipitation.Meteorology is the study of the atmosphere and the processes

that cause what we refer to as the “weather.”

Atmosphere - meteorology

If we think of the atmosphere as a reservoir for gas,the gas concentration in the reservoir will remain constant

so long as the input rate is equal to the output rate. Under such conditions, we say that the concentration of

the gas exists in a steady state.

Reservoir

The average length of time that individual moleculesof a given substance remain in the atmosphere

is called the residence time.The residence time is found by dividing the mass of the substance in the atmosphere (in kilograms)

by the rate at which the substance enters and exitsthe atmosphere (in kilograms per year).

Residence time

Atmospheric gases are often categorized as being permanent or variable,

depending on whether their concentration is stable.Permanent gases are those that form a

constant proportion of the atmospheric mass.

Permanent Gases of the Atmosphere

Permanent gases

Permanent gases account for the greater part of the atmospheric mass—99.999 percent—and occur

in a constant proportion throughout the atmosphere’s lowest 80 km (50 mi).

Because of its chemical homogeneity,this region within 80 km of Earth’s surface

is called the homosphere.

Homosphere

Above the homosphere is the heterosphere, where lighter gases (such as hydrogen and helium)

become increasingly dominant with increasing altitude.Because its composition varies with altitude,

the heterosphere contains no truly permanent gases.

Heterosphere

Variable gases are those whose distributionin the atmosphere varies in both time and space.

The most abundant of the variable gases, water vapor, occupies about one-quarter of 1 percent

of the total mass of the atmosphere.Most atmospheric water vapor is foundin the lowest 5 km of the atmosphere.

Variable Gases of the Atmosphere

Variable gases

Water is constantly being cycled between the planetand the atmosphere through the hydrologic cycle.

Water continuously evaporates from both open water and plant leaves into the atmosphere, where it eventually

condenses to form liquid droplets and ice crystals. These liquid and solid particles are removed from the

atmosphere by precipitation as rain, snow, sleet, or hail.

Water vapour: H2O

Another important variable gas is carbon dioxide (.037%).Increases in the carbon dioxide content of the atmosphere

may have some important climatic consequences that could greatly affect human societies.

Carbon dioxide is removed from the atmosphere by photosynthesis, the process by which

green plants convert light energy to chemical energy (Box 1-1).

Carbon dioxide: CO2

Since the 1950s, the concentration of carbon dioxidehas increased at a rate of about 1.8 ppm per year.

The increase has occurred mainly because of anthropogenic combustion and deforestation

of large tracts of woodland.

Carbon dioxide increase since the 1950s

Small solid particles and liquid droplets in the air(excluding cloud droplets and precipitation) are

collectively known as aerosols (Box 1-3).Aerosols play a major role in the formation

of cloud droplets because virtually all cloud droplets that form in nature do so on

suspended aerosols called condensation nuclei.

Aerosols

Satellite data for aerosols

The density of any substance is the amount of mass of the substance contained in a unit of volume.

At lower altitudes, there is more overlying atmospheric mass than is the case higher up.

Because air is compressible and subjected to greater compression at lower elevations, the density of the air

at lower levels is greater than that aloft.

Density

Meteorologists find it convenient to divide the atmospherevertically into several distinct layers. Some layers are

distinguished by electrical characteristics, some by chemical composition,

and some by temperature characteristics.Together with the change in density with height,

this layering of the atmosphere gives it its structure.

Vertical structure

Scientists divide the atmosphere into four layersbased not on chemical composition but rather on

how mean temperature varies with altitude.The average temperature profile, called the

standard atmosphere, shows the four layers:troposphere, stratosphere,

mesosphere, and thermosphere.

Standard atmosphere:

Average temperatur profile

Temperature profile of the atmosphere

The troposphere is the lowest of the four temperature layers. The troposphere

is where the vast majority of weather eventsoccur and is marked by a general pattern

in which temperature decreases with height.At the top of the troposphere, a transition zone called the tropopause marks the level at which

temperature ceases to decrease with height.

Troposphere

Temperature profile of the atmosphere

Despite the strong tendency for temperatureto decrease with altitude in the troposphere,it is not uncommon for the reverse situation

to occur. Such situations, where temperatureincreases with height, are known as inversions.

Above the tropopause is the stratosphere. Little weather occurs in this region.

In the lowest part of the stratosphere, the temperature remains relatively constant

up to a height of about 20 km.From there to the top of the stratosphere

(called the stratopause), the temperature increases with altitude. In the upper stratosphere,

heating is almost exclusively the result of ultraviolet radiation being absorbed by ozone.

Stratosphere

Temperature profile of the atmosphere

Ozone is the form of oxygen in which three O atomsare joined to form a single molecule.

The small amount of it that exists in the the stratosphere is absolutely essential to life on Earth

because it absorbs lethal ultraviolet radiation from the sun.Near Earth’s surface it is a major component

of air pollution, causing irritation to lungs and eyesand damage to vegetation (Box 1-2).

Ozone

The red areas reveal the “ozone hole” over Antarctica

Of the 0.1 percent of the atmosphere not containedin the troposphere and stratosphere,

99.9 percent exists in the mesospherewhich extends to a height of about 80 km.

Temperature in the mesosphere decreases with altitude.

Mesosphere

Temperature profile of the atmosphere

Above the mesosphere is the thermosphere, where temperature increases with altitude

to values in excess of 1,500 C. The temperatureof the air is an expression of its kinetic energy,

which is related to the speed at which its molecules move.

Thermosphere

Temperature profile of the atmosphere

An additional layer, called the ionosphere, can be defined based

on its electrical properties.This layer, which extends from the

upper mesosphere into the thermosphere, contains large

numbers of electrically charged particles called ions.

The ionosphere is important for reflecting AM radio waves back

toward Earth and is responsible for the aurora borealis

and the aurora australis.

Ionosphere