Chapter One Composition and Structure 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.”

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.

The average length of time that individual molecules of 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 exits the atmosphere (in kilograms per year).

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 account for the greater part of the atmospheric mass— 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.

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.

Variable gases are those whose distribution in 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 found in the lowest 5 km (3 mi) of the atmosphere. Variable Gases of the Atmosphere

Water is constantly being cycled between the planet and 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.

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).

Since the 1950s, the concentration of carbon dioxide has 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.

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.

Meteorologists find it convenient to divide the atmosphere vertically 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.

Scientists divide the atmosphere into four layers based 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.

Temperature profile of the atmosphere

The troposphere is the lowest of the four temperature layers. The troposphere is where the vast majority of weather events occur 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.

Despite the strong tendency for temperature to decrease with altitude in the troposphere, it is not uncommon for the reverse situation to occur. Such situations, where temperature increases 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 (12 mi). 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.

Ozone is the form of oxygen in which three O atoms are 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 eyes and damage to vegetation (Box 1-2).

The red areas reveal the “ozone hole” over Antarctica

Of the 0.1 percent of the atmosphere not contained in the troposphere and stratosphere, 99.9 percent exists in the mesosphere which extends to a height of about 80 km (50 mi). Temperature in the mesosphere decreases with altitude.

Above the mesosphere is the thermosphere, where temperature increases with altitude to values in excess of 1,500 C. The temperature of the air is an expression of its kinetic energy, which is related to the speed at which its molecules move.

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.

It is generally believed that Earth was formed perhaps 4.5 billion years ago. If an atmosphere formed with Earth, it must have consisted of the gases most abundant in the early solar system including large amounts of hydrogen and helium, the two lightest elements. If molecules move with sufficient speed, known as their escape velocity, they can overcome gravity and leave the atmosphere. Light gases are more likely to achieve escape velocity; thus, the hydrogen and helium were most readily lost.

Over time a new, secondary atmosphere formed, made up of gases released from Earth’s interior by volcanic eruptions—a process called outgassing. The gases spewed out during volcanic events are predominantly water vapor and carbon dioxide, with lesser amounts of sulfur dioxide, nitrogen, and other gases.

The transformation to an atmosphere high in oxygen depended on the advent of primitive, anaerobic bacteria about 3.5 billion years ago. These primitive life-forms were the first in a long line of organisms that removed carbon dioxide from the air and replaced it with oxygen. Ultimately, plant and later animal material sank to the ocean floor, where the organic carbon was locked away in sediments.

Atmospheric pressure is one of the most fundamental of weather characteristics. Air tends to blow away from regions of high pressure toward areas of lower pressure. The horizontal variation in air pressure generates winds. Air tends to rise in areas of low surface pressure and sink in zones of high pressure. Rising motions favor the formation of clouds, while sinking motions promote clear skies.

Atmospheric pressure is routinely plotted on maps by the use of lines called isobars. Each isobar connects points having equal air pressure with the pressure being expressed in units of millibars (mb) in the United States and kilopascals (kPa) in Canada. A surface weather map

Information regarding wind speed and direction can be obtained on weather maps by looking at the station models, which contain symbols and numbers giving weather information for particular locations. Station model symbols

Temperature is one of the most obvious weather components and varies from place to place systematically. Major changes in temperature often occur due to the presence of fronts, fairly narrow boundary zones separating relatively warm and cold air. Cold fronts are shown as a blue line with triangles while warm fronts are depicted by a red line with semicircles.

Relative humidity is one of several ways of expressing the amount of water vapor in the air. It indicates the amount of water vapor present relative to the maximum possible; thus, it is usually reported as a percentage. Another index called the dew point temperature is often preferred. The higher the dew point, the greater the amount of water vapor in the air.

Weather forecasters routinely employ state-of-the-art computer hardware and software systems that perform millions of calculations, based on input data and displayed at work stations employing the Advanced Weather Interactive Processing System (AWIPS), which allows forecasters to display maps of current weather conditions, computer models, satellite and radar images, as well as forecast advisories and discussions from other weather facilities. AWIPS graphical display monitors

The next chapter examines solar radiation and the seasons.