Chapter One Composition and Structure of the Atmosphere
Atmosphere - meteorology 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.”
Reservoir 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.
Residence time 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).
Permanent gases 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
Homosphere 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.
Heterosphere 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 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 of the atmosphere. Variable Gases of the Atmosphere
Water vapour: H2O 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.
Carbon dioxide: CO2 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
Aerosols 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.
Density 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.
Vertical structure 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.
Standard atmosphere: Average temperatur profile 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.
Troposphere 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.
Stratosphere 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.
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).
Mesosphere 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. Temperature in the mesosphere decreases with altitude.
Thermosphere 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.
Ionosphere 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.