The Earth and Its Atmosphere

Atmosphere: the envelope of gases that surrounds a planet and is held to it by the planet's gravitational attraction. The earth's atmosphere is mainly composed of nitrogen and oxygen. Although our atmosphere extends upward for many hundreds of kilometers, 99% of the atmosphere's mass is confined within a 30 km of the earth's surface.

Weather: the condition of the atmosphere at any particular time and location, defined as temperature, cloudiness, precipitation, and wind speed and direction. Meteorology is the study of the atmosphere and the processes that cause weather. A professional meteorologist is a person who has completed the requirements for a college degree in meteorology or atmopsheric science.

Figure 1.17, page #21 (Ahrens)

Climate: the representation of daily and seasonal weather events over a long period of time. The distribution of the weather element throughout the season or year, departures from long term averages, anomalies, and extremes in weather are also important aspects of climate. Climate usually employs the values of the weather elements at the earth's surface. For example, the climate of Baltimore, Maryland is characterized by the annual average precipitation of 41.8 in. and annual mean temperature of 55 F. Climatology is the study of climate, its control and variability. A climatologistis a person who stidues the interaction between land, ocean, and the atmosphere and its influence on planet Earth many years from now.

Figure 17.7, page #480-481 (Ahrens)

The earth's atmosphere today is the result of a longtime evaluation process that began billions years ago. The early atmosphere was likely composed of hydrogen and helium. More dense atmopshere than today was the second stage due to the escape of gases from Earth's interior in the presence of abudant carbon dioxide (CO2).

Outgassing: the release of gases from hot, molten rock during volcanic activity. It was the main source of atmospheric gases, which was mostly CO2, with some nitrogen (N2), water vapor, and trace gases of methane, ammonia, sulfur dioxide, and hydrochloric acid.

The decline in the atmospheric CO2 concentration was evident as the planet cooled enough to condensate the water vapor into clouds and rainfall. Carbon dioxide dissolves in rainwater and produces weak carbonic acid that reacts chemically with the bedrock; large-scale geo-chemical process.

The formation of molecular oxygen (O2) probably began as energetic rays from the sun split water vapor into hydrogen and oxygen during photodissociation. The hydrogen is lighter and probably escaped to the space and the oxygen stayed in the atmosphere.

Photosynthesis: the product whereby plants use sunlight, water, and CO2 to manufacture their food. It was the primary source of oxygen (O2) and became an important component of the atmosphere's evaluation by about 2-3 billion years ago.

After the plants evolved, the atmospheric content increased rapidly and probably reached its present composition about several hundred millions ago.

Breath of airthe volume of average size breath of air is one liter (10^-3 m3). At sea level, there are roughly 10^22 air molecules in a liter and there are nearly 10^44 molecules in the atmosphere.

Homosphere: the atmosphere up to 80 km (50 mi.) in which the proportions of the principal gases, N2 (78%) and O2 (21%) remain relatively constant. The homosphere also contains 1% of argon and small quantities of permanent gases of helium, neon, hydrogen, and xenon and variable gases of water vapor, CO2 (0.035%), methane, nitrous oxide, and ozone (O3) and particles of dust, aerosols, cloud droplets and ice crystals.

The significance of an atmospheric gas or aerosol is not necessarily related to its relative concentration. Water vapor, CO2, and O3 are minor in concentration but are extremely important in atmospheric radiative processes. Water vapor is the main source of water around the globe; CO2 is essential for photosynthesis; O3 protects live organisms from ultraviolet radiation of the sun.

Table 1.1, page #6 (Ahrens)

Heterosphere: the atmosphere above 80 km in which gases are stratified such that concentrations of the heavier gases decrease more rapidly than do concentrations of lighter gases. Within the heterosphere, above about 150 km, oxygen is the major atmospheric gas but it occurs primarily in the atomic (O) rather than diatomic (O2) form. Ultraviolet radiation from the sun photo-dissociates O2 into its constituent atoms.

Photo-dissociation: the breakdown of molecules by radiation. Because of low air density at these altitudes, the collision of two oxygen atoms into a molecule is infrequent. At lower altitudes, below 100 km, the rate of recombination of oxygen atoms exceeds the rate of photo-dissociation of oxygen molecules.

Exosphere:the upper limit of the atmoshere. It is the region where atoms and molecules shoot off into space.

Aerosols: tiny liquid or solid particles that occur suspended in the atmosphere. They originate through forest fires, as sea-salt crystals from ocean spray, from wind erosion of soil, in volcanic emissions, from industrial and agricultural activities. Some aerosols act as nuclei for the development of clouds, and precipitation, and have some influence on air temperature by interacting with solar radiation.

Air pollutant: a gas or aerosol substance that occurs in concentrations high enough to threaten human health, to disrupt physical and biological processes, or to toxify a given environment. The concentration of a gaseous pollutant is expressed as number of pollutant molecules per million molecules of air (parts per million, ppm), while aerosol concentration are given in mass of pollutant per unit volume of air (micrograms per cubic meter).

Acid rain:cloud droplets or raindrops combining with gaseous pollutants (e.g. SO2, NOx) to make rain acidic - pH less than 5.0. If the fog droplets combine with such pollutants, it becomes acid fog

Figure 19.24, page 559 (Ahrens)

Figure 19.25, page 560 (Ahrens)

Air pollutants are produced by natural events and human activities, anthropogenic. Natural sources of air pollutants include forest fires, pollen dispersal, soil erosion, volcanic eruptions, and decay of dead plants and animals. Anthropogenic sources of air pollutants include emissions from automobiles, oil refineries, chemical plants, etc.

Air pollutants are classified according to two categories: Primary air pollutants are those directly emitted into the air. The most important primary air pollutans are particulate matter, sulfur oxides, carbon monoxide, hydrocarbons, and nitrogen oxides.

Primary air pollutants can undergo chemical reactions within the atmosphere and produce new substances known as secondary air pollutants. The photochemical smog and ozone are the most important secondary air pollutants. Both are generated as a result of interaction between the intense sunlight and the primary pollutans. Smog is the combination of fog and smoke.

The Martian atmosphere consists of mainly CO2 (95%), N2, argon, and O2. It is much thinner; its surface pressure is only 0.7% of the Earth's average sea-level pressure (1013.25 mb). Volcanic eruptions are the primary source of gases of the Martian atmosphere.

The average surface temperature is about -60 C (-76 F). Water in unknown quantities mixed with solid CO2 (dry ice) exists in the polar ice caps and scattered patches of permafrost. There are no photosynthesis, and hence, no free oxygen in the Martian atmosphere.

Gravity is about 38% of the Earth. This is because Mars is smaller, 53% of the Earth's equatorial diameter (1.274x107 m) and is less massive, 10% of the Earth's mass (5.98x1024 kg).

The Venusian atmosphere consits of mainly CO2 (95%), N2, and water vapor. The atmopshere is thick through opaque acid cloud deck. The surface pressure is 90,000 millibars, which is 90 times greater than the Earth. The average surface temperature is 480 C (900 F). It is slightly smaller than the Earth.

The outer planets (Jupiter, Saturn, Uranus, Neptune) are greater than the Earth in our solar system. The Jupiter and Saturn are composed of H2 and He. Among the other plantes, Uranus are composted of methane and H2, while Neptune has mainly methane and N2. In terms of density, the inner plantes (Mercury, Venus, Earth, Mars) are denser than the outer plantes .

Figure 1.2, page #5 (Ahrens)

Table 1, page #14 (Ahrens)

Through the early part of the twentieth century, weather intruments borne by kites, aircraft, and balloons provided information about the lowest 5000 m of the atmosphere followed by rockets, radiosondes, satellites, and radar.

Radiosonde: the instrument that consists of an instrument package and radio transmitter carried by balloon to altitudes of about 30 km (19 mi.) It provides vertical profiles (soundings) of temperature, pressure, and relative humidity. The pressure, humidity, and temperature are measured by separate sensors attached to the box and transmited by radio. A tracking instrument with global positioning system provides the vertical profile of wind. It is launched twice a day corresponding noon and midnight at universal time.

Figure 4, page #16 (Ahrens)

Dropwindsonde: another instrument similar to radiosonde but it is dropped from an aircraft and descends on a parachute at about 18 km (11mi.) per hour. It provides vertical profiles of air temperature, pressure, humidity, and wind. Dropwindsondes were developed to obtain soundings over oceans.

Surface launched rockets that provided information at altitudes of 110 to 165 km was replaced by satellites which monitor patterns of temperature and water vapor concentration, upper-air winds, and the life cycles of severe storms. Radar, on the other hand, determines the location and movement and intensity of precipitation.

Based on the average vertical temperature profile, the atmosphere is subdivided into four concentric layers, troposphere, stratosphere, mesosphere, and thermosphere. Almost all weather takes place in the troposphere and lower stratosphere.

Troposphere: the lowest thermal layer that extends from the earth's surface to an average altitude ranging from about 6 km at the poles to about 16 kilometer at the equator. The temperature within the troposphere usually, but not always, decreases with height with a rate of 6.5 ûC per 1000 meters. Tropopause is the transition zone between the troposphere and the stratosphere.

Stratosphere: the thermal layer that extends from the tropopause up to 50 km. The temperature within the stratosphere is constant, isothermal, below about 20 km and increases with height above about 20 km at which ozone concentration reaches its maximum. It is the ideal layer for jet aircraft travel since it is above the weather. Stratopause is the transition zone between the stratosphere and the mesosphere.

Mesosphere: the thermal layer that extends from the stratopause up to 80 km. The temperature within the mesosphere decreases with height reaching its lowest average value of -95 C (-139 F). Mesopause is the transition zone between the mesosphere and thermosphere.

Thermosphere: the outmost thermal layer that extends from the mesopause outward to space. The temperature within the thermosphere is at first isothermal and then rise rapidly with height. It is the region in which sun dissociates oxygen and nitrogen and ionizes the atmospheric gases.

Figure 1.11, page #13 (Ahrens)

Ionosphere: the region within the thermosphere, between altitudes of 80 to 400 km, that contains relatively high concentration of ions, electrically charged particles. Within the ionosphere, the charged subatomic particles reflect outgoing radio waves. Multiple reflections between the ionosphere and the Earth's surface greatly extend the range of radio transmissions. The relatively good reception of distant radio signals at night results from the reflection of radio waves by the upper ionosphere.>

Based on electron density, the ionosphere is subdivided into three layers: D-layer (60 to 90 km), E-layer (90 to 140 km), and F-layer (above 140 km). Since electron density increases nearly continuously with altitude to a maximum at an average level close to 300 km, D- E-, and F-layers are not discrete, rather differ each other on the reflection of radio waves.

Figure 1.13, page #16 (Ahrens)

The production rate of ions and electrons depends on i) the density of atoms and molecules for ionization, which decreases with height, ii) the intensity of solar radiation which increases with height.

Aurora borealis (northern lights): greenish-white lights produced by electrical activity in the ionosphere. They are visible at night in the Northern Hemisphere at altitudes between 100 and 400 km.

Aurora australis (southern lights): greenish-white lights produced by electrical activity in the ionosphere. They are visible at night in the Southern Hemisphere at altitudes between 100 and 400 km.

Each atmospheric gages has its own sets of energy levels, therefore its own charactersitic color. The molecular nitrogen gives off violet and red, while atomic oxygen can emit red or green.

Figure 2.22, page #53 (Ahrens)

Auroral ovals: the geographic areas where the aurora is visible. They are situated between 20û and 30û of latitude from the geomagnetic poles. In the Northern Hemisphere, they are centered on the northwest tip of Greenland.

Figure 2.23, page #54 (Ahrens)

Auroral activity varies with the sun's activity. When the sun is quiet, the auroral oval shrinks, but when the sun is active, the auroral oval expands equatorward, and the aurora may be visible across southern Canada and northern United States, or rarely, even further south. The sun's activity peaks about every 11-year.

Solar flares: the gigantic disturbances on the sun that emit the high-velocity streams of electrically charged subatomic particles to space. They last perhaps an hour and produce a shock wave that propagates rapidly 500 to 1000 km/sec through the solar wind.

Magnetosphere: the region within the ionosphere in which solar wind is deflected by the Earth's magnetic field and is deformed into a teardrop-shaped cavity surrounding the planet. An aurora results from the interaction between the solar wind and the Earth's magnetic field. On the side facing to the sun, the pressure of the solar wind compresses the field lines. On the opposite site, the magnetosphere stretches out into a long tail - magnetotail- which reaches far beyond the moon's orbit.

Solar wind: a stream of electrically charged subatomic particles (protons and electrons) that continually emanates from the sun and travels into space at speeds of 400 to 500 km/sec.

Bow shock:a boundary at which the speed of solar wind abruptly drops as a results of its approach to the magnestosphere.

Figure 2.20, page #51 (Ahrens)

Airglow:light from ionized oxygen and nitrogen and other gases that have been excited by solar radiation. It is a faint glow at night, much weaker then the aurora, no correlation with solar activity.