The Atmosphere

I. Chemical Composition/Atmospheric Pressure

Most abundant gas is nitrogen (approximately 78%). Second most abundant gas is oxygen (approximately 21%). All other gasses are present in concentrations less than 1%. (Fig 16.3)

Other important constituents of the atmosphere - water vapor, aerosols (water droplets, dust, pollen), ozone.

Barometric Pressure - Pressure exerted by the atmosphere. Is highest at the Earth's surface, and decreases with increase in elevation. One-half of the mass of the atmosphere lies below 5.6 km (3.5 miles). So, the change in barometric pressure with altitude is not linear. (Fig 16.5)

II. Atmospheric Structure (Fig 16.7)

Atmospheric structure is defined with respect to temperature change with altitude.

Troposphere - The layer of air that lies closest to the Earth's surface and extends upward to about 12 kilometers. Temperatures decrease with elevation  (the environmental lapse rate) because heating of the troposphere occurs by emission of infrared radiation from the Earth's surface.

Tropopause - The boundary between the troposphere and the overlying stratosphere.

Stratosphere - The layer of air extending from the tropopause to about 47kilometers. Gradual temperature increase with elevation due to absorption of ultraviolet light by ozone.

Stratopause - The boundary between the stratosphere and the overlying mesophere.

Mesosphere - The layer of air that lies above the stratosphere and extends to approximately 80 kilometers. Temperature decreases with elevation because density of gasses is very low, and therefore heating by absorption  of solar radiation by atmospheric gasses is minimal.

Mesopause - The boundary between the mesophere and the overlying thermosphere.

Thermosphere - An extremely high and diffuse region of the atmosphere lying above the mesosphere. Temperature gradually increases with elevation to more than 1000^o C due to absorption of high energy x-rays and ultraviolet radiation. However, because the density of the thermosphere is very low, it would feel very cold in the thermosphere.

III. Temperature Changes at the Earth's Surface

Variations in temperature between different latitudes are largely due to differences in solar altitude and changes in seasons.

Solar Altitude - Angle of the sun above the horizon. Changes hourly during the day, and with changes in the seasons. Concentration of solar radiation received at the surface is highest at those latitudes where average solar altitude is high (incoming solar energy is concentrated into a smaller unit surface area). Highest concentrations of solar radiation per unit area are received at low latitudes (equatorial regions), and therefore average temperatures are highest at low latitudes. (Fig 16.10, 16.11)

Seasons - Because the North Pole is inclined at an angle of 23.5^o toward the North Star (Polaris), the sun's most direct rays strike the Earth at latitudes 23.5^o south (Tropic of Capricorn) on December 21 (Winter Solstice), 0^o (Equator) on March 21 (Vernal Equinox) and September 21 (Autumnal Equinox), and 23.5^o north (Tropic of Cancer) on June 21 (Summer Solstice). (Fig 16.12,16.13)

Variations in temperature at any given latitude are largely due to local/regional variations in A) heat transport by air masses, B) latent heat released or absorbed by evaporation, precipitation, melting and freezing of water, and C) heat storage in land and water, D) elevation above sea level, E) geographic position, F) cloud cover.

A) Heat Transport Mechanisms (Fig 16.16):

1. Conduction - Transfer of heat from one object to another by atomic or molecular motion. Air masses are poor transporters of heat by conduction. Example: Transfer of heat from a hot pan to your hand.

2. Convection - Transfer of heat by movement of a mass from one place to another. Air masses are good transporters of heat by convection. Example: Transfer of heat by convective currents in boiling water.

3. Transmission - Transfer of heat through a transparent medium. Example: Transfer of solar energy through the atmosphere to Earth's surface.

Solar energy traveling through Earth's atmosphere is primarily in the form of electromagnetic radiation - an oscillating electric and magnetic field .

Electromagnetic Spectrum  (Fig 16.17) - The entire range of electromagnetic radiation from very long wavelength (low frequency) radiation to very short wavelength (high frequency) radiation. As solar energy is transmitted through the atmosphere, it may be affected by one ore more of the following processes:

1. Absorption - The acceptance of electromagnetic radiation by an object and conversion to another form of energy. Example: Absorption of solar energy by your skin and subsequent transfer to heat energy. (Fig 16.19)

3. Reflection - The return of electromagnetic radiation off an object's surface. Albedo - the proportional reflectance of a surface. Example: Reflection of light off a lake surface. (Fig 16.19)

4. Scattering - The change in direction of electromagnetic radiation after striking an object. Degree of scattering is inversely proportional to wavelength. Example: The scattering of sunlight by atmospheric gasses, water droplets, and dust particles. (Note: The sky appears blue on a clear day because short wavelength visible light (blue) is more effectively scattered in the atmosphere than long wavelength (red) light.)  (Fig 16.19,16.20)

Approximately 50% of the incoming solar radiation (mostly visible and ultraviolet) is reflected by clouds, absorbed by clouds and atmospheric gasses, and scattered by atmospheric gasses and dust particles. (Fig 16.19)

Approximately 50% of the incoming solar radiation reaches the Earth's surface. A small amount of this radiation is reflected off the Earth's surface. Most is absorbed by the Earth's surface and emitted back into the atmosphere as infrared  (heat) radiation. Atmospheric gasses absorb much of the emitted infrared radiation, keeping the lower atmosphere warm (greenhouse effect). (Fig 16.21).

Most important greenhouse gasses - water, carbon dioxide, methane.

B) Latent Heat - Heat energy released (or absorbed) when a substance changes from one state to another. Example: Processes of  melting and evaporation of water absorb energy (lower surrounding temperature). Processes of freezing and of condensation of water releases energy (increase surrounding temperature) (Fig 17.2).

C) The amount of heat stored by land or water is dependent upon:

1. Specific Heat - the amount of energy needed to raise the temperature of one gram of rock/water by 1^oC. Water has a higher specific heat than rock, and therefore land surfaces heat more rapidly that bodies of water.

2. Heat transport within the material - Solids transfer heat primarily by conduction. Liquids and gasses transfer heat primarily by convection. Therefore, solid bodies tend to have highest temperatures at their surface. Liquid and gas bodies more effectively distribute heat throughout the entire body.

3. Latent heat - Because evaporation is a cooling process, bodies of water lose heat more readily than solid bodies.

D) Elevation above sea level - In general, higher elevations experience lower temperatures due to a) adiabatic cooling (see below for definition) and b) greater distance from average land surface elevation, from which u.v. radiation is emitted (i.e., lessened greenhouse effect). (Fig 16.25)

E) Geographic position - In general, windward coastlines (winds blow from the ocean onto shore; example: northern California) experience more moderate temperatures than leeward coastlines (winds blow from land toward the ocean; example: New England). However, geographic localities on the leeward side of large coastal mountain ranges may experience temperature extremes because the mountains act to block coastal winds (example: Spokane, WA) (Fig 16.24,16.26.16.27)

E) Cloud Cover - Cloud Cover reflects solar radiation back into space (albedo - the reflectivity of a surface), reducing daytime temperatures. At night, clouds absorb u.v. radiation emitted by Earth's surface resulting in higher near-surface temperatures. (Fig 16.28) 

IV. Humidity, Clouds and Fog

Humidity - The amount of water vapor in the air.

Absolute Humidity - The mass of water vapor in a given volume of air.

Relative Humidity - The amount of water vapor in a given volume of air relative divided by the maximum quantity of water that volume of air can hold at the same temperature. (Fig 17.4) Because the maximum quantity of water vapor air can hold is directly proportional to air temperature, relative humidity is inversely proportional to air temperature. (Fig 17.5,17.6)

Dew Point - The temperature at which relative humidity reaches 100%.

Dew - condensation of water on cool solid objects. Example: The night cooling of the solid Earth lowers the temperature of the air directly above the Earth's surface to its dew point, causing water to condense on grass, rocks, soil, etc.

Adiabatic Temperature Change - The change in air temperature as a mass of air expands to a lower density upon rising, or compresses to a higher density upon sinking, without the addition or removal of heat. (Fig 17.9)

Dry Air Adiabatic Rate = approximately 10^oC/km

Wet Air Adiabatic Rate = 5^oC to 10^o C/km

The wet air adiabatic rate is lower than the dry air adiabatic rate due to latent heat released/absorbed as water changes from the vapor to liquid or liquid to vapor state in the atmosphere.

Cloud - Visible concentration of water droplets which form as rising and cooling air masses condense water droplets. Cloud formation typically occurs as a result of :

1. Convection - One portion of the atmosphere becomes warmer that the surrounding air. As the warmer air mass rises, it cools and condenses water droplets. (Fig 17.9)

2. Orographic Uplift - The rise of an air mass as it flows over a mountain range, resulting in cooling of the air mass and condensation of water droplets. (Fig 17.10)

3. Frontal Wedging - The rise of warm, moist air over an advancing mass of cool, dry air. Results in cooling of the warm air mass and condensation of water droplets. (Fig 17.12)

4. Convergence - The rise of warm, moist air caused by a pile-up of horizontal air flow at the surface. (Fig 17.13)

Fog - A cloud that forms at or close to ground level. Formation occurs by one of the following processes:

1. Advection Fog - Forms when warm, moist air from the sea blows onto cooler land. The air cools below its dew point, and water vapor condenses at ground level.

2. Radiation Fog - Earth's surface and air near the surface cool below dew point by radiation at night, resulting in condensation of water as fog.

3. Upslope Fog - Air cools below its dew point as it rises along a land surface.

4. Evaporation Fog - Air is cooled below its dew point by evaporation from a body of water, or cooling along a weather front.

V. Pressure, Wind and Weather Fronts

Wind - Movement of air mass from a region of high barometric pressure to a region of low barometric pressure.

Pressure Gradient - The change in barometric pressure over horizontal distance on the Earth's surface. Wind speed increases in response to an increase in pressure gradient. (Fig 18.7)

Isobar - Line on a map denoting points of equal barometric pressure. (Fig 18.5)

The pressure gradient force acts in a direction at right angles to isobars of decreasing atmospheric pressure. However, the direction of wind is also affected by the Coriolis effect - the apparent deflection in the direction of air currents to the right of their path of motion. Accordingly, in the N. Hemisphere air currents rotate in a clockwise direction. In the S. Hemisphere, air currents rotate in a counter clockwise direction. (Fig 18.7, 18.9)

At high altitudes, the pressure gradient force will eventually balance the Coriolis effect, and winds will blow parallel to isobars. Such winds are called geostrophic winds. At low altitudes, friction between land surface and winds decreases the Coriolis effect so that surface winds blow toward isobars of lower pressure. (Fig 18.9).

Cyclone - A low-pressure region with its accompanying surface winds. In the Northern Hemisphere, winds blow in a counterclockwise rotation and inward. Rising warm air from center cools below its dew point, resulting in clouds formation. (Fig 18.10,18.12)

Anticyclone - A high-pressure region with its accompanying surface winds. In the Northern Hemisphere, winds blow in a clockwise rotation and outward. Descending dry, cool air results in clear skies. (Fig 18.10,18.12)

Front - The boundary between a warm air mass and a cool air mass. Four Types:

1. Warm Front - forms when moving warm air collides with stationary or slower moving cool air. Frontal boundary has a gradual slope. As a result, warm air rises slowly over boundary, typically resulting in cloud formation and scattered showers. (Fig 19.6)

2. Cold Front - Forms when collides with stationary or slower moving warm air. Frontal boundary has a steep slope. As a result, warm air rises rapidly typically resulting in heavy rain and thunderstorms. (Fig 19.7)

3. Occluded Front - Forms when a fast moving mass of cold air traps warm air between a second mass of cold air.  Warm air rapidly rises, causing precipitation along both frontal boundaries. (Fig 19.8)

4. Stationary Front - Forms along the boundary between two stationary air masses. Small amounts of precipitation may form as warm air rises along localized regions of the front.

Mid-Latitude Cyclone - A large region of low pressure which forms at mid-latitudes in North America. Formation begins along a stationary front when when warm air begins to move northward and cold air begins to move southward. Results in counter-clockwise rotation and the formation of a cyclone. In general, Warm air moves toward the NE along a warm front and cold air moves toward the SE along a cold front. If the mass of cold air overtakes the mass of warm air, an occluded front will form close to the center of the cyclone. (Fig 19.9)

VI. Global Winds

Global winds  are generated by heat-driven convection currents, and then their direction is altered by the Earth's rotation (Coriolis Effect). Results in three atmospheric convection cells (Hadley Cells) in each hemisphere. (Fig 18.15)

Equatorial Low - Warm , moist air rises at the equator. Splits to flow north and south at high altitudes. Note that some of Earth's wettest regions are located at latitudes affected by the equatorial low (Amazon basin and Congo basin rain forests).

Subtropical High - Cool, dry  air sinks to the surface at about 30^o north latitude and 30^o south latitude. Note that many of the Earth's major deserts are located at these latitudes (Example: Sahara, Kalahari). The descending air splits into two directions - toward the poles, and toward the equator.

Descending winds blowing toward the equator are deflected by the Coriolis effect, so they blow from the northeast in the Northern Hemisphere and the southeast in the  Southern Hemisphere (Trade Winds). Winds flowing toward the poles are also deflected by the Coriolis effect, flowing from the southwest in the Northern Hemisphere, and the northwest in the Southern Hemisphere (Westerlies).

Cold Polar air sinks to the surface at high latitudes (Polar High).  Sinking winds are deflected by the Coriolis effect to the east (Polar Easterlies). At about 60^o north and south latitude, the polar easterlies converge with the westerlies along the Polar Front. Warm air rises along the convergence (Subpolar Low) .

Jet Streams - Narrow bands of fast moving air at high altitude. Form at the boundaries between atmospheric convection cells.

Subtropical Jet Stream - Flows between the trade winds and the westerlies.

Polar Jet Stream - Flows between the westerlies and the polar easterlies, along the polar front. Storms commonly occur along the polar jet stream due to convergence between warm, moist air of the westerlies and the cold, dry air of the polar easterlies.

Study Questions - The Atmosphere