The atmosphere is the envelope of air surrounding the earth and consists mainly of nitrogen (78 percent) and oxygen (21 percent).
The hydrosphere includes all the water of the earth's surface.
The troposphere is the lowest and the densest layer of the atmosphere.
Temperature decreases with height in the troposphere.
All weather occurs in the troposphere.
The stratosphere is the layer of the atmosphere above the troposphere.
Ozone (O3) in the stratosphere absorbs harmful solar ultraviolet radiation.
Certain pollutants, especially the gaseous chlorofluorocarbons (CFCs), cause the breakdown of O3 molecules, thus allowing additional ultraviolet radiation to reach the earth's surface.
The portion of the atmosphere above the stratosphere extending from 50 to 80 km above the earth is called the mesosphere. The mesosphere is characterized by decreasing temperature with altitude, changing from about 10°C at 50 km to about -75°C at 80 km.
Above the mesosphere, extending from 80 km to about 600 km, is the thermosphere.
Temperature increases within the thermosphere to about 2000°C, although the density of the thermosphere is so low that there is very little total heat energy present.
An ionized region within the thermosphere called the ionosphere reflects radio waves back to earth and makes possible long-distance radio communication.
13-2. Atmospheric Moisture
Water vapor consists of water molecules that have escaped, or evaporated, from a body of water at a temperature below the boiling point of water.
Humidity refers to the water vapor content of air.
Air is said to be saturated when it holds the maximum amount of water vapor at a given temperature.
13-3. Clouds
When air that contains water vapor is cooled past its saturation point, some of the vapor condenses to form dew, fog, or clouds.
Rising, warm, moist air expands and cools as atmospheric pressure decreases. This causes water vapor to condense into clouds consisting of ice crystals or water droplets, depending on temperature.
Condensation nuclei, such as airborne salt and dust particles, must be present before condensation can occur.
Three processes in the atmosphere can cause cloud formation:
A warm air mass moving horizontally meets a land barrier such as a mountain and rises.
An air mass is heated by contact with a warm part of the earth's surface and rises by convection.
A warm air mass meets a cooler air mass and, being less dense, is forced upward over the cooler mass.
Rapid cooling of a cloud leads to rapid condensation and the precipitation of rain or snow, depending on temperature.
Sleet consists of raindrops that freeze as they fall; hail forms as violent up- and downdrafts associated with thunderstorms cause frozen raindrops to accumulate layers of ice as they alternately rise and fall.
Clouds are sometimes seeded with condensation nuclei of silver iodide crystals or "dry ice" (solid CO2) to induce precipitation.
13-4. Atmospheric Energy
Meteorology is the study of weather.
Weather refers to the temperature, humidity, air pressure, cloudiness, and precipitation at any given time at a given place.
Climate is a summary of weather conditions over a period of years over a large area, including how temperatures and rainfall vary with the seasons.
Solar energy arriving at the upper atmosphere is called insolation and is mainly in the form of visible light.
Insolation reaching the earth's surface is absorbed and becomes heat. This heat energy is radiated back into the atmosphere as long-wavelength infrared radiation.
This long-wavelength infrared radiation is readily absorbed by atmospheric CO2 and water vapor, thus preventing its rapid escape into space. This heating of the atmosphere is called the greenhouse effect.
Although temperatures vary around the earth, the earth's average temperature changes very little with time. There is a balance between incoming radiation and outgoing energy.
Winds and the ocean currents carry energy from tropical regions to the cooler polar regions.
13-5. The Seasons
The earth is nearest the sun in January and is farthest from the sun in July.
The earth's seasons are caused by the tilt of its axis together with its annual orbit around the sun.
Because of the tilt of the earth's axis, in half of the year one hemisphere receives more direct sunlight than the other hemisphere, and in the other half of the year it receives less.
On about June 22 the noon sun is at its highest in the sky in the northern hemisphere, and the period of daylight is longest.
The solstices occur on about December 22 the noon sun is at its lowest in the sky, and the period of daylight is shortest.
The equinoxes occur on about March 21 and on about September 23 the noon sun is directly overhead at the equator and the periods of daylight and darkness are equal.
13-6. Winds
Winds are horizontal movements of air that take place in response to the pressure differences in the atmosphere. The greater the pressure difference, the faster the wind.
Uneven heating of the earth's surface creates convection currents in which rising warm air is cooled and then moves downward toward the earth.
The coriolis effect is the deflection of moving things such as air currents due to the earth's rotation. The deflection is to the right in the northern hemisphere and to the left in the southern hemisphere.
Air rushing into a low-pressure region follows a counterclockwise spiral path in the northern hemisphere and a clockwise spiral path in the southern hemisphere.
13-7. General Circulation of the Atmosphere
The north and the south winds coming from the polar regions are deflected by the coriolis effect into large-scale eddies that lead to a generally eastward drift in the middle of each hemisphere and a westward drift in the tropics.
The steady easterly winds on each side of the equator are called the trade winds.
The region of light, erratic wind along the equator, where the principal movement of air is upward, constitutes the doldrums.
The largely calm belts that separate the trade winds in both hemispheres from the prevailing westerlies poleward of them are called the horse latitudes.
The jet streams are narrow, meandering, high-speed winds in the upper troposphere.
13-8. Middle-Latitude Weather Systems
The movement of warm and cold air masses through the belts of the westerlies determines the weather in North America.
An air mass is a large body of air that is relatively uniform in temperature and moisture content.
A cyclone is a weather system centered on a low-pressure region.
Cyclonic winds blow in a counterclockwise spiral in the northern hemisphere and in a clockwise spiral in the southern hemisphere.
Cyclones usually bring unstable weather conditions with clouds, rain, strong winds, and abrupt temperature changes.
An anticyclone is a weather system centered on a high-pressure region.
Anticyclonic winds blow in a clockwise spiral in the northern hemisphere and in a counterclockwise spiral in the southern hemisphere.
The weather associated with anticyclones is usually settled with clear skies and little wind.
Middle-latitude cyclones originate at the polar front, the boundary between the cold polar air mass and the warm maritime tropical air mass.
A warm front forms when a relatively warm air mass moves to a region occupied by colder air. A prolonged period of cloudiness and precipitation often follows a warm front.
A cold front forms when a cold air mass moves into a region occupied by warmer air. Rainfall, if it occurs, is heavier and of shorter duration than along a warm front.
An occluded front forms when a cold front overtakes a warm front and lifts the warm air off the ground.
13-9. Tropical Climates
Weather in the doldrums is hot with almost daily rains and light, changeable winds.
Weather in the horse latitudes is dry with light winds or calms.
Weather in the trade wind belts is for the most part dry, although seasonal shifts of the trade wind belts give rainfall during part of the year along their equatorial margins.
13-10. Middle-Latitude Climates
The belts of prevailing westerlies generally have moderate average temperatures, although continental interiors show pronounced seasonal variations.
In the United States, mountain ranges along the West Coast wring out moisture from the prevailing westerlies so that their western slopes receive abundant rainfall. Their eastern slopes and the continental interior are dry. Rainfall increases eastward across the country.
Polar weather is characterized by short summers and long cold winters.
13-11. Climatic Change
Evidence indicates that climates are subject to long-term change.
The ice ages were periods of extreme climatic change, the last one reaching its peak about 21,000 years ago.
The "Little Ice Age" was a period of cool summers and very cold winters that took place during the first half of the seventeenth century.
The late nineteenth century experienced a trend toward a warmer worldwide climate that peaked about 1940. Recently, global temperatures have again started upward.
Global warming has caused a rise in sea level, mostly due to the thermal expansion of sea water.
Continued melting of the earth’s ice sheets and glaciers contributes to rising sea level and other environmental changes.
13-12. Origins of Climatic Change
Fluctuations in the sun's energy output may account for some aspects of climatic change.
The Milankovitch theory of climatic change relates large-scale climatic changes to periodic changes that occur in the tilt of the earth's axis, the shape of earth's orbit, and the time of year when earth is closest to the sun.
Milankovitch hypothesized that it is the amount of insolation reaching the polar regions in summer, and not minor differences in total global insolation, that induce large-scale climatic changes such as the ice ages.
Observations that periods of ice sheet advance and retreat correspond to various periods of the earth's orbital variation, along with current theoretical models of the earth's climate, support Milankovitch's hypothesis.
The increased content of CO2 and other gases such as CFCs in the atmosphere have enhanced the greenhouse effect and could lead to global climate modification.
13-13. Ocean Basins
A continental shelf is a gently sloping surface extending seaward from a continental margin to a line marked by a sudden increase in slope.
A continental slope is a submerged, steeply sloping surface extending from the seaward edge of the continental shelf to the ocean floor.
The continental rise is the ocean floor beyond the base of the continental slope having a less pronounced slope.
The abyssal plain is the nearly level area of the ocean floor extending from the seaward margin of the continental rise.
The ocean basins have an average depth of 3.7 km.
The deepest point in the oceans occurs within the Marianas Trench and is 11 km below sea level.
The ocean floor consists of plains, valleys, isolated volcanic peaks, and mountain ranges such as the Mid-Atlantic Ridge.
The Antarctic ice cap contains about 90 percent of the world's permanent ice.
The world's oceans probably appeared about 4 billion years ago and have always been salty.
13-14. Ocean Currents
Oceans affect climate in two ways:
Oceans act as heat reservoirs that moderate the seasonal temperature changes of adjacent land areas.
Wind-driven ocean currents, by retaining the temperatures of their latitudes of origin, influence the temperatures of neighboring land areas.
Wind-driven surface currents parallel to a large extent the earth's major wind systems and aid the winds in their distribution of heat and cold over the surface of the earth.
Major ocean currents include the equatorial current, the Gulf Stream, the Labrador Current, and the Japan Current.
