ATMOSPHERE – The mixture of gases surrounding the Earth, held in place by gravity. It forms distinct layers at different heights. The Earth’s atmosphere consists, in ascending order, of the troposphere, the stratosphere, the mesosphere, the thermosphere and the exosphere. The atmosphere is composed primarily of nitrogen (78 percent) and oxygen (21 percent).

CLIMATE – The general or average weather conditions of a certain region, including temperature, rainfall and wind. On Earth, climate is affected most by latitude, the tilt of the Earth’s axis, the movement of the Earth’s wind belts, the difference in temperatures of land and sea, and topography.

CARBON CYCLE – The continuous process by which carbon is exchanged between organisms and the environment. Carbon dioxide is absorbed from the atmosphere by plants and algae and converted to carbohydrates by photosynthesis. Carbon is then passed into the food chain and returned to the atmosphere by the respiration and decay of animals, plants and other organisms. The burning of fossil fuels also releases carbon dioxide into the atmosphere.

CORIOLIS EFFECT – The deflection of objects or substances (such as air) moving along the surface of the Earth, rightward in the Northern Hemisphere and leftward in the Southern Hemisphere, because of the Earth’s rotation.

EL NINO – A warming of the surface water of the eastern and western central Pacific Ocean, occurring every four to 12 years and causing unusual global weather patterns, including heavy rain in western South America and drought in Eastern Australia and Indonesia.

GLACIER – A large mass of ice moving very slowly through a valley or spreading outward from a center under the influence of gravity. Glaciers form over many years from packed snow in areas where snow accumulates faster than it melts. A glacier is always moving, but when its forward edge melts faster than the ice behind advances, the glacier as a whole sinks backward.

GLOBAL WARMING – An increase in the average temperature of the Earth’s atmosphere, especially a sustained increase great enough to cause changes in the global climate. The Earth has experienced numerous episodes of global warming throughout its history, and appears now to be undergoing such warming. Expected long-term effects of current global warming are rising sea levels, flooding, melting of polar ice caps and glaciers, fluctuations in temperature and precipitation, more frequent and stronger El Ninos and La Ninas, drought, heat waves and forest fires.

GREENHOUSE EFFECT – The retention of part of the sun’s energy in the Earth’s atmosphere in the form of heat as a result of greenhouse gases. Solar energy penetrates the Earth’s atmosphere and is absorbed by the Earth’s surface. The heated surface then radiates some of that energy into the atmosphere. Although some of this radiation escapes into space, much of it is absorbed by greenhouse gases in the lower atmosphere, which in turn re-radiate a portion back to the Earth’s surface. The atmosphere thus acts in a manner roughly analogous to the glass in a greenhouse. The greenhouse effect is essential to life on Earth; however, the intensification of its effect because of increased levels of greenhouse gases in the atmosphere is considered a contributing factor to global warming.

GREENHOUSE GASES – Any of the atmospheric gases that contribute to the greenhouse effect. They include carbon dioxide, methane, nitrous oxide and water vapor. Although greenhouse gases occur naturally in the atmosphere, the elevated levels that have been observed in recent decades are directly related, at least in part, to human activities such as the burning of fossil fuels and the deforestation of tropical forests.

GULF STREAM – A warm ocean current of the northern Atlantic Ocean off eastern North America. It flows northward and eastward from the Gulf of Mexico, eventually dividing into several branches. A major branch continues eastward to warm the coast and moderate the climate of northwest Europe.

ICE AGE – 1. Any of several cold periods during which glaciers covered much of the Earth. 2. The most recent glacial period, which occurred during the Pleistocene Epoch and ended about 10,000 years ago. During the Pleistocene Ice Age, great sheets of ice up to 2 miles thick covered most of Greenland, Canada and the northern United States, as well as northern Europe and Russia.

JET STREAM – A narrow current of strong wind circling the Earth from west to east at altitudes of about seven to eight miles above sea level. There are usually four distinct jet streams, two each in the Northern and Southern hemispheres. They are caused by significant differences in the temperatures of adjacent air masses. These differences occur where cold, polar air meets warmer, subtropical air.

LA NINA – A cooling of the surface water of the eastern and central Pacific Ocean, occurring somewhat less frequently than El Nino, but causing similar, generally opposite disruptions to global weather patterns.

MONSOON – A system of winds that influences the climate of a large area and that reverses direction with the seasons. Monsoons are caused primarily by the much greater annual variation of temperature over land than over large areas of adjacent ocean water. This variation causes an excess of atmospheric pressure over the continents in winter, and a deficit in the summer. The disparity causes strong winds to blow between the ocean and the land, bringing heavy seasonal rainfall.

PRECIPITATION – A form of water, such as rain, snow or sleet, that condenses from the atmosphere, becomes too heavy to remain suspended and falls to the Earth’s surface. Different atmospheric conditions are responsible for the different forms of precipitation.

* THERMOHALINE CURRENT – A current driven by density differences caused by variation in temperature and-or salinity and responsible for deep ocean circulation

WEATHER – The state of the atmosphere at a particular time and place. Weather is described in terms of variable conditions such as temperature, humidity, wind velocity, precipitation and barometric pressure.

WIND – A current of air, that moves parallel to the ground, from an area of high pressure to an area of low pressure. The large-scale patterns of winds on Earth is governed primarily by differences in the net solar radiation received at the Earth’s surface, but it is also influenced by the Earth’s rotation, by the distribution of continents and oceans, by ocean currents and by topography. Prevailing global winds are classified into three major belts in the Northern Hemisphere and corresponding belts in the Southern Hemisphere. The trade winds blow generally east to west toward a low-pressure zone at the equator throughout the region from 30 degrees north to 30 degrees south of the equator. The westerlies blow from west to east in the temperate mid-latitude regions (from 30 degrees to 60 degrees north and south of the equator), and the polar easterlies blow from east to west out of high-pressure areas in the polar regions.

Sources: American Heritage Science Dictionary, 2005

* New Penguin Dictionary of Geology, 2001