Absorb: To take in; to transform radiant energy into a different form, resulting in a rise in temperature.

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1 FOSS Weather and Water Course Glossary (10/5/04) Absorb: To take in; to transform radiant energy into a different form, resulting in a rise in temperature. Air mass: A large body of air that has uniform temperature and humidity. Air masses have distinct boundaries and can extend hundreds or thousands of kilometers over Earth s surface. Air: The mixture of gases surrounding Earth and forming its atmosphere. Air pressure: The force exerted on a surface by the weight of the air above it; also called atmospheric pressure. Altimeter: A barometer used to measure how high an airplane is. Anemometer: An instrument used to measure wind speed. Aneroid barometer: An instrument used to measure air pressure; operates with a sealed bellows-like chamber containing a spring. Aphelion: The point in an orbit where a planet is farthest from the Sun. Atmosphere: The layer of gases that surrounds a planet or star. Atom: The smallest particle of an element that has the properties of the element and can exist either alone or in combination with a similar atom as a molecule. Axis: An imaginary line around which Earth spins or rotates. Barometer: An instrument used to measure air pressure. Bimetallic strip: A thin piece of two different metals stuck together. The two metals expand and contract at different rates as temperature changes, which makes them useful in some thermometers, such as an oven thermometer. Blizzard: A severe storm with low temperatures, strong winds, and large quantities of snow. Carbon dioxide: A variable gas composed of carbon and oxygen; plays an important role in photosynthesis. Climate: A pattern of weather conditions over a period of many years or centuries. FOSS Weather and Water Glossary 1

2 Climatology: The study of weather conditions over long periods from years to millions of years. Cloud: A visible accumulation of water on condensation nuclei, either as liquid or ice, usually high in the air. Compass: An instrument used to determine direction. Condensation: The process in which water vapor becomes liquid water. Condensation nucleus: A small particle on which water vapor condenses. Conduction: The transfer of energy from one place to another by contact. Contact: A meeting or touching of surfaces or materials. Contract: To take up less space. Convection: Movement of a fluid, such as air, that transfers heat from one place to another. Convection cell: Cycle caused by convection when cool, denser material sinks and replaces warmer, less dense material. Coriolis effect: The apparent deflection of winds due to Earth s rotation. Cumuliform: A puffy, sometimes fast-moving and rapidly growing type of cloud. Cumulus: A puffy cloud; means heap in Latin. Density: The amount of matter in a given volume of a material; mass per volume. Dew: Water that condenses on surfaces when the temperature drops below the dew point. Dew point: The temperature at which a volume of air is saturated with water vapor; condensation occurs when the temperature drops below the dew point. Downburst: A severe localized downdraft from a thunderstorm, causing damaging winds at or near Earth s surface. Drought: Less than normal precipitation over a long period of time. Dust devil: Small rotating wind not associated with a thunderstorm, visible when it collects dust and debris. FOSS Weather and Water Glossary 2

3 Dust storm: A condition in which strong winds carry dust over a large area, usually during drought conditions. Energy: The ability to do work. Equinox: When the Sun appears overhead at the equator and day and night is equal length everywhere. Equinox happens twice a year on about March 21 and September 23. Evaporation: The process in which liquid water becomes water vapor. Exosphere: The outermost layer of the atmosphere, where the atmosphere makes the transition to space. Expand: To take up more space. Eye: The center of low atmospheric pressure in a hurricane. Flash flood: A short, rapid unexpected flow of water, often caused by intense, heavy rainfall. Flood: The overflowing of a water body, such as a river, over its banks onto normally dry land. Fluid: A substance that flows or takes the shape of its container. Front: The leading edge of a moving air mass. Frost: Frozen dew. Frost forms when the dew point is below 0ºC. Glacier: A large, slowly moving sheet of ice. Global wind: Wind that affects large sections of Earth, such as trade winds. Greenhouse effect: The heating of the atmosphere by trapped solar radiation. Groundwater: The water in soil and porous rocks underground. Hail: Precipitation in the form of balls of ice. Heat: A form of kinetic energy that raises the temperature of a substance. Humidity: The water vapor in the air. Hurricane: A cyclone or moving wind system that rotates around an eye. FOSS Weather and Water Glossary 3

4 Hygrometer: An instrument used to measure relative humidity. Icecap: A sheet of snow and ice that permanently covers a large area of land. Ionosphere: A layer of the atmosphere within the thermosphere; noted for its large number of electrically charged ions. The aurora borealis and australis, or northern and southern lights, form in the ionosphere. Kinetic energy: Energy of motion. Land breeze: A local wind that blows from the land to the sea. Latitude: The distance north or south from the equator, measured in degrees. Lightning: A visible electric discharge produced by thunderstorms. Liquid crystal: A fluid material that changes color within a very narrow range of temperature; used in some types of thermometers. Local wind: A wind that changes with the season and time of day, usually affecting a particular region. Mass: The amount of matter in an object. Matter: The stuff that makes up the universe. Matter occupies space and has mass. Mesosphere: The layer of the atmosphere above the stratosphere, in which temperature decreases and meteors burn up when approaching Earth. Meteorologist: A scientist who studies the causes and effects of Earth s weather. Meteorology: The study of day-to-day weather conditions in the troposphere and lower stratosphere. Methane: A colorless, odorless, variable gas composed of carbon and hydrogen. Microburst: A severe localized wind blasting down from a thunderstorm, covering an area less than 4 kilometers in diameter and usually lasting less than 5 minutes. Millibar: A unit of measurement for air pressure. FOSS Weather and Water Glossary 4

5 Molecule: The smallest particle of a substance having all the characteristics of the substance. Nitrogen: The most abundant gas in Earth s atmosphere (N 2 ). Nitrogen is stable, meaning it doesn t react easily with other substances. Oxygen: A colorless, odorless, tasteless gas; the second most abundant gas in Earth s atmosphere (O 2 ). Ozone: A form of oxygen containing three atoms of oxygen (O 3 ). Ozone can exist either high in the atmosphere, where it shields Earth against harmful ultraviolet rays from the Sun, or close to the ground, where it is the main component of smog. Perihelion: The point in an orbit where a planet is closest to the Sun. Permanent gas: A gas whose amount in the atmosphere stays constant; for example, oxygen and nitrogen. Photosynthesis: A process used by plants to convert water, carbon dioxide, and sunlight into carbohydrates and oxygen. Prevailing wind direction: The direction from which the wind blows most often. Psychrometer: An instrument used to measure relative humidity. Radiant energy: Energy transmitted in the form of electromagnetic waves; for example, heat waves, light waves, radio waves, X rays. Radiation: Energy that is radiated or transmitted through space in the form of rays, waves, or particles. Radiosonde: A weather instrument package containing sensors for measuring temperature, relative humidity, and air pressure. Ray: A thin beam or line of radiant energy, such as light. Relative humidity: The amount of water vapor in the air compared to the amount of water vapor needed to saturate the air at a given temperature; expressed as a percentage. Reradiate: To radiate again. Revolution: The motion of one celestial body (such as Earth) around another (such as the Sun). Rotation: The spinning of a celestial body, such as Earth, about its axis. FOSS Weather and Water Glossary 5

6 Saturated: Holding as much water vapor as possible. Sea breeze: A local wind that blows from the sea to the land. Season: A period of the year marked by the equinoxes and solstices or by atmospheric conditions. Solar energy: The radiant energy of the Sun, which can be converted into other forms of energy, such as heat or electricity. Solstice: When the sun is at its greatest distance from the celestial equator; the point in the apparent path of the Sun at which the Sun is farthest north or south of the equator. Solstice happens twice a year on about June 21 and December 21. Step leader: Steps through which the electric charge of lightning travels downward. Straight-line wind: A strong wind with no rotation. Stratiform: A flat, layered type of cloud. Stratosphere: The layer of the atmosphere above the troposphere; contains a layer of ozone, but almost no moisture or dust. Stratus: A type of flat, layered cloud; means layer in Latin. Temperature: A measure of the average kinetic energy of the molecules in a material. Thermometer: An instrument used to measure temperature. Thermosphere: The thin layer of the atmosphere above the mesosphere, in which temperature may reach more than 1500 C. Thunder: The explosive sound that usually accompanies lightning. Thunderstorm: A storm that produces rapidly rising air currents and usually results in heavy rain or hail with thunder and lightning. Tilt: Slant. Tornado: A rapidly rotating column of air that extends from a thunderstorm to the ground. Wind speeds can reach 417 kilometers per hour or more in a tornado. Transfer: To move from one place to another. FOSS Weather and Water Glossary 6

7 Troposphere: The layer of the atmosphere next to Earth s surface, in which we live and in which weather takes place. Typhoon: A Pacific cyclone north of the equator and west of the international dateline. Variable gas: A gas whose amount changes in response to environmental activities; for example, water vapor and carbon dioxide. Volume: The space occupied by a material. Water cycle: The circuit of water movement from the oceans to the atmosphere to Earth s surface and back to the atmosphere through various processes, such as precipitation, runoff, infiltration, storage, evaporation, and transportation. Waterspout: A small tornado or whirlwind that forms a funnel cloud over a body of water. Water vapor: The invisible, gaseous form of water. Weather: The state of the atmosphere at a given time, including temperature, humidity, cloud cover, precipitation type, and presence of aerosols. Weather balloon: A balloon made of a thin membrane of rubber used to carry a radiosonde into the stratosphere. Weather factors: Properties of air that affect weather, including temperature, humidity, pressure, and wind. Wind: Movement of air. Windstorm: Strong winds not directly connected with thunderstorms, tornadoes, or hurricanes. FOSS Weather and Water Glossary 7