Weather Glossary 
Weather Glossary
Absolute zero: The coldest possible temperature, at which a substance has no heat energy.
Acid rain: Cloud or rain droplets containing pollutants, such as oxides of sulfur and nitrogen.
Amplitude: The height of a wave crest (or depth of a trough) measured from a point of equilibrium.
Aftershocks: Small temblors, caused when rocks continue shifting after an earthquake, that may last for two years after an earthquake. They usually decrease over time.
Air mass: A large body of air having similar horizontal temperature and moisture characteristics.
Anticyclone: A large area of high pressure around which the winds blow clockwise in the Northern Hemisphere.
Aphelion: A planet's farthest point from the sun during orbit (applies to comets, too).
Aurora borealis: Sheets of vibrant light visible at northern latitudes, caused by charged solar particles igniting gases in Earth's atmosphere.
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Barometer: Measures atmospheric pressure.
Big Bang Theory: States that the universe began by exploding from a cluster some 15 to 20 billion years ago, supported by the fact that objects are still moving away from the center. The event is often called a singularity.
Black hole: A region of space, believed to be the remnant of a collapsed star, where light cannot escape because the gravity is so strong.
Blizzard: A condition over a long period of time when snow is falling, winds blow faster than 35 mph, and visibility is one-quarter mile or less.
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Caldera: On some volcanoes, the magma chamber collapses after a violent eruption and a caldera forms, which is just a large, bowl-shaped crater.
Cenozoic era: Extends back 65 million years and includes the rise of Mammals.
Climate: Average weather conditions over long periods of time, decade or more. Note that a climatologists studies long-term patterns, whereas a meteorologist is more focused on short-term forecasting.
Clouds: The air around us always contains water, but it’s usually in the form of vapor. When air is warmed on a sunny day or by the interaction of colliding air masses, it becomes lighter and begins to rise. As it rises, the air cools. Colder air has less ability to hold water vapor, so it condenses into water droplets on small particles, such as dust. The water droplets join together to form clouds.
Cold front: The boundary between a cold air mass that is advancing and a relatively warmer air mass. Generally characterized by steady precipitation followed by showery precipitation.
Conjunction: The instant when two celestial bodies are closest to each other, a condition affected by their orbits.
Continental drift: A hypothesis that one large super continent, named Pangaea, broke apart to form today's seven continents. The idea, first proposed in 1858, led to today's theory of plate tectonics.
Convection: Transfer of energy by moving currents of fluid or gas.
Coriolis Effect: The apparent curving of wind or ocean currents caused by the rotation of the Earth under an air mass (or body of water). Imagine a spinning disk (like a CD or LP). If you scratched a line from the center to the edge while the disk was spinning, the resulting line would be curved. In case you were wondering, it is a myth that the Coriolis Effect impacts your bathtub drain; it is not strong enough to do that. Do a few tests and, unless your drain is possessed, you'll get roughly equal numbers of clockwise and counterclockwise rotations.
Corona: The Sun's outer atmosphere..
Cumulonimbus: A vertically developed cloud, often capped by an anvil shaped cloud. Also called a thunderstorm cloud, it is frequently accompanied by heavy showers, lightning, thunder, and sometimes hail or gusty winds.
Cumulus: A cloud in the shape of individual detached domes, with a flat base and a bulging upper portion resembling cauliflower.
Cyclone: An area of low pressure around which winds blow counterclockwise in the Northern Hemisphere. Also the term used for a hurricane in the Indian Ocean and in the Western Pacific Ocean. In the Southern Hemisphere, cyclones rotate clockwise.
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Dew: Moisture that has condensed on objects near the ground, whose temperatures have fallen below the dew point temperature. Explore more.
Dew point: The temperature to which the air must be cooled for water vapor to condense.
Doppler effect: A change in wave frequency (light, sound or other) caused by the compression or elongation of waves as the source moves toward or away from the listener or viewer. You've heard the change when an fire engine goes by: The compressed sound rises in pitch, then as the siren moves away, the pitch lowers.
Downburst: A severe localized downdraft from a thunderstorm.
Drizzle: Small, slowly falling water droplets, with diameters between .2 and .5 millimeters.
Dust devil: A small, rapidly rotating wind that is made visible by the dust, dirt or debris it picks up. Also called a whirlwind, it develops best on clear, dry, hot afternoons.
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Eccentric orbit: Elliptical or noncircular.
Elongation: Point of greatest separation of the inner planets from the Sun, as viewed from Earth.
Epicenter: Point on the surface directly above the focus of an earthquake.
Eye: The relatively calm center of the tropical cyclone (including hurricanes) that is more than one half surrounded by wall cloud. Characterized by calm winds and often clear conditions.
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Fault: A fracture in rock that separates it into two sections that have moved relative to each other.
Fireball: A large, splattering meteor that creates a flash brighter than Venus.
Focus: The place under the surface where an earthquake's energy originates.
Freezing rain: Super cooled water drops that freeze when they hit the ground or other surface.
Frost: The covering of ice that is formed on exposed surfaces whose temperature falls below freezing.
Funnel cloud: A rotating, cone-shaped column of air extending down from the base of a thunderstorm. If it reaches the ground it becomes a tornado.
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Galaxy: A conglomeration of dust, gas and stars, all held together by gravity. There may be as few as a million stars, or there may be several billion.
Gale: Wind speeds from 39 to 54 mph (34 to 47 knots).
Greenhouse effect: The natural warming of the atmosphere by the trapping of heat radiated to space after the Sun warms the Earth.
Gulf Stream: A warm ocean current flowing from the Gulf of Mexico and across the Atlantic to Europe.
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Hail: Precipitation in the form of balls or irregular lumps of ice. Tall cumulonimbus clouds are much warmer at the bottom than at the top. This causes tremendous pressure differences and strong rising air currents, which suck warm water droplets from the bottom of the clouds to the top. There, they freeze. If the currents are strong enough, a hailstone will fall and rise many times, causing several layers of ice to build up until the hailstone is heavy enough to fall from the cloud
Halos: Rings or arcs that seem to encircle the Sun or Moon. They are caused by the refraction of light through the ice crystals in cirrus clouds.
Haze: Fine dry or wet dust or salt particles in the air that reduce visibility.
Heat index: Combines air temperature and humidity to give an apparent temperature (how hot it feels).
High wind: Sustained winds greater than or equal to 40 mph or gust greater than or equal to 58 mph.
Humidity: The amount of water vapor in the atmosphere.
Hurricane: A warm-core tropical cyclone with one-minute sustained winds of 74 mph or more. The term hurricane is used for Northern Hemisphere cyclones east of the International Dateline to the Greenwich Meridian. The term typhoon is used for Pacific cyclones north of the Equator west of the International Dateline.
Hurricane season: The portion of the year having a relatively incidence of hurricanes. The hurricane season in the Atlantic, Caribbean, and Gulf of Mexico runs from June 1 to November 30. In the Eastern Pacific, May 15 to November 30. In the Central Pacific basin, June 1 to November 30.
Hurricane warning: A warning that sustained winds of 74 mph or higher associated with a hurricane are expected within 24 hours. A hurricane warning can remain in effect when dangerously high water or a combination of dangerously high water and exceptionally high waves continue, even though winds may be less than hurricane force.
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Indian summer: An unseasonably warm period near the middle of autumn, usually following a substantial period of cool weather.
Inversion: Cool air near the ground trapped by warm air aloft.
Isobar: A line of equal barometric pressure on a weather map.
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Jet stream: High-speed winds that race around the globe at between about four and six miles above the Earth, mostly from west to east.
Jovian Planet: Four largest planets, all gaseous: Jupiter, Saturn, Uranus, and Neptune.
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Knot: One nautical mile per hour (=1.15 mph).
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La Niña: A cooling of the equatorial waters in the Pacific Ocean caused when eastward-moving atmospheric and oceanic currents bring cold water to the surface. Explore El Niño & La Niña.
Land breeze: A wind that blows from the land and toward the sea.
Latent heat: When water evaporates, it stores energy, which is released as heat upon condensation. This latent heat adds feeds the rising columns of air near the center of hurricanes, thereby transferring energy from the ocean into the storm.
Lava: Magma that has risen to the surface.
Light, speed of: 186,291 miles per second
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Light year: Distance light travels in a vacuum in one year, about 5.88 trillion miles.
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Magma: Liquid, melted rock inside the Earth that can contain crystals and dissolved gases. Upon reaching the surface, it is called lava.
Mass: The amount of matter in an object (unrelated to size).
Mesocyclone: A rotating column of air in a thunderstorm that sometimes spawns tornadoes.
The Mesozoic era: Runs from 65 to 245 million years ago, and saw the rise of dinosaurs and the first modern birds.
Microburst: A strong localized downburst from a thunderstorm.
Micron: Short for "micrometer," a micron is one-millionth of a meter. A human hair is about 50 microns thick.
Milky Way galaxy: Like all galaxies, our own Milky Way contains billions of stars. Most of the points of light in the night sky are stars in the Milky Way, which is about 100,000 light-years across and about 10,000 light-years thick. The universe contains billions of other galaxies.
Millibar: A unit of atmospheric pressure. Normal surface pressure is approximately 1013 millibars (29.92 inches of mercury).
Monsoon: Persistent seasonal winds which, during summer, bring copious amounts of moisture from the ocean and dump it on land.
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Neap tide: Small tides that occur at the Moon's first and third quarters.
Nebula: Cloud of dust or gas in space which is sometimes made visible by the reflected light from nearby stars. Nebulae are the birthplace of stars.
Nor'easter: Strictly defined as any reasonably strong wind blowing from the northeast for an extended period of time. Nor’easters usually do most of their damage at the coast, in the form of beach erosion and flooding. The northeast winds come from a low-pressure system offshore.
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Occluded front: A complex frontal system that occurs when a cold front
overtakes a warm front. Also known as an occlusion.
Offshore breeze: A wind that blows from the land towards a body of water. Also known as a land breeze.
Onshore breeze: A wind that blows from a body of water towards the land. Also known as a sea breeze.
Opposition: The point in a planet's orbit when it is opposite the Sun in relation to the Earth.
Ozone: A form of oxygen containing 3 atoms, occurring naturally in the stratosphere, and responsible for filtering out much of the Sun's ultraviolet radiation. Another type of ozone, smog, is caused by the burning of fossil fuels and other human activities. Smog resides in the lower troposphere.
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Paleozoic era: Runs from 245 to 570 million years ago.
Perihelion: The point in a planet's orbit when it is closest to the Sun.
Plate tectonics: A theory that a dozen or so large, relatively rigid plates slide about on the Earth's surface; interactions at the plate borders cause the formation and changes in oceans, continents, mountains, volcanoes, and earthquakes.
Precambrian era: Prior to 570 million years ago, included very little life.
Precipitation: Liquid or solid water molecules that fall from the atmosphere and reach the ground.
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Quasar: Distant, small, bright objects at the core of a young galaxy.
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Radar: An instrument used to detect precipitation by measuring the strength of the electromagnetic signal reflected back.
Radiation: Any stream of energized particles that travels in the form of electromagnetic waves or photons.
Radiosonde: An instrument attached to a weather balloon that transmits pressure, humidity, temperature and winds as it ascends.
Rain: Liquid water droplets that fall from the atmosphere, having diameters greater than drizzle.
Relative humidity: The amount of water vapor in the air, compared to the amount the air could hold if it was totally saturated. (Expressed as a percentage).
Ridge: An elongated area of high pressure in the atmosphere.
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Santa Ana winds: Relatively warm, dry winds that blow into Southern California coastal areas from an anticyclone located over the high deserts of California or Nevada. The warmth and dryness are due to compressional heating.
Sea breeze: Wind that blows from the sea and toward the land.
Seismic waves: The disturbances that transmit an earthquake's energy to the surface.
Sleet: Frozen precipitation, consisting of small transparent pellets.
Smog: Pollution formed by the interaction of pollutants and sunlight (photochemical smog), usually restricting visibility, and occasionally hazardous to health.
Snow: Most precipitation, except in the tropics, starts high in the clouds where water vapor condenses into ice crystals. As the crystals fall, they freeze together with other crystals. If the air below the clouds remains below freezing, snow will fall. If not, the snow turns to rain.
Sound, speed of: 1,088 feet per second at sea level, in dry air that is 32 degrees F. The speed increases with temperature. Through water, sound speed up to 4,820 feet per second.
Squall line: A non-frontal band, or line, of thunderstorms.
Stationary front: A transition zone between air masses, with neither advancing upon the other.
Supernova: A massive and bright star that has lost most of its mass in a tremendous explosion.
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Thunder: The sound caused by a lightning stroke as it heats the air and causes it to rapidly expand.
Trade winds: Persistent tropical winds that blow from the subtropical high pressure centers towards the equatorial low.
Tropical cyclone: A warm-core, nonfrontal low pressure system that develops over tropical or subtropical waters and has a definite organized surface circulation.
Trough: An elongated area of low pressure at the surface or aloft.
Tule fog: Radiation fog in the central valley of California that forms during night and morning hours in late fall and winter months following the first significant rainfall. A leading cause of weather related casualties in California.
Turbulence: Disrupted flow in the atmosphere that produces gusts and eddies.
Typhoon: A warm-core tropical cyclone with one-minute sustained winds of 74 mph or more that forms in the Northern Hemisphere west of the International Dateline (the Western Pacific). The term hurricane is used for Northern Hemisphere cyclones east of the International Dateline to the Greenwich Meridian.
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Ultraviolet (UV): Electromagnetic energy with a wavelength just shorter than violet light, so it is not visible.
Umbra: The complete portion of a shadow in an eclipse.
Universe: Everything in space; all the planets, stars and galaxies, and everything between. Scientists believe the universe is expanding, though it is not known whether this will continue. Studies in 1998 made age estimates of 15 billion years, though other estimates vary from this number. Recent NASA research puts the figure at 12 billion years, though this has been disputed.
Upwelling: The rising of cold water from the deeper areas of the ocean to the surface.
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Virga: Precipitation falling from the base of a cloud and evaporating before it reaches the ground.
Volt: Unit that measures the difference in electrical potential between to points needed to move a current of 1 ampere through a resistance of 1 ohm.
Vorticity: A measure of the amount of "spin" (or rotation) in the atmosphere (especially in a hurricane).
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Warm front: A boundary between a warm air mass that is replacing a cooler air mass.
Waterspout: A tornado over water.
Wind shear: The change of wind speed or direction with distance, usually vertical.
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Zenith: Point in an orbit or celestial sphere that is directly above an observer.