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Helpful Weather Terms
(Listed in alphabetical order) - A to L

Acid Rain: Cloud or rain droplets containing pollutants, such as oxides of sulfur and nitrogen, to make them acidic (eg. pH <5.6).

Advisory: Advisories are issued for weather situations that cause significant inconveniences but do not meet warning criteria and, if caution is not exercised, could lead to life-threatening situations. Advisories are issued for significant events that are occurring, are imminent, or have a very high probability of occurrence.

Air Masses: Changes in all weather are caused by movements of large bodies of air.

Air Pressure: Any particular point on the Earth depends on the density of air.

Atmosphere: The blanket of air and gases surrounding earth.

Chance: A 30, 40 or 50 percent chance of occurrence of measurable precipitation.

Clear: Sky condition of less than 1/10 cloud coverage.

Clouds: Form when moisture in the air condenses on small particles of dust or solids in the air.

Cold Front: The leading edge of a cold air mass that is displacing the warmer air ahead of it.

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.

Drizzle: Small, slowly falling water droplets, with diameters between .2 and .5 millimeters.

El Nino: A major warming of the equatorial waters in the Pacific Ocean. El Nino events usually occur every 3 to 7 years, and are characterized by shifts in "normal" weather patterns.

Evaporation: Process by which water molecules escape into the air.

Fair: Less than 4/10 opaque cloud cover, no precipitation, and no extremes in temperature, visibility or winds.

Flash Flood: A flood that occurs within a few hours (usually less than six) of heavy or excessive rainfall, dam or levee failure. Flood High flow, overflow or inundation of a normally dry area which causes or threatens damage.

Fog: The visible aggregate of minute water droplets suspended in the atmosphere near the earth's surface. Essentially a cloud whose base is at the earth's surface, limiting visibility.

Front: Where two air masses meet that don't mix.

Greenhouse Effect: The warming of the atmosphere by the trapping of longwave radiation being radiated to space. The gases most responsible for this effect are water vapor and carbon dioxide.

Gust: A brief sudden increase in wind speed. Generally the duration is less than 20 seconds and the fluctuation greater than 10 mph.

Hail: Precipitation in the form of balls or irregular lumps of ice.

High: A region of air with higher barometric pressure and clockwise winds; usually associated with fair weather.

High risk (of severe thunderstorms): Severe weather is expected to affect more than 10 percent of the area. A high risk implies an unusually dangerous situation with the possibility of a major severe weather outbreak.

Hurricane: A severe tropical cyclone with wind speeds in excess of 74 mph (64 knots).

Instability: The tendency for air parcels to accelerate when they are displaced from their original position; especially, the tendency to accelerate upward after being lifted. Instability is a prerequisite for severe weather - the greater the instability, the greater the potential for severe thunderstorms.

Jet Stream: Strong winds in the higher levels of the atmosphere that separate areas of different temperatures, and act as steering current for fronts.

La Nina: A cooling of the equatorial waters in the Pacific Ocean.

Lightning: A discharge of atmospheric electricity accompanied by a vivid flash of light. During thunderstorms, static electricity builds up within the clouds. A positive charge builds in the upper part of the cloud, while a large negative charge builds in the lower portion. When the difference between the positive and negative charges becomes great, the electrical charge jumps from one area to another, creating a lightning bolt.

Low: A region of air with lower barometric pressure and counter-clockwise winds; usually associated with stormy weather.

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