What makes the storm a nor’easter? What is an atmospheric river?

A winter storm delivering heavy, wet snow — a true nor’easter — hit parts of the northeastern United States this week. Meanwhile, the latest in a series of atmospheric rivers has brought more moisture to California, a state where crippling drought has given way to a series of damaging floods and rains.

Some commonly used weather terms and their definitions, which are based on material from the National Weather Service:

atmospheric river — Long, broad plumes of moisture that form over an ocean and flow across the sky above land.

blizzard — Wind speeds of 35 mph or more and considerable snowfall and/or blowing with visibility of less than a quarter mile for three or more hours.

cyclone — A storm with strong winds that revolves around a moving center of low atmospheric pressure. The word is sometimes used in the United States for tornadoes and in the Indian Ocean area for hurricane.

derecho – A widespread and usually fast-moving straight-line windstorm. It is usually over hundreds of miles long and over 100 miles wide.

El Nino, La Nina – El Nino is a natural climatic phenomenon that begins with unusually warm waters in the central and eastern Equatorial Pacific and then changes the climate around the world. The flip side of El Nino is La Nina, which is an occasional but natural cooling of the equatorial Pacific that also changes the climate around the world.

hurricane or typhoon — A warm-core tropical cyclone in which the minimum sustained surface wind is 74 mph or greater. Hurricanes spawn east of the international date line. Typhoons develop west of the line. They are known as cyclones in the Indian Ocean and Australia.

microburst – Occurs when a mass of cooled air rushes down from a thunderstorm, hits the ground, and rushes outward in all directions.

polar vortex – Usually refers to the giant circular upper air weather pattern in the Arctic region, which engulfs the North Pole (but can also apply to the South Pole). It’s a regular pattern that is strongest in the winter and holds some of the coldest weather bottled up near the North Pole. The jet stream usually encloses the polar vortex and keeps it north. But sometimes part of the vortex can break off or move south, bringing unusually cold weather south and allowing warmer weather to creep north.

blizzard — An intense but short-lived period of moderate to heavy snow, with high winds and possible lightning.

storm — An abnormal rise in water above the normal tide caused by a storm.

tornado – A violently rotating column of air that forms a pendant, usually from a cumulonimbus cloud, and touches the ground. On a local scale, it is the most destructive of all atmospheric phenomena. Tornadoes can appear from any direction, but in the United States most move from the southwest to the northeast. Measured on an F-scale from EF0 to EF5, which considers 28 different types of damage to structures and trees. An EF2 or greater is considered a significant tornado.

tornado warning – National Weather Service issues to warn the public of existing tornado.

Tornado Watch: Alerts the public to the possibility of tornadoes forming.

tropical depression – A tropical cyclone in which the maximum sustained surface wind is 38 mph (33 knots) or less.

tropical storm – A warm-core tropical cyclone in which maximum sustained surface winds range from 39 to 73 mph (34 to 63 knots).

tsunami — A large sea wave or seismic sea wave caused by an undersea disturbance such as an earthquake, landslide, or volcano.

nor’easter — The term used by the National Weather Service for storms that exit or move northward along the east coast, producing northeasterly blowing winds.

waterspout — A tornado over water.

wind chill factor — A calculation describing the combined effect of wind and cold on exposed skin.

wind shear — A sudden shift in wind direction and/or speed.


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