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Jet Stream: Relatively strong winds which are concentrated in a narrow band in the atmosphere. Jet Streams are usually 1000s of miles long and 100's of miles wide but are only a few 1000 feet thick.
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La-Nina: A large scale weather pattern which occurs when the ocean waters over the eastern pacific cool below normal readings. This pattern usually causes the subtropical jet stream to weaken or migrate north over the northern US and Canada. This weather pattern brings dry and warm conditions to most of the United States.
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Lightning: Originates within a thunderstorm when ice crystals collide with graupel or snow, which generates an electric charge via friction. The charge differences between the different regions in a cloud, or the cloud to ground, can generate a lightning bolt to complete the connection.
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Mesocyclone: A cyclonic (counterclockwise) or anti-cyclonic (clockwise) rotating column of air found within a thunderstorm updraft. This rotating column of air may be located at either the mid-levels or low-levels, or at both the mid and low-levels simultaneously within a thunderstorm.
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Meteorology: Study of the state and processes of the atmosphere
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Occluded front: A complex front which forms when a cold front over takes a warm front.
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Phased Array RADAR: Operators can control how, when, and where the radar scans to sample a particular storm, rather than rotate a full 360 degrees like a typical WSR-88D where storms may not be present in the other directions.
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Precipitation: The accumulated depth of rain or drizzle, and also melted water containing snow and other frozen precipitation including hail.
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Psychrometer: An instrument for measuring the moisture content of the air by use of a dry- and wet-bulb thermometer.
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RADAR: Acronym for Radio Detecting and Ranging. National Weather Service Weather Surveillance RADAR 1988-Doppler (WSR-88D) networks operate at S-band (10-cm wavelength).
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Relative Humidity: The ratio of water vapor contained in air compared to the maximum amount of moisture that air can hold at a given temperature and pressure.
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Ridge: An elongated area of high pressure.
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Rime: A deposit of ice from when supercooled water droplets freeze on contact with an object.
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Squall line: A solid line of thunderstorms which form ahead or along a surface boundary, such as a front, dryline, or outflow boundary.
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Storm-Relative Wind: The storm-relative (SR) winds are the winds a storm actually experiences given its motion relative to the background/environmental wind profile. The SR wind is also important in determining potential supercell types: LP, classic, or HP. Typically, weak SR winds (<=30 knots) in the anvil-level of the supercell indicate an HP supercell mode, while 30-50 knot anvil SR winds indicate a classic mode, and strong SR winds (>=50 knots) in the anvil-level indicate an LP supercell mode.
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Stratosphere: The second lowest of the main layers of the atmosphere; it is characterized by more or less isothermal conditions (temperature is constant with height) and a highly stable stratification.
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Stratus: A continuous horizontal layer of cloud.
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Sublimation: The process in which ice changes directly to water vapor without melting.
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Supercell: A thunderstorm which exhibits mid-level rotation (i.e. a mid-level mesocyclone) and a steady-state updraft. These thunderstorms are usually severe, producing any of the following: large hail (>=1.0" diameter), severe winds (>=58mph), and tornadoes. There are three types of supercells: low-precipitation (LP), classic, and high-precipitation (HP).
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Thunder: A sound wave produced when lightning super heats a volume of air. The super heated air rapidly expands producing a sound wave which travels through the atmosphere at the speed of sound (approximately every 4.6 seconds is one mile for the sound to travel, meaning if it takes 10 seconds to hear thunder, the lightning occurred roughly 2 miles away).
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Thunderstorm: Cumulonimbus cloud from which lightning occurs.
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Tornado: A violently rotating column of air which makes contact from the ground to cloud base. Tornadoes are usually spawned by the low-level mesocyclone of a supercell thunderstorm.
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Trough: An elongated area of low pressure.
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Updraft: The area of a thunderstorm where the maximum vertical motion is occurring.
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Upper Low: A low pressure center located in the mid and upper levels of the atmosphere which moves from west to east within the jet stream. These disturbances are also called upper level troughs or short-wave troughs.
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Virga: Precipitation that falls from clouds but evaporates in dry air beneath the cloud before reaching the ground.
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Vertical Wind Shear: The turning of wind direction and/or the increase of wind speed with height. A clockwise turning (veering) with height along with an increase of wind speed with height is a key ingredient for the development of a supercell given a high degree of atmospheric instability.
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Wall Cloud: A lowered cloud base under a thunderstorm. Wall clouds that rotate around a vertical axis are called low-level mesocyclones and may be precursors to funnel clouds and potential tornadoes.
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Wind: The movement of air (both direction and speed) which is caused mainly by horizontal pressure differences. Wind direction is measured from which it's blowing.