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Thunderstorms, also known as electrical storms, or lightning storms, are a form of weather characterized by the presence of lightning and its effect: thunder. Supercell ThunderstormThis image shows a supercell thunderstorm with a rotating updraft indicated by the striations. Supercells can produce hail, rain, strong winds, and substantial downbursts.  The source of this material is Windows to the Universe, at http://www.windows.ucar.edu/ at the University Corporation for Atmospheric Research (UCAR). © 2010 The Regents of the University of Michigan. Windows to the Universe® is a registered trademark of UCAR. All Rights Reserved. In order to understand storms, especially thunderstorms, we must present a little elementary meteorology, a term invented by Aristotle about 350 years BCE, to mean study of the atmosphere. The study begins with the air , made of molecules of Nitrogen, about 78%, Oxygen about 21% and traces of other gasses especially a variable amount of water vapor, about 1%. Air pressure is the weight of the atmosphere above a unit area. This is about 14.7 pounds per square inch. The pressure can be measured by a barometer, invented by Evangelista Torricelli in 1643. Torricelli filled a four-foot long glass tube with mercury and inverted the tube into a dish. Some of the mercury did not escape from the tube and Torricelli observed the vacuum that was created. He observed that the height of the mercury column changed with time and he credited the change to the changing atmospheric pressure. The pressure is given as inches of mercury. At sea level the air pressure is about 29.9 inches of mercury; this amounts to about 1013 millibars (Mb). There are 1000 Mb in one bar which is the pressure of a million dynes per square centimeter. The "standard" atmospheric pressure at 15 degrees C and latitude 45 degrees is defined as 1013.2 Mb. An air mass is a large body of air with uniform temperature and humidity throughout. A front is a zone between two air masses of different density and pressure. A high pressure mass is heavier than a low pressure mass. If two masses collide, a cold front hitting a warm front, the cold and heavier mass will tend to dive below the warmer and lighter mass. Wind flows from high pressure to low pressure along the pressure gradient, which is the change of pressure across a given distance. But there is a complication called the Coriolis effect which simply arises because the earth rotates below the air masses; this tends to change the wind direction to the east in the Northern Hemisphere. Clouds are formed from warm water vapor and can reach great heights. The temperature drops rapidly as the cloud rises and when the temperature drops below zero centigrade (32 Fahrenheit) the water vapor freezes to produce sleet, snow and freezing rain. Fair weather clouds are called cumulus; they have a flat base, look like cotton candy and last up to 40 minutes. When conditions are just right the cumulus, fueled by strong updrafts, can morph into very destructive cumulonimbus and can reach heights of 39,000 feet or even higher. Thunderstorms develop here and produce lightning, high wind and flash floods The top of the cumulonimbus often develops into a black anvil from which tornadoes are spawned. The setting sun captures the top of a classic anvil-shaped thunderstorm cloud in east central Nebraska. Picture from Wikipedia  KnowYour Winter Storm and Extreme Cold Terms. Table contents Credit:FEMA Familiarize yourself with these terms to help identify a winter storm hazard:
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