General Overview

��Thunderstorms can lead to many other deadly types of weather, and are relatively common in the United States. Like I previously mentioned, thunderstorms can create tornadoes, hail, flash floods, heavy winds, and (of course) lightning. Thunderstorms occur in the warmer states with unstable weather (the Great Plains, the Rocky Mountains, the South) usually during the summer months. Actually, ²¤₃ of the nations' thunderstorms occur between the months of June and September, with most of them occuring in July.

��In one instance, during April 3^rd and 4^th, 1974, thunderstorms in the Great Plains created the "Super Outbreak" of tornadoes -- the largest, most damaging outbreak in history. This outbreak included 127 tornadoes, six of which had winds that were greater than 261 miles per hour, otherwise known as category 5 tornadoes. All of the tornadoes were caused by cold air spreading over the Rocky Mountains combining with warm humid air moving north from the Gulf of México. Some forecasts also showed that the jet stream was pushing extremely dry air towards the east and the warm humid air northward.

��Thunderstorms are also "convective storms," using convection currents as their main energy source. In order for these currents to happen, there needs to be unstable air, warm near the ground and cold above it. First of all, in order for a thunderstorm to occur, warm and humid air needs to rise from the ground, which are called updrafts.

��When the water drops become large enough to weigh down the updraft, they start to fall to earth, carrying the air with them. In summer, even if there are ice crystals, they melt on the way down. The water droplets rocketing toward the earth and pulling the air with it causes downdrafts. The updrafts continue supplying the warm and humid air into the thunderstorm.

��At this point in the storm, with both the updrafts and downdrafts occuring, this is the most destructive stage of a thunderstorm. Downdrafts eventually grow too strong and overpower the updrafts. Now that the major supply of warm air is cut off, the thunderstorm becomes wear and the rain begins to stop. A few other interesting points are that thunderstorms begin with an updraft of warm air, called a cell. Thunderstorms that have one cell are rare; usually they have more than one cell. Also, at the beginning of a storm, people usually feel a gentle wind blowing toward the storm cloud, which is the warm updraft.

Weather Instruments

��There are many instruments used to measure those five things. First of all, you need an anemometer to measure wind speed. It is a metal pole with three or four cups mounted on a vertical axis and it rotated at a rater based directly on wind sspeed. The wind direction is measured by a weather vane, which is a metal pointer that swings with the wind and is attached to a compass (in order to tell the compass direction).

��Air pressure is measured by a barometer, a flexible metal vacuum box that expands and contracts based on pressure. If you would like to measure continuous pressure, a barograph is needed. A barograph is a barometer that moves a pen across a rotating drum, going up and down on the paper based on the pressure.

��Temperature is obviously measured by a thermometer, which is a hollow glass tube filled with a liquid (usually mercury), and the liquid expands (rises) and contracts (falls) with change in temperature. Continuous temperature is measured by a thermograph, a heat-sensitive metal coil that moves against a rotating drum, similar to a barograph.

��If humidity is what you want, then a psychrometer is in order. A psychrometer is composed of two thermometers, one with a dry bulb and one with a wet bulb. The humidity is directly related to the difference in temperature between the two bulbs. Hopefully, with all of these instruments, it is possible to find out about conditions in a thunderstorm, and possibly predict them.

��There is a French electronic system used for recording thunderstorm data called Météorage. It "measures the impact of every one of the 800,000 or so lightning bolts that strike France each year." People who use the system know within seconds where and when it struck, the amplitude, polarity, and number of return strokes of the bolt. It helps out the power company Electricité de France and other businesses such as EuroDisney and insurance companies.

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