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    Home / Science lessons / Storms
    • Storms

      We've all experienced thunderstorms at one time for another. Although some parts of the U.S. average 10 or less a year, a few areas have 100 or more to make up for it! Meteorologists, scientists who study the weather, think that Kampala, Uganda (in East Africa), probably holds the world record for annual thunderstorms: at least 242 occurred in just one year! Although we already know what a thunderstorm is like, here's how it actually works.

      The first thing to note is that warm air rises. Thunderstorms form either when a cold front pushes a warm mass of air up, or when a warm surface temperature (from a hot summer day, for example) heats the air above it. This causes an updraft and the warm air, which holds moisture, forms a cloud as it reaches cooler temperatures. This is called the cumulus stage. The cloud continues to grow, reaching a mature stage. Cold air enters the cloud from the sides and causes a downdraft, although the updraft continues. Ice crystals form at the top and start falling downward through the cloud. Precipitation starts, with rain or even hail, and thunder and lightning.  Usually after 30 minutes, the final or dissipating stage begins, with the cold downdrafts pulling the storm apart. There is lighter precipitation as the storm breaks up, usually ending after just an hour.

      Lightningoccurs because of electrical charges, usually the attraction of positive and negative charges between the thundercloud and the ground. However, lightning can occur within a cloud or even run from the ground up to the cloud. Thunderis the result of the lightning super-heating the molecules in the air, causing shock wave vibrations that are the sound waves we hear. Even though the sound waves begin with the lightning, it takes longer for sound to travel than for light and so we hear a clap of thunder after we've seen the lightning bolt.

      Tornadoes are closely associated with thunderstorms. A tornado forms within a thundercloud, where winds whirling in opposite directions cause a long funnel cloud to drop out of the main cloud. If the funnel cloud touches the ground, then it's classified as a tornado. The violent updraft in the middle of the tornado can cause severe damage, since in the biggest tornadoes (as much as 2000 feet across), it is strong enough to rip up houses. Thankfully, almost 70% of tornadoes are weak ones that do little harm.

      Hurricanes form near the equator, in the intertropical convergence zone (ITCZ)where contrasting winds meet. A combination of heat and moisture is necessary for the storm to get started: as the winds swirl around they cause moisture to condense in the air. As this happens, heat increases and gradually a cyclone forms over the water. With hurricanes, there is an endless supply of water while they are over the ocean, so they can grow continuously. From the first stage, called a tropical depression, the storm can develop into a tropical storm, and then, when winds reach 74 mph or more, it becomes a hurricane. Hurricanes, in spite of their devastating nature, are very orderly storms. They have a calm spot, the eye, in the center (usually about 15 miles wide), surrounded by the strongest winds and dark nimbostratus storm clouds just outside it and then spiral rain bands of precipitation and clouds moving outward from that. In the Northern Hemisphere (the top half of the world), hurricane winds move counter-clockwise, but in the Southern Hemisphere they move in the opposite direction.

      As a hurricane approaches land, barometric pressure drops, causing the sea level to rise: in the vacuum brought about by the lowered pressure, seawater rushes up onto the land, causing flooding along with the destruction from the storm. The lowest barometric pressures ever recorded have occurred within hurricanes.

      The Saffir-Simpson scale is used to measure the force of hurricanes: 1 is the least destructive (with winds from 74-95 mph) and 5 is the most destructive (wind speeds over 155 mph).

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