~~Storms are created when a center of low pressure develops with a system of high pressure surrounding it. This combination of opposing forces can create winds and result in the formation of storm clouds, such as the cumulonimbus. Small localized areas of low pressure can form from hot air rising off hot ground; resulting in smaller disturbances such as dust devils and whirlwinds. On a hot summer day the surface of the Earth is heated by the sun. The Earth's surface heats the air just above the surface through the process of conduction.
~The action of warm air rising and cold air sinking (convection) plays a key role in the formation of severe thunderstorms. If the warm surface air is forced to rise, it will continue to rise, because it is less dense than the surrounding air. In addition, it will transfer heat from the land surface to upper levels of the atmosphere through the process of convection.
~`Two of the most important ingredients for thunderstorm formation are instability (unstable air) and moisture.
Storms are caused by the sun. Since the earth isn't perfectly uniform, the sun's radiation heats the earth differently in different places. If a parcel of air is warmer than its surroundings, it will rise. More air then has to move in to replace the rising air, creating horizontal winds. Storms in the middle latitudes generally form in those areas where there are large temperature gradients to enhance the air circulation. (Tropical storms are also caused by solar heating, but their energy source is the heat content of the ocean rather than a temperature gradient.
~Tropical Storm Formation
Tropical storms generally form in the eastern portion of tropical oceans and track westward. Hurricanes, typhoons, and willy-willies all start out as weak low pressure areas that form over warm tropical waters (e.g., surface water temperature of at least 80 oF). Initially, winds and cloud formations over the warm tropical waters are minimal. Both intensify with time. Formation of tropical storms also requires a significant Coriolis effect to induce proper spin in the wind formation. As the storm begins to organize itself into a coherent pattern, it will experience increased activity and intensity.
When a storm develops a clearly recognizable pattern, it is referred to as a tropical depression. When wind speeds reach 35 knots (40.3 mph), it is called a tropical storm and is given a name. When wind speed equals or exceeds 74 mph, the storm is called a hurricane. In the western Pacific, a hurricane is referred to as a typhoon. In waters around Australia it is called a cyclone or willy-willy.
Hurricanes intensify when moving over areas of increased water temperatures, and weaken over colder water surfaces. Upper atmosphere wind shear (different wind direction and speeds at different elevations) will frequently prevent or slow intensification of tropical storms by "spreading out" the storm horizontally and preventing the formation of strong updrafts of warm, humid air. Movement over a land-mass will weaken hurricane winds but will result in large-scale rain that can result in large-scale flooding. When encountering a strong frontal system (such as a polar front) the hurricane will curve and track along the leading edge of the front or become implanted in it.
Satellite infrared imagery can identify surface water temperatures that will foster tropical storm development.
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