Darbas:
thunderstorm cell. A draft is a large-scale vertical current of air that is continuous over many thousands of feet of altitude. Downdraft speeds are either relatively constant or gradually varying from one altitude to the next. Gusts, on the other hand, are smaller scaled discontinuities associated with the draft proper. A draft maybe compared to a great river flowing at a fairly constant rate, whereas a gust is comparable to an eddy or any other random motion of water within the main current.2.2 THE NECESSARY INGREDIENTS FOR THUNDERSTORMS
Every thunderstorm needs three ingredients:
· Moisture - to form clouds and rain
· Instability - relatively warm air that can rise rapidly
· A lifting mechanism- fronts, sea breezes, and mountains are capable of lifting air to form thunderstorms.
· Sources of moisture
Typical sources of moisture are large bodies of water such as the Atlantic and Pacific oceans as well as the Gulf of Mexico. Winds bring moisture
Instability
An unstable air mass is characterized by warm moist air near the surface and cold dry air aloft. In these situations, if a bubble or parcel of air is forced upward it will continue to rise on its own. As it raises it cools and some of the water vapor will condense, forming the familiar tall cumulonimbus cloud that is the thunderstorm. Characteristics of an unstable air mass with warm moist air near the surface with colder and drier air aloft. Air that is forced upward will continue to rise, and air that is forced downward will continue to sink.
Sources of Lift (upward)
Typically, for a thunderstorm to develop, there needs to be a mechanism, which initiates the upward motion, something that will give the air a nudge upward. This is done by several methods.
Differential Heating
This heating of the ground and lower atmosphere is not uniform. For example, a grassy field will heat at a slower rate than a paved street. The warmest air, called thermals, tends to rise.
Fronts, Dry lines and Outflow Boundaries
Fronts are the boundary between two air masses of different temperatures. Fronts lift warm moist air. Cold fronts lift air the most abruptly. The cold-front thunderstorm is caused by the forward motion of a wedge of cold air into a body of warm, moist unstable air. Cold-front storms are normally positioned aloft along the frontal surface in what appears to be a continuous line. Under special atmospheric conditions, a line of thunderstorms develops ahead of a cold front. This line of thunderstorms is the prefrontal squall line. Its distance ahead of the front ranges from 50 to 300 miles. Prefrontal thunderstorms are usually intense and appear menacing.
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