Darbas:

67,000 feet and some severe thunderstorms attain an even greater height. More often the maximum height is from 40,000 to 45,000 feet. In general, air-mass thunderstorms extend to greater heights than do frontal storms. Rising and descending drafts of air are, in effect, the structural bases of the 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 from the ocean over the land area...lift is provided by approaching cooler, drier, more dense air (a cold front)
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. Bases of the clouds are very low. Tornadoes sometimes occur when this type of activity is present.

The warm-front thunderstorm is caused when warm, moist, unstable air is forced aloft over a colder, denser shelf of retreating air. Warm-front thunderstorms are generally scattered; they are usually difficult to identify because other clouds obscure them

Dry lines are the boundary between two air masses of different moisture content and separate warm moist air from hot dry air. While the temperature may be different across the dry line, the main difference is the rapid decrease in moisture behind the dry line.
It is the lack of moisture, which allows the temperatures to occasionally be higher than ahead of the dry line. However, the result is the same as the warm moist air is lifted along the dry line forming thunderstorms. This is common over the plains in the spring and early summer.
Outflow boundaries are a result of the rush of cold air as a thunderstorm moves overhead. The rain-cooled air acts as a "mini cold front", called an outflow boundary. Like fronts, this boundary lifts warm moist air and can cause new thunderstorms to form.
Terrain
As air encounters a mountain it is forced up the slope of the terrain. Upslope thunderstorms are common in the Rocky Mountain west during the summer.

2.3 LIFE CYCLE OF THE CLOUD

The building block of all thunderstorms is the thunderstorm cell. The thunderstorm cell has a distinct life cycle that lasts about 30 minutes.

2.3.1 Single cell.
The life cycle of a single cell can be separated into three stages:
· Developing stage
· Mature stage
· Dissipating stage



2.3.1.1 DEVELOPING STAGE:
Every thunderstorm begins life as a cumulus cloud. The developing stage lasts 5 to 10 minutes. A cumulus cloud begins to grow vertically, perhaps to a height of 40,000 to 60,000 feet. The cloud starts growing upward, driven by the latent heat as water vapor condenses. Strong updrafts prevail throughout the cell and it rapidly builds up into a towering cumulonimbus cloud. The diameter of the cell is between 2 and 8 km. Temperatures within the cell are higher than temperatures at the same level in the surrounding air, intensifying still more the convective currents within the cell. There is usually no precipitation from the storm at this stage of its development since the water