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Up and Down
Air containing sufficient moisture must rise and cool for rain, snow or thunderstorms to form. The air flow can be lifted by solar heating of the earth's surface, flowing over a mountain, low pressure zones and cold fronts. Wind meeting at the ground must go upward, but air currents converging aloft may go up or down. A rising air mass aloft can be associated with "upper level lows" and divergence aloft, and may create clouds and rain. A sinking air mass aloft is related to upper highs, which produce convergence aloft and divergence at the ground, suppressing clouds and rain. Upper highs often bring heat waves in the summertime, fair weather in the fall and spring, and cold weather in the winter.
Fronts and Lows
A cold front is a boundary between two differing air masses at the Earth's surface. Fronts extend upward one to three miles depending on intensity. A front is a region of convergence where two or more air currents flow into one area. A typical example of three currents coming together is with a low-pressure center, a cold front and a warm front. The cold front extends outward from the low-pressure area. Cold air is on the northern and western sides of the low with warmer, more humid air to the south and east of the low. A third air mass, to the northeast of the low, generally possesses qualities in between the cold and warm air. These three air currents meet as they spiral in around the low pressure area. A large area of clouds and rain normally accompanies a mature low pressure area as described here.
Dry Line
The dry line is a common feature over the High Plains during the spring and summer months. It separates dry and hot air to the west from a more humid air mass to the east. The wind blows from the west on the dry side of the dry line and often from the south on the humid side. A cold front generally extends from a low-pressure area just north of the dry line. The meeting of the cold front and dry line is called the triple point, because three different air flows are converging. Dry lines are often associated with severe thunderstorms and tornadoes due to very strong lifting of the air mass along with low-level wind shear.
Thunderstorm Enhancement
A Supercell thunderstorm may form more readily when fast winds several miles above-0ground interact with rising air inside a thunderstorm and lower-level wind shear. A National Weather Service case study of a 1998 central Florida tornado outbreak showed a strong correlation between a vigorous jet and Supercell formation. The rising air, the spiraling air and the jet stream air all work to lower the storm pressure and cause convergence, which further lowers the pressure. If the change in wind speed or direction is great enough as the air rises, the storm rotates, producing damaging winds, hail or tornadoes.
Hurricanes
A hurricane is a giant heat engine converting heat energy released from condensing water droplets into kinetic energy (wind). Air spirals inward counter-clockwise in the lower parts of the storm, upward through the eyewall to about 30,000 feet, where it spins outward in a clockwise fashion. The three air currents of inward, upward and outward all work together to strengthen the storm and maintain it. An outflow of clockwise-flowing air aloft is essential to "vent" the air rising from below and keep the pressure dropping inside the eye. Inflow of air spinning counter-clockwise in the lower three mils of the hurricane is essential to maintain low pressure in the center. The upward-moving air around the eye serves as a transport link between the top and bottom of the storm.