Thunderstorm
From Pilots Almanac
Contents |
Introduction
It is estimated that there are as many as 40,000 thunderstorm occurrences each day world-wide. This translates into an astounding 14.6 million occurrences annually. The United States certainly experiences its share of thunderstorm occurrences.
The frequency of occurrence is greatest in the southeastern states, with Florida having the highest incidence (80 to 100+ thunderstorm days per year). It is in this part of the country that warm, moist air from the Gulf of Mexico and Atlantic Ocean (a necessary ingredient for thunderstorm development) is most readily available to fuel thunderstorm growth.
Development
All thunderstorms require three ingredients for their formation:
- Moisture,
- Instability, and
- a Lifting Mechanism.
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. The southeastern U.S. and especially Florida has access to two moisture sources in the Atlantic ocean and the Gulf of Mexico which helps explain why there are so many thunderstorms in that region.
Instability
Air is considered unstable if it continues to rise when given a nudge upward (or continues to sink if given a nudge downward). 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 rises it cools and some of the water vapor will condense, forming the familiar tall cumulonimbus cloud that is the thunderstorm.
Air that is forced upward will continue to rise, and air that is forced downward will continue to sink.
Sources of Lift
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, Drylines 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. If the air is moist and unstable thunderstorms will form along the cold front.
Drylines 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 dryline, the main difference is the rapid decrease in moisture behind the dryline. It is the lack of moisture which allows the temperatures to occasionally be higher than ahead of the dryline. However, the result is the same as the warm moist air is lifted along the dryline 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.
Life Cycle
The building block of all thunderstorms is the thunderstorm cell. The thunderstorm cell has a distinct life-cycle that lasts about 30 minutes.
Towering cumulus stage
The first stage is the towering cumulus stage. A cumulus cloud begins to grow vertically, perhaps to a height of 20,000 feet. Air within the cloud is dominated by updraft with some turbulent eddies around the edges.
Mature stage
The second stage is the mature stage. The storm has considerable depth, often reaching 40,000 to 60,000 feet. Strong updrafts and downdrafts coexist. This is the most dangerous stage when large hail, damaging winds, and flash flooding may occur.
Dissipating stage
The final stage is the dissipating stage. The downdraft cuts off the updraft. The storm no longer has a supply of warm moist air to maintain itself and therefore it dissipates. Light rain and weak outflow winds may remain for a while during this stage, before leaving behind just a remnant anvil top.
