I've Seen Fire and I've Seen Rain

We've all marveled at the strength of a thunderstorm on a warm summer night, as lightning illuminates the sky and thunderclaps shake the earth. Incredibly, just one lightning bolt exceeds the power output of a major city, and a fully developed thunderstorm releases more energy than a nuclear explosion.

Three elements are necessary for the development of a thunderstorm. The first requirement is moisture. Because warm air can hold more water vapor than cold air, the warmer the air, the more likely the development of a thunderstorm. Second, a thunderstorm needs an unstable atmosphere. The stability of the atmosphere can be measured by how fast the temperature drops as altitude increases. This decrease is called the lapse rate. The warmer the air at the surface of the Earth, the faster the temperature drops, and the greater the lapse rate. The final ingredient for a thunderstorm is an uplifting atmospheric force. This force can be created by mountains, by the blunt edge of an approaching cold front, or by solar heating of the lower levels of the atmosphere.

Imagine a warm humid afternoon with cold dry air aloft. The sun warms the Earth and through conduction the air at the Earth's surface is heated. A bubble of warm air breaks free and begins to rise like a hot-air balloon. As this warm bubble rises, its temperature drops as it expands into the reduced atmospheric pressure at higher altitudes. Because the temperature of the air is colder at higher altitudes, the growing bubble of air continues to be warmer than its surroundings, and thus continues to rise. Eventually the temperature of the rising air drops to the dew point, where water droplets condense. This condensation releases heat energy that feeds the process. The water droplets are supported by the updrafts of the rising air column.

The first stage of the thunderstorm is the cumulus stage, indicated by puffy white clouds. As the process continues, the water droplets bump into each other, coalescing into larger drops, eventually becoming so heavy that they fall, or precipitate. As the rain falls into warmer air, some of the drops evaporate. Evaporation, which cools the air, is the opposite of condensation. The cool air then begins to descend, adding downdrafts that mix with the updrafts already present. The mature stage is reached when the rain reaches the Earth's surface.

As the drops of water are carried to altitudes where the temperature is below freezing, the droplets clump together to form frozen hailstones. Strong thunderstorms can sustain very large hailstones that have cycled up and down through the thunderstorm cell several times, growing larger and larger. The rubbing together of ice particles in the cell creates a static electrical charge. When this charge reaches a significant level, the discharge to other clouds or to the ground is observed as lightning. The rapid expansion of air, heated by lightning many times hotter than the surface of the sun, creates a pressure wave heard as thunder.

Eventually the storm's updrafts reach the tropopause, which starts 5 to 11 miles above the planet, where the temperature stops dropping. At this altitude the cloud spreads out in all directions. If upper-level winds are present, the top of the cell will be pushed into the classic anvil shape, with the anvil pointing in the direction of thunderstorm movement. The downdrafts eventually prevail and the storm reaches the final dissipating stage.

Thunderstorms are dangerous and are therefore treated very cautiously by pilots. Passengers on commercial airliners rarely notice when aircraft bypass storms encountered along the way to a destination. A storm near an airport, however, is much more noticeably inconvenient because it often must be waited out, sometimes causing major delays in aircraft arrivals and departures. Air travelers need to be patient in these situations and to keep in mind that delays occur because of the respect that aviation professionals have for thunderstorms, and, more importantly, for passenger safety.

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