Essay

Introduction 51 psc lap.docx

Introduction:

A thunderstorm is a storm that contains both thunder and lightning and usually accompanied by gusty surface winds and heavy rain. The storm may result from single cumulonimbus cloud or a cluster of clouds extending for more than 100 kilometers.　 Thunderstorms are born when warm, humid air rises in a conditionally unstable atmosphere.　 Thunderstorms develop when converging surface winds are coupled to diverging upper-level winds.

STAGES IN THE DEVELOPMENT OF THUNDERSTORM:

There are three stages in the life-cycle of a thunderstorm. They are the cumulus stage, mature stage, and dissipating (decay) stage.

1. Cumulus Stage:

This is the development stage. The warm and moist air rises in a series of convective updrafts. As this occurs the air begins to condense forming a cumulus cloud. These interactions between the rising and cooling air causes the development of a positive feedback mechanism. The warm air within the cloud continues to rise and eventually condenses. The condensation releases heat into the cloud and causes warming of the air. This, in turn, causes it to rise adiabatically. The cloud edges during this stage are sharp and distinct, showing that the cloud consists primarily of water droplets. The process continues and works to form a towering cumulus cloud. The convective cloud continues to grow upward, eventually growing above the freezing level where supercoiled water droplets and ice crystals coexist. Precipitation begins to form via the Bergeron process once the air rises above the freezing level. Falling precipitation and cool air entrainment from the environment start the initiation of cool downdrafts leading to the second stage.

2. Mature Stage:

The mature stage is the height of the storm's strength when it is most intense . During this stage there is both an updraft and downdraft which constitutes a cell, precipitation, and possible severe weather. The downdrafts are initiated by the downward drag of falling precipitation. The downdraft is strengthened by evaporative cooling, as the rain falling with the downdraft enters drier air below the cloud base and evaporates. This cold descending air in the downdraft will often reach the ground before the precipitation. As the mature-stage thunderstorm develops, the cumulus cloud continues to increase in size, height and width. Cloud to ground lightning usually begins when the precipitation first falls from the cloud base.

During this phase of the life cycle, the top of the resulting cumulonimbus cloud starts to flatten out and forms an anvil shape at the top of the troposphere Within 15 to 30 minutes of entering the mature stage, the storm begins to dissipate.

3. Decaying (Dissipating) Stage:

This is the weakening stage and during this stage damaging weather is still possible.

The decaying stage is dominated by downdrafts throughout the entire cloud. Decay stage begins when the supercoiled cloud droplets start freezing and the cloud becomes glaciated, indicating that it contains ice crystals. Glaciation first appears in the anvil, which becomes more pronounced in this stage. The glaciated cloud appears filmy, or diffuse, with indistinct cloud edges. The cloud begins to collapse since no additional latent heat is being released after the cloud droplets freeze, and because the shadow of the cloud and rain cooled downdrafts reduce the temperature below the cloud. The decaying of a thunderstorm can also be initiated when the precipitation within the storm becomes too heavy for the updrafts to support, when the source of moisture is cut off, or when the lifting ceases.