THUNDERSTORMS Brett Ewing October, 2003

Transcription

1 THUNDERSTORMS Brett Ewing October, 2003

2 A natural hazard that occurs often on a daily basis in the lower and mid-latitudes is thunderstorms. Thunderstorms is a weather system that can produce lightning,tornadoes, high wind and heavy precipitation. This is a natural hazard that is taken lightly because they are common but they have the potential to be very destructive. Thunderstorms are formed by unstable or rising air. Lifting- Thermal lifting/convection- Thermal convection occurs in the summertime when you get warm surfaces on the earth. These places are localized with a series of warm rising air bubbles. As the air rises, it expands and the expanding air then cools with height until it cools down to its dew-point. This is the lifting condensation level which is the bottom of the cloud. The height where the rising parcel is cooled to the temperature of the surrounding air then that is the top of the cloud. This is called the equilibrium level. Orographic lifting- This is when air is forced up because of a barrier and the air has no where to go but up. Usually the only significant barrier is a mountain range. When the air rises it cools forming clouds with the unstable conditions. Frontal lifting and convergence- This lifting is from the convergence of two different fronts. This forces air to rise which causes unstable conditions. The warm front has light

3 rain and a long duration while a cold front with heavy rain and a short duration. There are two types of thunderstorms. Air mass thunderstorms are self-extinguishing and severe thunderstorms which are self-propagating. Air mass thunderstorms- These types of thunderstorm are least destructive than severe storms. They have a short life span which lasts less than one hour. Air mass gets its name because they occupy only a small portion of an air mass rather than the entire air mass itself. This thunderstorm has three stages in its life cycle until it extinguishes itself. Cumulus stage- In the cumulus stage there is no precipitation occurring from cloud to ground. The cloud receives water vapor from the surface up. As the cloud thickens it grows upward. It can grow upward at a rate of 5-20meters per second. Mature stage- The mature stage begins when the precipitation starts to fall. This marks the strongest part of this thunderstorm system. The precipitation, lightning and thunder is the most intense during this stage. Dissipative stage- This stage is when this thunderstorm system begins to cut off its own supply of energy and begins to kill itself. As heavy

4 precipitation has been coming down with it brings heavy downdrafts. These downdrafts become so intense that they cut off any additional water vapor trying to go up into the cloud itself. This eventually kills the cloud. -Severe thunderstorms- Severe thunderstorms are defined by having wind speeds exceeding 58 miles per hour, having hailstones larger than 1.9 centimeters and spawn tornadoes. A severs thunderstorm can cover several counties and can last up to 12 hours in duration. Severe storms account for 60 percent of the annual precipitation. Severe thunderstorms can also form squall lines which is a number of individual storms forming a linear band. These usually form 300 miles out in front of cold fronts. Supercell storms are also in the severe category. These storms can reach diameters from 12 to 30 miles. These are smaller than squall lines but are more localized and more intense. They can last 2-4 hours long. Its updraft and downdraft bend around each other forming large rotation in the clouds. This rotation forms very large tornadoes. -Frequency and seasonality- Thunderstorms can happen almost daily if the conditions are unstable and you get rising air. The low-latitudes receive thunderstorms daily because of the warm temperature. The late

5 afternoon is prime for most thunderstorms because the earth has been heated all day and would give off its most heat. These unstable conditions fall into thermal convection. Thunderstorms is a hazard itself but they can also spawn other natural hazards themselves. -Tornadoes- Tornadoes are spawn out of severe thunderstorms which have intense upward and downward drafts which form large rotation -Flood- Flooding can occur during the mature stage of an air mass storm or in a severe storms when the rain is at its most intense time -Lightning- Lightning is formed from the initial separation of positive and negative charges in different areas of the cloud. Cloud to ground lightning strikes can cause fires and even kill people. -Wind damage- During a severe thunderstorm the winds can exceed 58 miles per hour which as strong as tropical storm winds. Thunderstorms can be formed any where on the earth. The most common places though are the low to mid-latitudes where you have a lot of unstable air. When the conditions are in place then you will get thunderstorms. Thunderstorms happen all

6 the time so people have been used to them but you must remember how dangerous these can get and how much of a hazard it is to society.