Under the right conditions, rainfall from thunderstorms causes flash flooding, killing more people each year than hurricanes, tornadoes or lightning. Lightning is responsible for many fires around the world each year, and causes fatalities. Hail up to the size of softballs damages cars and windows, and kills livestock caught out in the open.
Strong up to more than mph straight-line winds associated with thunderstorms knock down trees, power lines and mobile homes. Tornadoes with winds up to about mph can destroy all but the best-built man-made structures. Where are severe thunderstorms most common? The greatest severe weather threat in the U. But, no place in the United States is completely safe from the threat of severe weather.
A watch can cover parts of a state or several states. Warnings mean there is a serious threat to life and property to those in the path of the storm. ACT now to find safe shelter! A warning can cover parts of counties or several counties in the path of danger. How does a thunderstorm form? As the air rises, it transfers heat from the surface of the earth to the upper levels of the atmosphere the process of convection.
The water vapor it contains begins to cool, releases the heat, condenses and forms a cloud. A cyclone is a system of winds rotating counterclockwise in the Northern Hemisphere around a low pressure center. The swirling air rises and cools, creating clouds and precipitation. There are two types of cyclones: middle latitude mid-latitude cyclones and tropical cyclones.
Mid-latitude cyclones are the main cause of winter storms in the middle latitudes. Tropical cyclones are also known as hurricanes. An anticyclone is the opposite of a cyclone. Air comes in from above and sinks to the ground. High pressure centers generally have fair weather. Mid-latitude cyclones , sometimes called extratropical cyclones, form at the polar front when the temperature difference between two air masses is large. These air masses blow past each other in opposite directions.
Coriolis Effect deflects winds to the right in the Northern Hemisphere, causing the winds to strike the polar front at an angle. Warm and cold fronts form next to each other. Most winter storms in the middle latitudes, including most of the United States and Europe, are caused by mid-latitude cyclones Figure below. A hypothetical mid-latitude cyclone affecting the United Kingdom. The arrows point the wind direction and its relative temperature; L is the low pressure area. Notice the warm, cold, and occluded fronts.
The warm air at the cold front rises and creates a low pressure cell. Winds rush into the low pressure and create a rising column of air. The air twists, rotating counterclockwise in the Northern Hemisphere and clockwise in the Southern Hemisphere.
Since the rising air is moist, rain or snow falls. Mid-latitude cyclones form in winter in the mid-latitudes and move eastward with the westerly winds.
These two- to five-day storms can reach 1, to 2, km to 1, miles in diameter and produce winds up to km 75 miles per hour.
Like tropical cyclones, they can cause extensive beach erosion and flooding. Figure below. Tropical cyclones have many names. By any name, they are the most damaging storms on Earth. The warm seas create a large humid air mass. The warm air rises and forms a low pressure cell, known as a tropical depression. Thunderstorms materialize around the tropical depression. As the air rises, water vapor condenses, releasing energy from latent heat. If wind shear is low, the storm builds into a hurricane within two to three days.
Hurricanes are huge with high winds. The exception is the relatively calm eye of the storm where air is rising upward.
Rainfall can be as high as 2. The release of latent heat generates enormous amounts of energy, nearly the total annual electrical power consumption of the United States from one storm. Hurricanes can also generate tornadoes.
Hurricanes are assigned to categories based on their wind speed. The categories are listed on the Saffir-Simpson hurricane scale Table below. Hurricanes move with the prevailing winds. In the Northern Hemisphere, they originate in the trade winds and move to the west. When they reach the latitude of the westerlies, they switch direction and travel toward the north or northeast. Hurricanes may cover km miles in one day.
Damage from hurricanes comes from the high winds, rainfall, and storm surge. Flooding can be devastating, especially along low-lying coastlines such as the Atlantic and Gulf Coasts. Hurricane Camille in had a 7. Hurricanes typically last for 5 to 10 days.
When a hurricane disintegrates, it is replaced with intense rains and tornadoes. There are about hurricanes around the world each year, plus many smaller tropical storms and tropical depressions. As people develop coastal regions, property damage from storms continues to rise. This circuit of rising and falling air is called a convection cell. If this happens a small amount, a cloud will form. If this happens with large amounts of air and moisture, a thunderstorm can form.
Thunderstorms can consist of just one convection cell, multiple convection cells, or even one extremely large and powerful convection cell. Below is a description of three types of thunderstorms, classified by their structure: single-cell, mulit-cell and supercell. All thunderstorms begin with air rising into the atmosphere to form a convection cell, but the air can be lifted in different ways.
Another way to classify thunderstorms is by the location where they form and the reason that air rises.
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