When two fronts collide, rainstorms and snowstorms form. When a warm front and a cold front meet, heavy storm clouds often develop. In the summer, the result is a steady rainfall that may last for several hours. In the winter, a heavy snowfall may occur. If the wind speed is above 56 kilometers an hour and the temperature is below -7 degrees Celsius, a BLIZZARD forms.

When a cold front meets a warm front, storm clouds may for to produce thunderstorms. A thunderstorm is a heavy rainstorm with thunder and lightning. These storms can be very violent. During a thunderstorm, areas of positive and negative electrical charges build up in the storm clouds. Lightning is the sudden discharge, or spark, of electricity between these cloud areas, or between a cloud and the ground. The lightning causes the air to heat and expand explosively, which is the cause of thunder. Sometimes hail is produced during a thunderstorm.

 

Air pressure has a very great effect on the weather. An area of low pressure containing rising warm air is called a CYCLONE. In a cyclone, cooler air moves in to take the place of rising warms air. The air currents begin to spin. Winds spiral around and into the centers of the cyclones. they move in a counterclockwise direction in the northern hemisphere. Cyclones usually cause rainy, stormy weather. A high pressure area containing cold, dry air is called an ANTICYCLONE. Winds spiral around and out from the centers of these anticyclones. The move in a clockwise direction in the northern hemisphere. The weather they bring is usually clear, dry and calm. Cyclones and anticyclones can be very small or very large. They travel across the United States, forming areas of high or low air pressure called HIGHS AND LOWS. Many changes in the weather are related to the movement of these Highs and Lows.

A hurricane is a powerful cyclone. It forms over tropical oceans. During late summer and early fall, low pressure areas form over the ocean. Warm, moist air begins to rise rapidly. Cooler air moves in, and the air begins to spin. As the air pressure in the center drops, more air is drawn into the spinning system. It begins to spin faster and faster. The rapidly spinning and rising air forms a cylindrical wall of strong winds, clouds and rainfall. Inside the wall, the air is calm. This calm center is called the EYE of the hurricane. Outside the eye, the winds may reach speeds between 75 and 155 mph! As long as the hurricane remains over the warm, tropical waters, it will continue to gain strength. The water temperature has to be at least 80 degrees Fahrenheit! That 80 degree water is the key! As hurricanes move over cooler water, or over land, they begin to lose their force and power. Heavy rains and high winds can still cause lots of damage though, even though the storm is weakening. High waves and the STORM SURGE can cause lots of damage to coastal properties too! Hurricanes are the most violent and destructive type of storm in the world!

Hurricane season reaches its peak in our area during August, September and early October. Comparable storms occur in the Indian and South Pacific oceans, and are called Typhoons.

West Indian Hurricanes originate between Florida and the South American coast. They move northwards, with their central generally off the gulf coast. They usually cause gale force winds from Texas to Nova Scotia. When hurricanes first appear as distinct storms, they are much smaller than the other types of cyclonic storms.

A tornado is a violent, whirling, funnel-shaped wind that moves in a narrow path, usually over land. They are uncommon in most parts of the world, and most parts of our country. In areas likely to experience a tornado, the storms are most common during the Springtime, in the late afternoon or early evening hours. Tornadoes are most common in the Great Plains section of the U.S., where cool, dry air from the west collides with warm, moist air from the Gulf of Mexico. Tornadoes are more abundant in the Mississippi Valley than anywhere else in the United States. More tornadoes occur in the U.S. than anywhere else in the world!

An average tornado is only 0.4 km in diameter and lasts only an average of fifteen minutes. The length of its path is generally only about 6km. If a tornado moves over the water, it draws large quantities of the water into the spout and is termed a WATERSPOUT. Tornadoes are associated with thunderstorms, along cold fronts and sometimes with hurricanes.

Viewed from the side, a tornado consists of a spout of black cloud spreading out to an umbrella shape above. These clouds are called CUMULONIMBUS CLOUDS, and are generally seen around sites of severe weather. Rain and hail will typically fall from the edge of the storm and the storm will be accompanied by thunder and lightning. Except for near the spout itself, winds will not be violent. Within the spout, wind speeds can reach over 350 km/hour. This wind is strong enough to tear apart strong buildings, uproot trees and lift heavy objects such as houses, automobiles and even railroad cars. No rain will fall within the spout, since the air will be rising too rapidly. The atmospheric pressure here will be very low. Sometimes, the expansion of air within a house as this low pressure moves by will be enough to blow the walls of a house outward.

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