For a tornado to form, there must be a cold front and a low level of humidity. Tornadoes are associated with large, strong thunderstorms. A column of warm, moist air will rise rapidly. Scientists don't understand how it starts to rotate, but what they think is that it is two different winds with different heights and speeds. This creates wind shear. The column of air sometimes gets caught in an updraft of the supercell. The updraft will strengthen and accelerate. When it starts to rain and hail, the precipitation will push down the column of air creating a tornado. Tornadoes occur primarily in the southern half of the United States. Multiple thunderstorms occur that are needed for a tornado to form. They normally move from south-west to north-east. The intensity of a tornado is measured using the Fujita-Pearson Tornado Scale. Safety strikes are very important during the time of a tornado. When you are in a public place, go to the designated shelter of which you will be informed. When you are at home, go to the basement or innermost room. If you are caught outdoors, lie down in a ditch and cover your head with your hands to protect yourself from flying debris. Hurricanes need a warm front to trigger. Air from other areas with higher atmospheric pressure will be pushed into the low pressure area. Then the air also becomes warm and humid and rises. As the warm air continues to rise, the surrounding air circulates and takes its place. When warm, moist air begins to cool, water in the air forms clouds. All the clouds will spin and grow, using the ocean's heat as fuel. Hurricanes occur over tropical oceans. When they start, they usually move westward, when they turn, they turn clockwise. They are measured from the Saff... at the center of the chart... the cloud base and the ground are below 40 degrees Fahrenheit. But if it were to fall at a higher temperature, it would turn the ice crystals into rain or sleet. Of course, snowstorms happen in cold places like Minnesota in the winter. They will normally head south as the large, heavy, cold air mass is blown in from the north. The intensity is measured by the amount of snow that falls. For example: it could be a light snowfall or a blizzard. When it snows, you should try to stay off the roads. If you drive your car in the snow, be sure to bring emergency supplies such as: gloves, jackets, blankets, matches and water. Make sure you keep your home and yourself warm if you're outside. Don't go out during a snowstorm. The blizzard could be so extreme that you could only see white and would be lost.