Just about everyone can identify with the events leading up to a severe thunderstorm. Even from early morning, you can tell that there will possibly be storms today, just by the humid feel in the air. The "muggy" feeling is oppressive, even sometimes making it harder to breathe. Then by mid-afternoon, during the hottest part of the day, you sense the first signs of an oncoming storm. First, you hear that distant rumble of rolling thunder. Curiously you peek out the window just as a flash of lightning catches your attention. It is coming from a towering mass of black clouds sitting in the western sky. Moments later the wind increases, rattling the window panes. The sun is soon overtaken by the giant thunder-head, and a wall of rain creeps ever closer, bringing water droplets down in sheets.
But what will be next? Will there be hail, flooding, a tornado? How can you find out, and what safety tips do you need to follow? The following page will tell you just that.
A thunderstorm develops when warm, moist, unstable air rises in the atmosphere (This usually occurs along areas called fronts, the point at which masses of cold air and warm air meet, the specific point of collision being the dry line.). As this air meets cooler air that has a tendency to sink, a horizontal circulation occurs between the two masses, causing the air to rise to greater altitudes. As the warm, moist air condenses, huge mushroom or anvil-shaped clouds (often called supercells) filled with rain, lightning, wind, hail, and tornadoes to form. The more extreme this meeting is, the more intense the energy exchange is, and therefore more intense storms form. As a rule, the taller the cloud tops, the more intense the thunderstorm is. Specifically, a thunderstorm needs to have at least one of the following to be classified as severe: winds of at least 58 mph, 3/4 inch hail, or the occurrence of a tornado.
Lightning, hail, wind, and tornadoes, are all bourne from the energy and movement that is created and occurs within a storm cloud. As the movement of warm and cold air increases, wind is created. This movement also causes the positively and negatively charged particles within the cloud to polarize (collect at the ends of the cloud - negative at the base, and positive at the top). Because opposite particles attract, these particles will eventually move toward one another, resulting in lightning, whether it be within the same cloud, cloud to cloud, or cloud to ground (the ground is positively charged). Thunder is the rumbling sound made when the atmospheric gases are rapidly heated by a lightning bolt.
Hail, another manifestation within a thunderstorm, is formed from the rapid movement of water particles within a cloud. The air is moving them so quickly around the cloud that they do not immediately fall. Instead, they begin to freeze and form balls of ice. When their weight is able to overcome the wind speed, they fall to the ground as hail. In some of the most severe thunderstorms, hail has fallen that was the size of softballs.
Tornadoes are probably the most feared of all factors of a severe thunderstorm. Movement within a thunderstorm becomes so great that a column of air extends down to the ground, rotating violently, leaving destruction behind as it moves along the ground. Please see our tornado page for more information and safety tips.
It is important to be aware of any severe thunderstorm watches or warnings posted for your area so that you may be able to take the appropriate safety precautions. A severe thunderstorm watch means that conditions are favorable for the formation of severe thunderstorms in your area. At this time you should review safety plans of action and tips. A severe thunderstorm warning, means that a severe thunderstorm has developed in your area, and you should take cover. Please see "Thunderstorm Safety Tips" for more information