Severe Weather: Thunderstorms, Tornadoes, and Hurricanes Chapter 16: Understanding Weather

Thunderstorms Thunderstorms are frequently spawned along cold fronts where warm, moist air gets pushed up rapidly. Typical conditions are heavy downpours, lightning, thunder, and hail. The U.S. experiences about 100,000 a year.

More on Thunderstorms… All thunderstorms require warm moist air. Most occur in the late afternoon and early evening in late spring and early summer. 3 stages of development: Cumulus: warm updrafts of air, cloud height increases. Mature: updrafts of warm air and downdrafts of cool air where precip is falling, most severe conditions. Dissipating: downdrafts of cool air allow the cloud to evaporate.

Grab Toto!! A tornado is defined as a violently rotating column of air extending from a thunderstorm to the ground. These destructive forces of nature are found most frequently in the United States east of the Rocky Mountains during the spring and summer months. This is called Tornado Alley. During the spring in the Central Plains, thunderstorms frequently develop along a "dryline," which separates very warm, moist air to the east from hot, dry air to the west. April-June is “tornado season” because most happen during these months. However, they can happen any time of the year.

Tornado Destruction & Prediction The destruction depends on the wind speed, which is rated on the Enhanced Fujita Scale. Predicting them is not easy because they are so spontaneous. Meteorologists look for a “hook” on Doppler radar. Watches and warnings are also issued to alert the public of potential danger.

Hurricanes A hurricane is a severe tropical storm that forms in the North Atlantic Ocean, the Northeast Pacific Ocean east of the dateline, or the South Pacific Ocean east of 160E. Hurricanes need warm tropical oceans, moisture and light winds above them. When hurricanes move onto land, the heavy rain, strong winds and heavy waves can damage buildings, trees and cars. The heavy waves are called a storm surge.

More on Hurricanes… Hurricanes can often live for a long period of time -- as much as two to three weeks. With a depression, wind speeds are 23-39 mph. With a tropical storm, winds are 39-73 mph. It finally becomes a hurricane when winds reach 74 mph. The eye is the focus of the hurricane, the point about which the rest of the storm rotates and where the lowest surface pressures are found in the storm. Skies are often clear above the eye and winds are relatively light. It is actually the calmest section of any hurricane.

Tracking and Prediction Tracking hurricanes is much easier than tornadoes because hurricanes last many days longer. The ones that affect the U.S. track west along the equator and curve north up the east coast. The Saffir-Simpson Scale is used to rank hurricane intensities. Thanks to satellites, we know about the existence of a tropical cyclone immediately. Meteorologists at the Tropical Prediction Center work to constantly monitor these systems as they move, issuing hurricane watches and warnings with adequate time for the public to prepare.