Tornado facts and ratings
A tornado is a violently rotating column of air, pendant from a cumuliform cloud or underneath a cumuliform cloud, often — but not always — visible as a funnel cloud.
To be classified as a tornado, it must have contact with the ground and the cloud base.
Tornadoes can appear from any direction. Most move from southwest to northeast or west to east. Some tornadoes have changed direction or even backtracked.
Hail, or any type of rain, lightning or calm, is not a reliable predictor of a tornado.
Tornadoes can last from several seconds to more than an hour. Most last less than 10 minutes.
The sound of a tornado depends on what it is hitting. The most common sound is a rumble, like a nearby train. Sometimes a tornado produces a sound like a waterfall or open car windows when you’re driving fast.
To watch for tornadoes, forecasters keep an eye out for temperature and wind patterns that can cause enough moisture, instability, wind shear and lift to produce a tornadic thunderstorm.
Winds from strong tornadoes far exceed winds from strong hurricanes, but tornadoes cause only a tenth as much damage each year. Hurricanes are more destructive because of their larger size and duration.
The worst tornado in U.S. history was the tri-state tornado of March 18, 1925, in which nearly 700 people in Missouri, Illinois and Indiana were killed.
Want to know more? There are many library books and Web sites about tornadoes. One of the best Web sites is spc.noaa.gov/faq/tornado.
Scientists use the Enhanced Fujita (EF) Scale to measure the strength of tornadoes based on the damage they did. The rating is based on wind estimates. Monday’s tornado in Boone County was determined to be an EF3.
EF0: Wind speed 65-85 mpg. Some damage to siding and shingles.
EF1: Wind 86 to 110. Considerable roof damage, uprooted trees, overturned mobile homes, bent flagpoles.
EF2: Wind 111-135. Single-wide mobile homes destroyed. Permanent homes shifted off foundation. Flagpoles collapse. Softwood trees debarked.
EF3: Wind 136-165. All but small portions of homes destroyed. Hardwood trees debarked.
EF4: Wind 166-200. Complete destruction of well-built homes, large sections of school buildings.
EF5: Wind above 200. Major structural damage of mid- and high-rise buildings.
Sources: USA Today, National Oceanic and Atmospheric Administration
Compiled by the Rockford Register Star