Dear Tom,
During many of the thunderstorms in Chicago this past summer, strong winds often
started up a few minutes before the rain began. Were any of those winds associated
with tornadoes?
Thad Courter
Dear Thad,
The winds that you have described are called thunderstorm outflow winds, and they are
not associated with tornadoes. Raindrops falling from a thunderstorm literally drag
down with them the air through which they are falling. Much more importantly,
raindrops also evaporate as they fall, cooling the air they pass through and making it
heavier. A current of strongly descending air thus develops in the rain area of the
storm. When that current strikes the ground, it rushes out ahead of the storm,
producing the strong (and sometimes damaging), gusty, cooling winds that typically
occur at the onset of a thunderstorm.
