1)Two air masses that differ vastly in temperature and humidity.
2) A fast-moving jet stream above.
3) Winds blowing in different directions at different altitudes.
The reason spring is such an apt time for tornadoes in the U.S. is due to the contrasts in warm and cold air masses, as I indicated above. In the middle of the U.S., topography (the Rocky Mountains) is an added ingredient for creating deadly tornadoes-in particular, F3s F4s and the deadliest-F5s, which contain winds of up to 318 mph. The F-scale was created by Dr. T. Theodore Fujita. The instability that results from the cold and warm air masses colliding creates violent supercell thunderstorms. The winds going at different speeds in the atmosphere create a twist, which cause the thunderclouds to spin. The jet stream acts as a bottle opener, allowing the rising moist warm air that feeds the thunderclouds to rise explosively. Combine all that, and you get a tornado.
With the Rocky Mountains, they add a further bit of imbalance. Cold air usually goes under warm air, causing it to rise. When cold dry air comes off of the Rocky Mountains, it can sometimes come off at such a speed that when it collides with the warm air, part of the cold air mass goes over a section of the warm air, creating an even greater imbalance. When the jet stream removes this cap, the results are even more explosive, leading to large tornado outbreaks and some monster tornadoes that can be up to 2 miles wide.
For those folks who live in Tornado Alley, make sure you have a weather radio. I own one-I've owned them for over 30 years. Last summer, I had a tornado-warned thundercloud go right over my house!! It was Doppler-radar indicated, but it didn't drop any funnels. It did put out a LOT of lightning, though. In the past, tornadoes were very rare in my area, but in recent years, I've been noticing an increasing number of them. And they are not just increasing in number-they are also getting more ferocious. Last summer, twin tornadoes struck Brooklyn, New York, causing extensive damage. Tornadic winds were determined by the National Weather Serice to be around 150 mph!!
Here's a bit from an email I sent to a friend:
going to happen eventually-I had a tornado-warned storm go right over
my house!! Eeeep!! Doppler radar indicated strong rotation inside
the parent thunderstorm, hence the tornado warning. It put out
continuous, WICKED lightning and the dark clouds were churning, but no
funnel clouds, fortunately. That summer, Brooklyn NY, a city just
southeast of me, was struck by twin tornadoes, with winds of 150 mph.
They did extensive damage.
The summer before that, I DID see a funnel cloud not far from my home.
It was the most humid day I have EVER felt in my life-summers around
here are hot and sticky. A thunderstorm EXPLODED into being over my
house. The cloud was developing rapidly. You could see dark fingers of
moisture feed from the seemingly clear air into the parent
thundercloud. A little off to the northeast, I saw a very
small, weak funnel twist from the cloud; it only lasted a few seconds.
But I knew it for what it was. I've been a severe weather buff for
years (a fantasy of mine would be to go storm chasing), and I've seen
PLENTY of tornado video. And later that day, there were tornado
warnings northeast of me. That was the first funnel cloud I'd seen in
Tornadoes around here used to be rare, but in recent years. we've had
one or two every year, and they seem to be increasing-and not just in
number, but ferocity. They are no longer just F0s or F1s (twister
intensity; the F is for the Fujita scale; read up on it-fascinating
stuff). Those are relatively weak tubes. But they are getting stronger
and more dangerous. 150 mph is around an F3, I think. That's what
investing in one. They could save your life one day. I've used weather
radios for over 30 years-and all of them from Radio Shack. There are
250(?) National Weather Service radio stations that broadcast around
the U.S. Go onto the Weather Underground site and type in your
city/zip. On there, you can hear a broadcast from your local NWS radio
station. Years ago, they used to be taped recordings of forecasts,
looped every hour. Now, they are digitized voices. Weather radios come with alarms
that you can set so that you can be
warned of approaching dangerous weather.
Working with the Federal Communication Commission's (FCC) Emergency Alert System , NWR is an "All Hazards" radio network, making it your single source for comprehensive weather and emergency information. In conjunction with Federal, State, and Local Emergency Managers and other public officials, NWR also broadcasts warning and post-event information for all types of hazards – including natural (such as earthquakes or avalanches), environmental (such as chemical releases or oil spills), and public safety (such as AMBER alerts or 911 Telephone outages).
Edited by Analog Kid, 18 April 2011 - 10:08 AM.