Warmth is a key ingredient for tornadoes, but the events of February 22, 1975 put this to the test. Between 12:30 and 3:00 am, southwest Oklahoma experienced a total of 6 tornadoes, leaving 3 dead and 34 injured. Temperatures were in the 30’s and 40’s, and a period of sleet and snow followed. Was this a momentary lapse in the laws of science or is there more to this story? A closer examination reveals this was an extreme event but not a rule-breaker.
While temperatures near the tornadoes were near 40° that night, if you hopped in a car and drove 50 miles east, you were in the 50’s. Turn south toward Dallas? It was near 70° at midnight. That in itself bears the mark of strong cold front. Strong fronts occasionally trigger severe weather, but these tornadoes occurred behind the front, which is unusual.
Tornadoes need warm, moist air to form, and cold fronts are boundaries between relative warmth and cold. Air behind this front was much colder and drier, but not all fronts are the same. The cold front that night was particularly shallow, meaning the dome of cold air was not very tall. In fact, observations suggest a strong current of warm air was rushing north over the cold air dome. Tornado ingredients were not at ground level but elevated, so thunderstorms that formed were essentially disconnected from temperatures below.
Once thunderstorms matured, they began rotating due different wind direction and speed at different heights, known as wind shear. Normally, elevated thunderstorms don’t produce tornadoes, but since this cold front was so shallow, rotation was able “drill” through the cold air and make contact with the ground. The result was series of destructive tornadoes that didn’t break the laws of physics but caught a lot of people by surprise.
SOURCES: http://www.tandfonline.com/doi/pdf/10.3200/WEWI.58.2.398, http://bangladeshtornadoes.org/UScases/022275/022275pth.gif, http://bangladeshtornadoes.org/UScases/022275/02227504zsf.gif, http://bangladeshtornadoes.org/UScases/022275/02227500zh7.gif, http://www.tornadohistoryproject.com/tornado/1975/2/22/table