In a response to a comment on Open Mind:

MF: Why was no mention made of the fact that unseasonably cold weather exacerbated the recent tornadoes?

T: Did it? Are you an atmospheric scientist? I’m not — so I would never make such a claim unless I had the information from the most reliable sources (the peer-reviewed scientific literature). What’s your source for that information?

MF: Early flurry of killer tornadoes linked to cold winter

[Ed: I have formatted the reply but kept the meaning unchanged.]

From the New Scientist article, “Tornadoes in the US form when a front of dry, cold air descending from the north meets warm, moist air coming up from the south. Sometimes, a body of the cold air slides over the top of the warm air, trapping it underneath.”

Although everything is not known about the formation of a tornado, we do know that tornadoes tend to form with intense thunderstorms and that a conditionally unstable atmosphere is essential for their development… [In a conditionally unstable atmosphere] [a]t the surface, we find an open-wave middle-latitude cyclone with cold, dry air moving in behind a cold front, and warm humid air pushing northward from the Gulf of Mexico behind a warm front. Above the warm surface air a wedge of warm, moist air is streaming northward. Directly above the moist layer is a wedge of colder drier air moving in from the southwest. Higher up, at the 500-mb level, a trough of low pressure exists to the west of the surface low and, at the 300-mb level, the polar front jet stream swings over the region. At this level, the jet stream provides an area of divergence that initiates surface convergence and rising air. The stage is now set for the development of severe storms. (Ahrens 2000)

That maybe a little complicated, so I’ll attempt to explain. At the surface, we have a typical winter mid-latitude cyclone. What’s happening in the atmosphere is more important to whether there will be thunderstorm development. As it said above, we need a conditionally unstable atmosphere. This means that the atmosphere is stable, but that it could become unstable based on other variables. For a good primer on atmospheric stability, see this Wikipedia article.

Above the surface cyclone, there is warm, moist air coming from the Gulf of Mexico, which provides the temperature inversion necessary for the low level inversion. Above that, there in very cold dry air coming from the North or Northwest. This provides the necessary condition for the conditionally unstable air. As soon as rising air from the surface breaks into this layer of the atmosphere, it will become positively buoyant and continue to rise until the next inversion (the tropopause). A positively buoyant parcel will continue to rise because it’s density will be less than the surrounding atmosphere.

Thus, the article is correct that cold air is necessary for severe storm development, from which tornadoes may form. However, it is not cold temperatures at the surface which are important. At the surface, there needs to be warm, humid air. And in the upper atmosphere there needs to be cold, dry air. In essence, the article is correct. But it is simplified in such a way that makes it appear that cold surface temperatures cause tornadoes, which is not true.

References:
Ahrens, C.Donald. Essentials of meteorology: an invitation to the atmosphere, 3rd edition (2000) pp. 277-278.

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