The strength of the collision affects the strength of the storm.
Also, the upper atmosphere affects the degree to which the warm air rises. If the upper winds are diverging, for example, they can pull air up more efficiently, Harrington said.
And curvature in the wind pattern or interactions with the jet stream can greatly magnify the effect.
On Friday night there was an unusually strong boundary between humid air and dry air. In addition, Harrington said, "all three of the upper-air mechanisms were there."
The result, he said, is that by 3 p.m. forecasters were predicting major storm activity in the region where the tornado later struck.
"They definitely had a handle on what was going to happen," he said.
But, he added, "It's one thing to say it's going to happen. It's another to have to deal with the consequences."
U.S. Leads in Tornadoes
Tornadoes, Harrington adds, have occurred on every continent except Antarctica. But they are most common in the U.S.
Tornado reports have also been increasing.
(Learn about tornado safety tips.)
But Harrington believes that this is simply a factor of more people living in twister-prone areas and reporting more accurately what they see.
For these reasons, he said, the increase in sightings has been limited to small tornadoes.
"We're now seeing things that may have occurred before in open country and not been counted," he said.
"We're seeing more of the ones that may not have done much damage but were pretty to photograph."
(See a photo gallery of tornadoes.)
Overall, he added, global warming seems to have little impact.
"The processes involved [in tornado formation]," he said, "are not necessarily related to slight changes in planetary temperature."
Free Email News Updates
Sign up for our Inside National Geographic newsletter. Every two weeks we'll send you our top stories and pictures (see sample).
SOURCES AND RELATED WEB SITES