Page 7 - May 2019

Another term you might see is storm-relative helicity, which gives a measure of the rotational potential of a thunderstorm updraft. This gives forecasters an indication of an environment that is favorable for supporting the development of thunderstorms with rotating updrafts, a precursor to super-cell thunderstorms and tornado development. For mesocyclone development, storm- relative winds typically have speeds greater than 20 knots and turn clockwise with height by at least 90 degrees in the lowest three kilometers of the atmosphere. Values of helicity greater than +150 are considered significant, although there is no “magic” value that indicates whether a rotational thunderstorm will develop.
Since we’re talking about Tornado Alley, here are some fas- cinating facts about this destructive weather event. (Courtesy of Horne’s “Flying America’s Weather”) Tornados tend to form between 4 and 8 p.m. and most form when surface temperatures are between 65 and 84 degrees Fahrenheit. The 700-plus observed tornadoes each year in the United States last on average of about a half-hour and their average ground track is six miles.
Tornadoes also tend to form in groups, and some of them can be very large. Once they’re on the ground, they tend to move along at 25 to 45 mph southwest to northeast. They have an average width of 400 yards, although its width can be much less at the surface.
And finally, more than half of all observed tornadoes occur in – you guessed it – Tornado Alley. Although, as Dorothy says, there’s no place like home, I am good with sustaining my “no tornado witnessed” record.
Select Airparts
Dianne White can be contacted at editor@diannewhite.com
Turbines 1/2 page
May 2019 / TWIN & TURBINE • 5