Wind Shear and the Temp/Dewpoint Spread

Know how to read the numbers and avoid a dangerous situation.

Windshear

Windshear

We all know that a very small temperature/dewpoint spread creates the perfect conditions for low visibility. But did you also know that a very high temperature/dewpoint spread associated with convective activity can be an indication of an imminent danger to light airplanes?

When the spread between the air temperature and dewpoint is between 15 and 30 degrees C and convective storms are in the area, that’s a recipe for wind shear.

Wind shear is the difference between wind speed and direction over a short distance in the atmosphere, and can be broken into horizontal and vertical components. The danger of low-level wind shear is that the sudden decrease in airspeed can rob an airplane of lift, possibly leading in extreme cases to an unrecoverable stall.

If the spread between the outside air temperature and dew point is large, then there is a good chance of windshear from microbursts associated with thunderstorms, even if the storms themselves have passed. The point to keep in mind is that in cases when there is convective activity lurking about and you notice the temperature/dewpoint spread is high, you may want to consider delaying your takeoff or landing to give the windshear time to dissipate.