Light airplanes can fly in the rain. There is very little different about flying in the rain versus flying in dry air, except for a bit of noise caused by the droplets hitting the windscreen. But how do you know if that blob on the radar is simply rain or a thunderstorm with the requisite strong turbulence that is a danger to all aircraft?
Here are 4 clues to help you out:
1. Is there a convective sigmet in place?
If the weather service has issued a convective sigmet for your route of flight, you should assume that anything but the lightest rain showers are indicative of a thunderstorm or a building thunderstorm.
2. Is there lightning in the area of precipitation?
Lightning means thunder, and thunder means thunderstorm. Use weather products like Foreflight that overlay lightning strikes onto radar returns to make this determination.
3. What are the intensity of the echoes?
Different radar products use different color codes. What is red on one product, may be yellow on another. Compare the color codes with the map legend to determine the decibel range represented on the map. As a general rule, non-convective precipitation tops out at around 40DBZ; anything higher than that should be assumed is convective.
4. How steep are the precipitation gradients?
Green that immediately turns to yellow that immediately turns to red is an indication of a thunderstorm. Green that extends for many miles, that turns into yellow that extends for many miles, may not be convective.
Check out the Air Safety Institute’s presentation on IFR Flight Planning, and their interactive course on Cockpit Weather.