In summer time, we get pop up cumulonimbus clouds that often develop into thunderstorms. So is there a defined point at which it would be considered a thunderstorm to stay 20 miles away from? Is it the “thunder” aspect (I.e. lightning)? Or is it a personal judgement call? Or is there a clearly defined definition I’ve missed?
Well, you won't get a ticket if you get too close, but please remember thunderstorms can produce very violent winds, and the cloud is produced by the updraft.
Sinking air and turbulence may be present in the clear air around it.
Although lightning can strike "out of the blue" many miles from a thunderstorm, it is the unpredictable wind conditions in and near the storm that pose the greatest risks to aviators.
Also, thunderstorms can rapidly develop, and radar information even 20 minutes old may be inaccurate.
Any area of instability should be avoided, meaning don't fly there that day unless you absolutely have to.
The FAA Advisory Circular AC-0024 C section 10 a. Thunderstorm Avoidance, line 14 states the 20 mile limit is for storms of a "severe" nature or giving an "intense" radar echo. Line 15 also recommends avoidance of areas where thunderstorm coverage is 6/10.
I've come across a rule of thumb that says to stay as many NM away from a storm as its tops in '000s of feet. So 20NM would be a minimum distance for a storm that tops out at FL200. For a really severe storm topping out at say FL450, give it a 45NM berth. This reflects the horizontal distance that hazards like hail and turbulence can be found from a big storm, especially on the downwind side.