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?
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2$\begingroup$ The definition I would use is "when it reaches the mature stage". Because before that it is a cloud. You are probably familiar, but here's a good link: aviator.zone/pireps/2021/1/14/3-stages-of-a-thunderstorm And even at the mature stage, 20 miles on the upwind side cruising at medium altitude is pretty conservative. You are more vulnerable low and slow during approach. $\endgroup$– Michael HallCommented Jun 10, 2023 at 16:58
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2 Answers
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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.
answered Jun 10, 2023 at 13:48
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$\begingroup$ I appreciate your answer, I’m gleaning that your interpretation falls under the “personal judgement” side of things. I’m just curious of whether or not the FAA has laid out a clear definition. I’m guessing not, as they often leave certain terminology vague seemingly on purpose. $\endgroup$ Commented Jun 10, 2023 at 14:09
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$\begingroup$ I would also say that nearly every day in the summer at my location has pop ups at some point in the day, so it’s unavoidable to continue flight training over the summer and to completely avoid areas of instability. Unless you just mean avoid all cumulonimbus development to which I completely agree with. The question is, by how far should they be avoided. $\endgroup$ Commented Jun 10, 2023 at 14:12
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1$\begingroup$ @RattMuscle I guess this is why checking the METARS is so important. 20 miles would be more towards a developed or severe storm, and, indeed, thermals underneath fair weather cumulus are gliders friends. It's the forecast that's important, especially if there is an approaching front. $\endgroup$ Commented Jun 10, 2023 at 14:19
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$\begingroup$ This is a good answer to “what are the hazards associated with flying too close to a thunderstorm?” Maybe the OP should change their question? $\endgroup$ Commented Jun 10, 2023 at 14:30
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2$\begingroup$ It’s a good answer, but I was more interested in finding out if there was an FAA definition for what defines a thunderstorm that would require significant avoidance. $\endgroup$ Commented Jun 10, 2023 at 14:44
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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.
