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It's my understanding that if a pilot is flying over an ocean or otherwise uncontrolled airspace and they encounter a cumulonimbus, they navigate around it at their discretion.

How does this happen in controlled airspace though? What instructions do ATC and a PIC typically exchange if a PIC wants to request navigation around a thunderhead?

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    $\begingroup$ Keep in mind that "over an ocean" is usually still controlled airspace, even if there's no radar coverage. Most flights aren't crossing the North Atlantic at 5,000 ft in Class G airspace. There will be procedures in place (here are the ones for the North Atlantic) for deviations for weather, but absent emergency situations, it's all still controlled. $\endgroup$ – Zach Lipton Mar 1 at 1:22
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If you are in controlled airspace its very possible ATC will direct you around weather, you can find out more about that in this question.

However what ATC can see on their scope varies from place to place and weather moves quickly. As such pilots can, and often do request deviation for weather. Lets say you are flying along dodging some clouds here and there, popping in and out, and all of a sudden you pop out with a cell forming in front of you. You may request something like:

N123AB requesting left turn 20 degrees for weather

N123AB left 20 approved for weather avoidance

It may also not be a bad idea to file a PIREP to help others out.

Ultimately as pilot in command you can deviate from any rule to ensure a safe flight. If you were about to fly straight through a storm and ATC was not allowing a deviation or not issuing one a case could be made for deviating any way. You can read more about that here.

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    $\begingroup$ Phraseology-wise, the use of the word "Cleared" in your example is wrong. It would either be something like "Left turn [20 degrees] approved, [report turning inbound XXX again]" or "Turn left 20 degrees, vectors, [report clear of weather]" $\endgroup$ – J. Hougaard Mar 1 at 7:25

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