I've developed the impression that often commercial planes will bank and turn as they climb across thin cloud layers that appears to be stratus or stratocumulus.

I think these clouds indicate a separation between different air masses. So my question is two-fold:

  • Are planes actually doing this or is it a coincidence?
  • If not a coincidence, what is the reason? and what would happen if they didn't?

Welcome to Aviation SE.

There is no need or reason to turn when going through clouds. Aircraft frequently climb (or descend) and change direction at the same time. If it happens in cloud, it's just a co-incidence.

As an example, on an approach, ATC often issue instructions such as:

Turn left heading 300 degrees, descend and maintain 4000 feet.

The pilot will start turning towards heading 300 and start descending at the same time.

Resume own navigation, climb flight level 370.

Following a previous instruction to deviate from the heading on the flight plan, the pilot is now cleared to turn back onto the planned track and climb to the planned flight level.

There is a high probability that you will see aircraft turning in clouds since arrival and departure routes, which is when most of the turning, climbing and descending is done, are when you are most likely to see aircraft in and out of clouds and manoeuvring.

In cruise, when an aircraft turns, it is also common to descend or climb since the altitude flown according to the heading is determined by the quadrantal and hemispheric rules.

  • $\begingroup$ You're most likely to go through a cloud layer when you're climbing or descending. You're quite likely to climb or descend when you turn because it is common to separate traffic going in different directions into different altitudes to make separation simpler and more effective. $\endgroup$ – David Schwartz Jan 18 '17 at 10:26
  • $\begingroup$ @DavidSchwartz Good point. Mind if I edit it in? Or perhaps you could add this as another answer since it's entirely relevant to the question. $\endgroup$ – Simon Jan 18 '17 at 11:15
  • $\begingroup$ Feel free to edit it in. Then I'll delete my comment if it's not longer relevant (and/or this one depending on how you edit). $\endgroup$ – David Schwartz Jan 18 '17 at 11:16

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