One of the major causes of turbulence in the free atmosphere is associated with the strong winds and steep temperature gradients that generally occur in the vicinity of the jet stream at an altitude of 30,000-40,000 ft. When the velocity of the jet stream begins to drop off away from its core, air parcels of different temperature mix to produce levels of turbulence that can be extreme. Clearly, then, to avoid this aspect of turbulence, the jet stream should be far from the observing site.

The atmosphere undergoes considerable motion when a cold front is nearby. Where two dissimilar air masses meet, warm air is set in motion by being lifted just ahead of the front. The colder air behind the front is more unstable, causing pronounced levels of convection. The resulting turbulence can be extremely detrimental to seeing.

Note: In developing these tables, I used suggestions by Eric J. Douglass in his article How to Predict Seeing (Sky & Telescope, January 2000)

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