Re: clear air turbulence

Date:         25 Jan 98 03:27:10 
From:         "Kwuan Tzee" <ktlim@cyberway.com.sg>
Organization: personal
References:   1
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Philip Miller <channelz@ix.netcom.com> wrote in article
<airliners.1998.120@ohare.Chicago.COM>...
> Can someone explain how this phenomenon occurs?  Is it simply a change
> in density of the air due to temperature?

The following is taken from "Aviation weather AC 00-6A"
Chapter 13 page 142.

Clear Air Turbulence(CAT) implies turbulence devoid of clouds. However, we
commonly reserve the term for high level wind shear turbulence, even when
in cirrus clouds.

Cold outbreaks colliding with warm air from the south intensify weather
systems in the vicinty of the jet stream along the boundary between the
cold and warm air. CAT develops in the turbulent energy exchange between
the contrasting air masses. Cold and warm advection along with strong wind
shears develop near the jet stream, especially where curvature of the jet
stream sharply increases in deepening upper troughs. CAT is most pronounced
in winter when temperature contrast is greatest between cold and warm air.

A preferred location of CAT is in an upper trough on the cold side of the
jet stream.Even in the absence of a well defined jet stream, CAT often is
experienced in wind shears associated with sharply curved contours of
strong lows, troughs, and ridges aloft, and in areas of strong, cold or
warm air advection. Also mountain waves can create CAT. Mountain wave CAT
may extendfrom the mountain crests to as high as 5 000ft above the
tropopase, and can range 100miles or more downstream from the mountains.

CAT can be encountered where there seems to be no reason for its
occurrence. Strong winds may carry a turbulent volume of air away from its
source region. Turbulence intensity diminishes downstream, but some
turbulence still may be encountered where it normally would not be
expected. CAT forecast areas are sometimes elongated to indicate probable
turbulence drifting downwind from the main source region.
<snip>

I hope the above answer your question. You can read this book for more
information on CAT.

Kwuan Tzee.