Page 132 - ภาษาอังกฤษสำหรับสายวิทยาการอุตุนิยมวิทยา

127

b. This shear can produce hazardous turbulence for aircraft landing or departing in an airport
area when frontal passage takes place. The air ahead of a fast moving cold front is forced upward by
strong lowlevel winds. This results in gradually increasing low-level turbulence as the front
approaches the airport.

12. Clear Air Turbulence

The term "clear air turbulence" (CAT) generally refers to high-level wind shear turbulence
which frequently occurs in the area of a jet stream. The intensity of CAT may be such that it
causes serious problems for aircraft and passengers.

13. Characteristics of CAT

High-level clear air turbulence tends to occur in limited areas for short periods of time.
The size of the turbulent area is quite variable, but it is thought to be around 2,000 feet
thick, 20 miles wide, and 50 miles or more long. The areas become longer in the direction
of the wind. The exact position of one of these areas is difficult, if not impossible to locate.
The determination of the locations is largely dependent on pilot's reports; however, the
meteorologist can predict general areas where the turbulence might occur.

14. CAT by Seasons
There are from three to four times more CAT occurrences during winter han sunmer.
The most frequent altitude of jet stream CAT en counters is 30,000 feet in winter and
34,000 feet in summer.