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CAT near significant vertical windshear

CAT sometimes occurs in areas where there are abrupt wind changes in the vertical direction (such changes are termed as vertical windshear).

On 22 June 2000, an aircraft flying near Xisha (see map below) reported encountering CAT at around 9:43 p.m. Hong Kong time, at the 37,000 ft (around 11 km) level.  The following wind profile, measured over Xisha at 8 p.m. Hong Kong time that evening, reveals a significant wind change (red dotted line) between 30,000 to 39,000 ft (around 9 to 12 km).  There, southwesterly winds of about 12 knots (22 kilometres per hour) at the lower level gave way to 50 knots (93 kilometres per hour) northeasterlies higher up, indicating relatively strong vertical windshear.

Wind Profiles of Xisha, Hong Kong and Dongsha
at 2000 Hong Kong Time on 22 June 2000

Wind Profiles of Xisha, Hong Kong and Dongsha on 22 June 2000

The location of Xisha and Dongsha