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High in the sky, the 'fasten seat belt' sign is on inside the plane.  When you look out from the window, the nearest cloud is kilometres away.  The weather is brilliant.  You may wonder, what could you possibly run into at this height?  Something out there, something called clear-air turbulence - an invisible trouble-maker for aircraft.
 

What is turbulence?

Turbulence is caused by irregular motion of air.  It brings about rapid bumps or jolts to an aircraft.  In severe cases, the aircraft might go momentarily out of control.  Turbulence usually occurs in areas where air masses with different speed, direction or temperature meet each other.
 

What causes turbulence? and clear-air turbulence?

Turbulence is sometimes associated with thunderstorms and cold and warm fronts where clouds and weather provide visible clues to the existence of turbulence.  However, turbulence can also occur in places where clouds are not present.  This kind of turbulence is called clear-air turbulence (CAT).  CAT usually occurs at relatively high altitudes of 20,000 feet (around 6 kilometres) or above.  It typically occurs near jet streams (i.e. narrow bands of strong winds) and other regions of significant wind changes in the vertical direction.  It can also occur when strong winds blow across mountain ranges.
 

How often is CAT encountered near Hong Kong? When does it occur most frequently in the year?

During the past couple of years, on average CAT was reported in about 15 days per year in the vicinity of Hong Kong. Out of these 15 days, severe CAT was reported in one day. A majority of the CAT events were reported in the winter months of December, January and February.
 

How does the Hong Kong Observatory (HKO) alert aircraft to CAT?

Weather forecasters at the HKO's Airport Meteorological Office constantly monitor the weather conditions in the vicinity of Hong Kong.  They are on the lookout for likely signs of CAT on satellite imageries, on winds aloft obtained with balloons, and from results of computer modelling of the air.

To alert the pilot to possible CAT along the way, the forecasters issue the following products: (a) significant weather information for aircraft in flight; and (b) significant weather charts for flight planning by airline operators and pilots.
 

What should aircraft passengers do to avoid injury?

A majority of turbulence-related injuries worldwide are related to passengers not having fastened their seat belts.

Passengers should buckle up at all times.  Make sure hand baggage are safely stowed away.  When the aircraft encounters turbulence, stay calm, listen to the aircrew and follow their instructions.

Remark: Here in this article, a turbulence report was classified as a Clear Air Turbulence when there was no thunderstorm nor layer cloud except cirrus within 1 degree latitude and longitude from the aircraft position.