Mixing Cloud in Winter

Written by: LEE Boon-ying

Can you think of other instances of mixing?

You can also see mixing clouds coming out from a car's front cover during a rainy day in winter. In this instance, the warm, saturated air (B) arises from evaporation from the car, forming a cloud when it meets the cold air (S) outside.

Another well-known example involves the appearance of aircraft contrails (Figure 2) in winter time. The word "contrail" comes from condensation trails. The burning of fuel in the aircraft's jet engines produces water vapour. When this hot water vapour mixes with the surrounding air, a cloud in the form of a contrail may appear.

Figure 2

Mixing clouds can be found in many other places. Further examples are: exhaust from a car; plume from a power station; air rising from a cup of hot coffee; and an approaching cold front.

Figure 3 - Mixing clouds associated with an approaching cold front at the Hong Kong International Airport on 4 May 2004

[Reference: C.F. Bohren, "Clouds in a glass of beer", Dover Publications, Inc., 1987.]