Notes and Comments
155
There are several ways in which the raising and cooling of the air may take place. A damp wind may be forced up a hillside, and form a cloud over the top; this is the reason for the heavy mass of Cumulus cloud which so often lies on the Peak during the SW monsoon. Again, air may rise vertically owing to convection currents over heated ground; this mode of cloud formation is illustrated by the isolated Cumulus or "Woolpack" clouds of a warm summer afternoon in England; each cloud marks the summit of a rising column of air. Thirdly, a whole layer of air may be forced up the sloping surface of a wedge of denser air lying beneath it, in which case a uniform layer of Stratus cloud may form.
If the process of cooling and condensation proceeds far enough, the minute droplets forming the cloud grow in size as more water is added to them. It is probable that they collide with each other and coalesce, and eventually they become sufficiently large to fall out of the cloud as rain. At first sight it may seem strange that a cloud remains floating in the sky while a shower of rain falls to the earth; both consist of water drops, so why should not the cloud fall as well as the rain? To explain why a cloud remains floating, we must examine the forces acting on a body falling through a resisting medium such as the air. When a body falls through air, it will continue to accelerate until the force of gravity is exactly balanced by the air resistance, and thereafter will fall at a constant speed known as its "terminal velocity". Now for objects of the same density, the air resistance depends on the surface area of the body, and so increases as the square of the linear dimensions, while the weight increases as the cube of the linear dimensions; consequently a large body must attain a much higher terminal velocity than a small one before its weight is balanced by the air resistance. A man, for instance, falling through air attains a terminal velocity of about 100 miles. per hour-however high the aeroplane may be when you fall out of it, you cannot hit the ground at a greater speed than this; if that is any comfort! A large raindrop may have a diameter of 0.2 inch, and its terminal velocity is about 18 m.p.h. The minute droplets which form a cloud may have diameters as small as 0.001 inch, and a terminal velocity of only 150 feet per hour.
Even if a cloud were not buoyed up by rising air-currents, its descent at this slow rate would be imperceptible from the ground.
More water-vapour is needed to saturate warm air than cold, and the amount of water-vapour which may be contained by the heated air of the Tropics is colossal. For example, the air over the Colony, covering an area of about 350 square miles, can hold when saturated about 50 million tons of water-vapour, if we take the temperature at ground level as 85° F. No wonder it rains hard in summer. If all this water-vapour were condensed and precipitated as rain, the fall would amount to about 3 inches; it is evident that to produce the tremendous falls of 20 inches or more in 24 hours, which have sometimes been recorded here, the damp air over the Colony must be renewed many times over.
In winter the air over Hong Kong, even if saturated, can contain a mere 20 million tons of water-vapour; as a rule it is very far from being saturated throughout its height at this time of year, and the precipitation. which falls from it can in truth be described as "the gentle rain from heaven".
July 1935.