IN calm conditions, a raindrop falls vertically at a speed dictated largely by its size.
When there is a wind, however, the drops are also carried horizontally on their journey earthwards, producing in the eye of the beholder the familiar illusion of oblique lines that hit the ground at an angle. And when the wind is very strong, as it has been in recent days, the result is an unpleasant phenomenon known as "driving rain", which lashes the unfortunate pedestrian almost horizontally and approaches buildings in its path in a Wagnerian swirl of turbulent sheets and eddies.
A typical raindrop during a spell of light but steady rain is about 2 mm in diameter. Heavy rain, of course, comprises larger drops, but they rarely exceed 5 mm because those that are larger become deformed as they fall through the air, and ultimately disintegrate into a number of smaller ones. This, in effect, puts an upper limit of about 8 mm on a raindrop's size.
Now all bodies, including raindrops, fall to earth under the influence of gravity, but they also meet with some resistance from the air. Their speed increases steadily at first, but the faster they move - the greater the air resistance until eventually they reach a stage where it exactly counteracts, the pull of gravity, and they continue downwards at a constant speed they are said to have reached their terminal velocity". Large raindrops have a smaller "surface area to weight" ratio than smaller ones, must travel faster before air resistance builds up sufficiently to balance gravity, and therefore reach a higher terminal velocity. In practice, a 2 mm raindrop falls through the lair at about 12 mph, while a large 5 mm raindrop falls earthwards at about 20 miles per hour, the latter, obviously, has much greater wetting power.
These speed limits, however, do not apply to "driving rain". In this case, the drop, which may be large, is carried along at speeds approaching that of the highest gust, which, not unusually, is 60 or 70 mph. In addition, much of the rain in this case approaches its target almost horizontally, and has great penetrating power on surfaces not prepared for such an onslaught.
Water from driving rain, for example, can quickly penetrate the exterior wall of a building through small cracks, mostly occurring at the interface between the blocks and mortar. As the block work becomes saturated, further water penetration occurs by capillary action through pores in the material itself, perhaps ultimately affecting the structural fabric of the building or causing damage to interior decoration.