The sole ambition of Padraic Colum's Old Woman of the Roads was to have "a little house - a house of my own/ Out of the wind and the rain's way".
A modest enough requirement, to be sure! But four walls and a roof do not in themselves guarantee complete protection from the elements. In particular, the coincidence of heavy rain and strong winds can be a severe test for even the most sophisticated of building methods and materials.
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 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.
All bodies, including raindrops, fall to earth under the influence of gravity, but they also meet 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 2mm raindrop falls through the air at about 12 m.p.h., while a large 5mm 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 m.p.h. In addition, as we have noted, 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 blockwork 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.