Overview

In the absence of wind, rain and snow fall vertically downwards. The effect of wind is to give the rain drops and snow flakes a horizontal component of velocity. Unfortunately, to date, little data on the simultaneous records of wind and rain are available, and consequently data on the correlation of these two events is vague.

There are three consequences of this horizontal movement. The first is on the building where the rain can now impinge on non-horizontal surfaces and so cause staining, or allow mosses and lichens to grow, or can cause damp to penetrate the walls to the detriment of its inhabitants. The second effect is on the comfort of people because the rain can penetrate beneath canopies and other protective devices. The third is a combination of building and people: in the past the materials of which buildings were made could absorb water, and during a storm, the surface of a large building would absorb tons of water, water which would be evaporated by the wind once the rain had stopped. Modern buildings are often clad in impervious materials, which keep the water on the surface whence it can be blown off by the wind or can flow down the surface. If an arcade occurs under the face of a building, then the rain descending the face can drip at the edge of the arcade in large drops which are annoying to the people passing under: gutters must be placed in this situation.

Canopies are placed...