When designing roof gutters, or surface drainage for building sites, The rain shadow can make a big difference. Especially for a tall building.

The Plumbing Codes assume rain is coming down at an angle of 2:1.

That is 2 units vertical to 1 unit horizontal. (63.4 degrees).

So from the diagram on the left, you can see the effect of the shadow.

The area of the shadow is **half the area of the vertical face** of the building. (2:1remember).

But what if rain comes from the other direction?

From the diagram on the right, you can see that half the vertical face of the building has been added to the catchment area.

The catchment area is always measured on the horizontal plane. Because that’s how they measure rainfall when they talk about ‘mm’ or ‘inches’ of rain. So to make all our hydraulic formulas work, we must also use this method if we want to use rainfall figures calculated by the local Weather Bureau.

Now consider the interesting case where we have a building on both sides. One side has the shadow, and the other has the added catchment. If the buildings were of equal height, the effects would cancel each other out. No matter which direction the rain came from. Even if the rain came down the middle.

Got your head around that one yet?

However if the rain came from all directions at once, we are in diabolical trouble, and our roof gutter design, or site storm water design would be the least of our worries.