Fast winds lower the pressure, and roofs fly

A strong wind can blow the roof off a house. How can moving air do that?

Try it (Activity 6.6). Blow up two balloons of the same size, tie a string to each, and hang them from a stick about 6 to 10 cm apart. Now blow hard into the gap between them.

They swing towards each other! Blowing makes the air in the gap move fast, and that creates low pressure there. The normal, higher air pressure on the outside pushes them together. Blow harder and they come together faster.

So the faster the air moves, the lower its pressure. High-speed winds come with low air pressure.

Now picture a house in a storm. Fast wind over the roof makes low pressure above it, while the air inside the house is at ordinary pressure. So the pressure under the roof is higher than the pressure above it.

If that difference is big and the roof is weak, the roof is pushed up and blown away (Fig. 6.14 a).

That's why it's safer to keep doors and windows open in a storm with very strong winds. Wind then moves through the house too, so the pressure inside drops as well. The difference between inside and above the roof becomes much smaller, and the roof stays on (Fig. 6.14 b).