Why can winds blow down houses?
Wind is a current of air that is moving across the earth's surface.
Wind is a current of air that is moving across the earth's surface. The motion of an object produces energy. The faster it moves, the greater the energy. For example, if you drive a car slowly toward a wall, you may not knock the wall down. However, if you drive at a fast enough speed, you can knock the wall down. By the same token, although the air is very light in mass, with enough high speed, lots of energy will generate to blow down houses and trees. That is, when moving air - wind - is stopped by a surface - the dynamic energy in the wind is transformed to pressure. The pressure acting on the surface transforms to a force. It is the force that can blow down houses.
Based on the calculated result, it is approximately:
A wind with a speed of 20 m/s, there is a pressure of 50 kilograms per square meter.
A wind with a speed of 30 m/s, there is a pressure of 110 kilograms per square meter.
A wind with a speed of 40 m/s, there is a pressure of 190 kilograms per square meter.
A wind with a speed of 50 m/s, there is a pressure of 300 kilograms per square meter.
The wind speed of a typhoon is usually around 40 m/s, which causes about 190 kilograms per square meter. That’s why run-down houses can be blown down.
The conversion of wind scale, wind speed and wind pressure is shown in the following table.
| Wind Scale | Wind Speed（m/s） | Wind Pressure(kg/m2) |
| 0 | 0. to 0.2 | 0 |
| 1 | 0.3 to 1.5 | less than 1 |
| 2 | 1.6 to 3.3 | 1 |
| 3 | 3.4 to 5.4 | 1 to 3 |
| 4 | 5.5 to 7.9 | 3 to 7 |
| 5 | 8.0 to 10.7 | 7 to 14 |
| 6 | 10.8 to13.8 | 14 to 23 |
| 7 | 13.9 to 17.1 | 23 to 35 |
| 8 | 17.2 to 20.7 | 35 to 52 |
| 9 | 20.8 to 24.4 | 52 to 72 |
| 10 | 24.5 to 28.4 | 72 to 97 |
| 11 | 28.5 to 32.6 | 97 to 128 |
| 12 | 32.7 to 36.9 | 128 to 164 |
| 13 | 37.0 to 41.4 | 164 to 206 |
| 14 | 41.5 to 46.1 | 206 to 256 |
| 15 | 46.2 to 50.9 | 256 to 312 |
| 16 | 51.0 to 56.0 | 312 to 377 |
| 17 | 56.1 to 61.2 | 377 to 449 |
| Greater than 17 | Greater than 61.2 | Greater than 449 |
Note 1: The wind speeds listed in this table are equivalent to the average wind speed in 10 minutes.
Note 2: Wind pressure calculation method: wind pressure ≒ 0.0611*(V*C)*(V*C), V is the average wind speed, and C value will be quite different depending on the factors affecting wind speed. C is calculated as 1.4 in this table.
