To estimate the energy in wind, let’s imagine holding up a hoop with area

*A*, facing the wind whose speed is *v*. Consider the mass of air that passes

through that hoop in one second. Here’s a picture of that mass of air just

before it passes through the hoop:

And here’s a picture of the same mass of air one second later:

The mass of this piece of air is the product of its density *ρ*, its area *A*, and

its length, which is *v* times *t*, where *t* is one second.

The kinetic energy of this piece of air is

(B.1)

So the power of the wind, for an area *A* – that is, the kinetic energy passing

across that area per unit time – is

(B.2)

This formula may look familiar – we derived an identical expression on

p255 when we were discussing the power requirement of a moving car.

What’s a typical wind speed? On a windy day, a cyclist really notices

the wind direction; if the wind is behind you, you can go much faster than

miles/ hour |
km/h | m/s | Beaufort scale |
---|---|---|---|

2.2 | 3.6 | 1 | force 1 |

7 | 11 | 3 | force 2 |

11 | 18 | 5 | force 3 |

13 | 21 | 6 | force 4 |

16 | 25 | 7 | |

22 | 36 | 10 | force 5 |

29 | 47 | 13 | force 6 |

36 | 58 |
16 | force 7 |

42 | 68 | 19 | force 8 |

49 | 79 | 22 | force 9 |

60 | 97 | 27 | force 10 |

69 | 112 | 31 | force 11 |

78 | 126 | 35 | force 12 |