Velocity and Pressure of the Wind.
The pressure varies as the square of the velocity or P ∝ V². The old formula for wind blowing against a normal plane was P = 0·005 × V². The latest or Eiffel Tower formula gives a much smaller value, being P = 0·003 × V², where V represents the velocity in miles per hour, and P the pressure in pounds per square foot.
| Velocity. | Pressure on a Sq. Foot. | Character of the Wind. | |||||||
|---|---|---|---|---|---|---|---|---|---|
| Per Hour. | Per Minute. | Per Second. | |||||||
| Miles. | Feet. | Feet. | Lbs. | ||||||
| 1 | 88 | 1 | ·5 | ·003 | Barely observable. | ||||
| 2 | 176 | 2 | ·9 | ·012 | - | Just perceptible. | |||
| 3 | 264 | 4 | ·4 | ·027 | |||||
| 4 | 352 | 5 | ·9 | ·048 | Light breeze. | ||||
| 5 | 440 | 7 | ·3 | ·075 | - | Gentle, pleasant wind. | |||
| 6 | 528 | 8 | ·8 | ·108 | |||||
| 8 | 704 | 11 | ·7 | ·192 | |||||
| 10 | 880 | 14 | ·7 | ·3 | Fresh breeze. | ||||
| 15 | 1,320 | 22 | ·675 | Brisk breeze. | |||||
| 20 | 1,760 | 29 | ·4 | 1 | ·2 | Stiff breeze. | |||
| 25 | 2,200 | 36 | ·7 | 1 | ·875 | Very brisk breeze. | |||
| 30 | 2,640 | 44 | 2 | ·7 | - | High wind. | |||
| 35 | 3,080 | 51 | ·3 | 3 | ·675 | ||||
| 40 | 3,520 | 58 | ·7 | 4 | ·8 | Very high wind. | |||
| 45 | 3,960 | 66 | 6 | ·075 | Gale. | ||||
| 50 | 4,400 | 73 | ·4 | 7 | ·5 | Storm. | |||
| 60 | 5,280 | 88 | 10 | ·8 | - | Great storm. | |||
| 70 | 6,160 | 102 | ·7 | 14 | ·7 | ||||
| 80 | 7,040 | 117 | ·2 | 19 | ·2 | Hurricane. | |||
| 90 | 7,920 | 132 | 24 | ·3 | - | Tornado. | |||
| 100 | 8,800 | 146 | ·7 | 30 | |||||
| 110 | 9,680 | 161 | ·2 | 36 | ·3 | - | “Washoe zephyrs.”[2] | ||
| 120 | 10,560 | 176 | 43 | ·2 | |||||
| 130 | 11,440 | 191 | 50 | ·7 | |||||
| 140 | 12,320 | 205 | ·3 | 58 | ·8 | ||||
| 150 | 13,200 | 220 | 67 | ·5 | |||||
[2] With apologies to Mark Twain.
Fig. 72a.—Angles and degrees compared. It will be observed that an angle of 1 in 4 is practically 14°.
Table of Equivalent Inclinations.
| Rise. | Sine of Angle. | Angle in Degrees. | |||
|---|---|---|---|---|---|
| 1 | in | 30, | ·0333 | 1 | ·91 |
| 1 | „ | 25, | ·04 | 2 | ·29 |
| 1 | „ | 20, | ·05 | 2 | ·87 |
| 1 | „ | 18, | ·0555 | 3 | ·18 |
| 1 | „ | 16, | ·0625 | 3 | ·58 |
| 1 | „ | 14, | ·0714 | 4 | ·09 |
| 1 | „ | 12, | ·0833 | 4 | ·78 |
| 1 | „ | 10, | ·1 | 5 | ·73 |
| 1 | „ | 9, | ·1111 | 6 | ·38 |
| 1 | „ | 8, | ·125 | 7 | ·18 |
| 1 | „ | 7, | ·143 | 8 | ·22 |
| 1 | „ | 6, | ·1667 | 9 | ·6 |
| 1 | „ | 5, | ·2 | 11 | ·53 |
| 1 | „ | 4, | ·25 | 14 | ·48 |
| 1 | „ | 3, | ·3333 | 19 | ·45 |