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According to one source, [39] the wind gradient is not significant for sailboats when the wind is over 6 knots (because a wind speed of 10 knots at the surface corresponds to 15 knots at 300 meters, so the change in speed is negligible over the height of a sailboat's mast). According to the same source, the wind increases steadily with height ...
In many cases the length of the unit was not uniquely fixed: for example, the English foot was stated as 11 pouces 2.6 lignes (French inches and lines) by Picard, 11 pouces 3.11 lignes by Maskelyne, and 11 pouces 3 lignes by D'Alembert. [47] Most of the various feet in this list ceased to be used when the countries adopted the metric system.
RVSM was implemented in China at 16:00 UTC on 21 November 2007, and in Mongolia at 00:01 UTC on 17 November 2011. Aircraft flying in feet according to the table below will have differences between the metric readout of the onboard avionics and ATC cleared flight level; however, the differences will never be more than thirty metres.
1 inch per second is equivalent to: = 0.0254 metres per second (exactly) = 1 ⁄ 12 or 0.08 3 feet per second (exactly) = 5 ⁄ 88 or 0.056 81 miles per hour (exactly) = 0.09144 km·h −1 (exactly) 1 metre per second ≈ 39.370079 inches per second (approximately) 1 foot per second = 12 inches per second (exactly)
The troy ounce is the only unit of the system in current use; it is used for precious metals. Although the troy ounce is larger than its avoirdupois equivalent, the pound is smaller. The obsolete troy pound was divided into 12 ounces, rather than the 16 ounces per pound of the avoirdupois system.
The reference wind pressure q is calculated using the equation q = ρv 2 / 2, where ρ is the air density and v is the wind speed. [19] Historically, wind speeds have been reported with a variety of averaging times (such as fastest mile, 3-second gust, 1-minute, and mean hourly) which designers may have to take into account.
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Air, a mixture of oxygen and nitrogen, constitutes a non-dispersive medium. However, air does contain a small amount of CO 2 which is a dispersive medium, and causes dispersion to air at ultrasonic frequencies (greater than 28 kHz). [8] In a dispersive medium, the speed of sound is a function of sound frequency, through the dispersion relation.