Fluctuations of temperature, atmospheric pressure and humidity may introduce a variation of up to 10 % on future wind energy earning estimates.

1) 10 % air density error due to weather (temperature, pressure and humidity) may severely limit accuracy of your wind energy calculations rendering your anemometer accuracy irrelevant.

• Wind energy measurement standards (IEC 61400-121) require recording air temperature and barometric pressure along with wind speed and wind direction.

• Air humidity measurement is highly recommended for locations that regularly have high air temperatures. (Humidity effect on density increases with temperature.)

• Changes in air density can be calculated from their mathematical relationships using this Web air density calculator.

Air Density Change Examples:

• Relative Humidity change from 30 % to 90 % can change air density by 1 % at 31 °C (88 °F). At 22 °C (72 °F) air density change is 0.6 % and 0.4 % at 15 °C (59 °F).

• Air temperature change from winter to summer (5 °C to 32 °C, 41 °F to 90 °F) can change air density by as much as 10 %.

• Elevation height change of only 82 m (270′ vertical feet) from sea level can change air density by 1%. (101325 Pa -100344 Pa = 981 Pa)

• Weather front pressure changes range from 980 and 1050 hPa which corresponds to a 7 % fluctuation in air density.

See our air density calculator for meteorology and wind energy applications: It calculates the impact of altitude, temperature, humidity and barometric air pressure on air density. Alternate version of the calculator can be found here: http://www.wind101.net/air-density/Air-Density-Calculations-Impact-of-Altitude-Temperature-Humidity-and-Pressure.htm

2) Altitude, weather (barometric pressure), temperature and humidity have important influence on wind energy calculations.

Wind Energy is proportional to air density. According to Betz’ law windpower is:

• Power from Wind = 1/2 *ρ *A * V³ (where ρ = air density, A =wind turbine propeller disk area (propeller radius squared times π (Pi =3.14)), V = wind speed)

Air Density depends on Pressure and Temperature according to:

• Air Density =P / (R * T) (thus, as P = pressure decreases due to weather or elevation so does air density (linearly) and inversely as T = temperature decreases air density linearly increases) (R = Universal Gas Constant = 8.3145 J/mol/K)