WPS 264 Humidity and pressure

Humidity

• amount of water molecules in gas phase

Determining humidity

• Wet bulb dry bulb hygrometer:

  • 2 thermometers, 1 dry, 1 with cloth sock on bulb which kept wet from tube of distilled water

  • Wet bulb temp drops due to evaporative cooling

  • Temp difference give humidity

• Errors:

  • contamination of wick from tap water or dirt (large positive error)

  • too rapid flow of water around sensor (large negative error)

Atmospheric pressure & RH with altitude

• p_atm inversely proportional to altitude

• RH directly proportional to altitude

Carrier equation

• p = p_0w - [(p_atm - p_0w)(T_d-T_w)] / (1546 - 1.44T_w)

• RH = p / p_0d * 100

• p_0w is saturated vapour pressure at wet bulb temp, T_d is dry bulb temp, T_w is wet bulb temp, p_0d is saturated vapour pressure at dry bulb temp

Dewpoint

• temp at which water in air starts to condensate

• p₀ = existing vapour pressure

• indep of altitude

• T_dew < T_d always

• T_dew = T_d only at H = 100%

• H can be determined from dewpoint & T_d