Question

What is the dew point dp temperature of air at 5000 ft above sea level at 95 deg.F and 25% rh? Compare this with the dp of air at sea level at the same conditions, explain the difference.

PSYCHROMETRIC CHART 5,000 Feet Barometric Pressure: 24.896 Inches of Mercury COOLERADO 75 65 SATU So -10 75 80 90 100 110 105 DRY BULB TEMPERATURE-F 115 120 ENTHALPY BTL RER ROUND OF DRY AIR dWHINS и ENTHALPY т7 BTU PER POUND OF DRY lIR atpeynze трmтт

Answer 1

From the fundamentals of dew point temperature and relative humidity and approximations related to it,

T_DP T - [(100 - RH) * (9/25))] where T and T_DP are in faranheit and RH in percent, which is actually an approximation made. Based on this, from the given data, T_DP comes out to be 68^o F.

From the psychrometric chart given, it comes out close to around 50^o F, which actually is a major difference but we have to understand that the calculation/approximation which is generally a simpler form of a complex equation which uses various constants and finally formalised. This particular approximation usually validates any problem for RH > 50%. For more accurate readings, using a psychrometric chart is the best possible way.

Coming to the second part of the question, if the relative humidity and temperature are same at ground level and at altitude, the dew point (the temperature at which water starts condensing out) is the same. The air pressure does not affect the dew point if RH and T are the same (except to some very small degree) but at the ground level RH is always bound to change and as a result, there is a change in the T_DP. And this can be clearly understood if we compare the psychrometric charts of both of them (one above sea level and another at the sea level), we can get a clear picture.