INDOOR  RELATIVE  HUMIDITIY
 

SUMMER  VERSUS  WINTER – AN  EXAMPLE

Suppose that on a hot, dry summer day the outside temperature is 95°F and the relative humidity is 15%.  Hence, since the saturation vapor pressure at 95°F is 56.2 mb (see Table  1, p. 102 of Ahrens), the actual vapor pressure is 15% × 56.2 mb = 8.4 mb.  Now suppose that the indoor temperature is
70°F for which the saturation vapor pressure is 25.0 mb (Table  1, p. 102).  The indoor relative humidity is therefore 8.4 mb/25.0 mb = 33.6%.

Now suppose that on a cold, damp winter day the outside temperature is 25°F and the relative humidity is 85%.  Hence, since the saturation vapor pressure at 25°F is 4.6 mb (see Table  1, p. 102), the actual vapor pressure is 85% × 4.6 mb = 3.9 mb.  Now suppose that again the indoor temperature is 70°F.  The indoor relative humidity is now 3.9 mb/25.0 mb = 15.6%.

 In locations where winter heating is necessary, indoor relative humidities are almost always much lower in winter than in summer, even in locations such as Utah where outdoor relative humidities are relatively high in the winter and very low in the summer!