Winter is coming to a close here in Pennsylvania and signs of spring are popping up everywhere. However; we are now in for the worst season of PA – the transition season. During this season it is easy to be lured into a sense of joy by the 75-degree days only to be hit with 45 and rainy the next. These 45-degree rainy days are not the cozy dreams, as seen in movies, but ones that chill you to the bones to a greater extreme than any snowy day could strive for. Though I am bias from my love of skiing, I believe that a 20-degree day with snow is far more comfortable than the rain. Since, ACT works in the HVAC industry and is an expert to all things thermal we are going to evaluate why 45 and rainy feels colder than 20 and snowing.
First, we will start with humidity. Humidity is the amount of water vapor present in the air. As the temperature of the air drops the amount of water it can hold lessens. This means that the warmer the air the more moisture it can hold. So, a 45 degree and rainy day will hold more water vapor than a 20 degree and snowy day.
So why does the water vapor in the air make a difference? For that we will have to look at the heat transfer properties of Water (H2O) vs. Air (21% O2, 79% N2). The thermal capacity of water is equal to 4.187 kJ/kg-K. The thermal capacity of air is equal to 1.004 kJ/kg-k. This means as air touches your skin it will absorb 1.004 kilo-joules of your body’s thermal energy per kilogram of air for every 1 degree Celsius it rises until both you and the air are at an equal temperature. Whereas water will absorb 4.187 kilo-joules of your body’s thermal energy per kilogram of water for every 1 degree Celsius. Since this is per kilo-gram of the molecule, we need to calculate out the true difference based on molecular density. Pure water vapor is roughly 62% the density of air. This means that as air pulls 1.228 kJ/m3-K (see calculation below), water will pull 3.436 kJ/m3-K. This means that on a rainy, 40-degree day you will experience about 2.8 time more heat loss due to the thermal properties of water vapor than on a cold, 20-degree, snowy day.
This proves the point that, although April shower bring May flowers, they also bring a colder feel to air.
Molecular Density of Air: 1.225 kg/m3
Molecular Density of water vapor: (0.62*1.225) = 0.75kg/m3
Amount of thermal energy absorbed by volumetric unit (air): 1.225kg/m3*(1.004 kJ/kg-K) = 1.228 kJ/m3-K
Amount of thermal energy absorbed by volumetric unit (water vapor):0.75 kg/m3*(4.187kJ/kg-K) = 3.436kJ/m3-K
Borgnakke, Claus, and Sonntag, Richard E. “Fundamentals of Thermodynamics”, Seventh Edition, John Wiley & Sons, Inc. 2009.