I have found that turning on all the burners of my stove on a cold winter day makes the kitchen feel moderately warm but putting a pot of water on to boil as well makes it feel much warmer, even if I use fewer burners. Why is that? — PM, Little Rock, Arkansas
When you simply heat the cold air, you lower its relative humidity—the heated air is holding a smaller fraction of its maximum water molecule capacity and is effectively dry. Dry air always feels colder than humid air at the same temperature. That’s because water molecules are always evaporating from your skin. If the air is dry, these evaporating molecules aren’t replaced and they carry away significant amounts of heat. On a hot day, this evaporation provides pleasant cooling but on a cold day it’s much less welcome. If the air near your skin is humid, water molecules will return to your skin almost as frequently as they leave and will bring back most of the heat that you would have lost to evaporation. Thus humid air spoils evaporative cooling, making humid weather unpleasant in the summer but quite nice in the winter.