Why 72 degrees feels different in July than in January

Comfort is not just temperature

Your body sheds heat largely by evaporating moisture from your skin. How fast that happens depends on how much moisture the surrounding air is already carrying.

In humid air, evaporation slows and you feel warmer than the thermometer says. In dry air it speeds up and you feel cooler. This is why 72 degrees in a humid July house and 72 degrees in a dry January house are genuinely different experiences rather than a matter of imagination.

Summer: too much moisture

In cooling season the air conditioner is your primary dehumidifier — moisture condenses on the evaporator coil and drains away. That only works if the system runs long enough, which is the sizing problem covered in the oversizing guide.

If the house feels cold and clammy rather than cool and dry, pushing the thermostat lower usually makes it worse: you get colder air that is still humid, and a bigger bill. The variable to address is the moisture.

Winter: too little

Cold outdoor air holds very little moisture. Bring it inside and heat it and its relative humidity falls further. That is why houses get dry in winter even though nothing is actively removing water.

Very dry indoor air produces static shocks, dry skin and sinuses, gaps opening in hardwood floors and furniture, and instruments that will not hold tune. It also feels cooler than it is, which tends to push the thermostat up.

Adding humidity in winter can improve comfort at a lower thermostat setting. It has limits — too much and you get condensation on cold window surfaces, which causes its own problems — so the goal is a sensible range rather than as much as possible.