It’s all about energy (heat) balance.

First, we need to remember that Heat is simple energy transferred by three different methods. Conduction, convection, and radiation. Our bodies “feel” those methods of heat in different proportions.

It feels heat roughly thru 25% conduction, 25% convection, and 50% infra-red radiation.

Think about how you can stand in the sun on a freezing day and feel warm, and yet be freezing in the shade around the corner. The air hasn’t changed.

But what’s at play, are the surfaces (notably windows) around you – they get cooler (or warmer) as the seasonal temperatures change, and the balance of IR energy shifts (the emit less when they are cold, and more when they are warm).

When am I Comfortable?

There are optimal points where you feel comfortable – with warm surfaces, the air around you can actually be kept a few degrees cooler and you’ll feel fine. Likewise, if the surfaces around you are cooler, you’ll need to crank the air temperature up to compensate.

It’s also why in the summer, your house can feel suffocatingly hot at night, because all the inside surfaces have built up energy over the day and are releasing it, despite the air temperature being cool.

Humidity does play a small role, but only because water droplets can hold more heat energy than air, and give off a bit more IR.

The problem is the thermostat/air temperature in a space has a poor correlation to human comfort. A far better indicator is a black globe thermometer, which takes more account of the heat loss/gain by radiation to other surfaces.

Give me an Example

Example; a new heating system fired up in winter. The thermostat said it was 70 and I should be comfortable. I was COLD because all the walls, floors and ceilings were cold and you were losing heat to them by radiation. It took several days for the structure to warm up and for 70 to feel comfortable.

Winter time; you’re losing heat by radiation to cold surfaces and feel cold at 70. Summer time the same 70 feels warm, all the surfaces have been warmed up.