When you’re getting too hot under a blanket, why does sticking out even one hand cool you off so much?
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Blankets work not just by capturing warm air, but by presenting a barrier to radiative cooling.
All objects give off heat. If one object gives off more heat than the other, the portion which radiates toward the cooler object heats that object. The smaller the difference in heat, the less heat each object loses to the other.
This is why a car parked under a tree frosts over much later than a car parked under open sky. Heat from the tree (which might be cold, but far less cold than the sky, which is effectively outer space and nearly absolute zero) slightly heats the windshield, the net effect being that the windshiled loses heat slower.
Now, imagine looking through an infrared camera. Black is cold. White is hot.
If you look around your room, anywhere that is black, such as walls, windows and the like, are cold. Heat from your hand radiates toward the walls, but no heat radiates back because the walls are cold.
Now, put a blanket over yourself. Quickly, the blanket heats up until it radiates back as much heat as it receives from your body. The air between its fibers prevents a lot of this heat from radiating away from the blanket, so you stay warm.
When you expose your hand from under the blanket, its heat is now free to radiate toward the wall, and is not opposed by heat from the wall . . . because there is none to receive. Heat from the rest of your body moves to replace heat which radiated away from your hand.
Suddenly, the heat balance you realized under the covers is disrupted, and you lose heat to that little hand sticking out.
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