It’s cold outside and you’re trying to decide how much to bundle up. Should your decision be based on the air temperature, the wind chill or the “feels like” number?

The last one, but don’t get too fixated on it.

As mammals, people need to keep a consistent internal temperature to stay alive but moving air carries away our body heat faster. “Wind chill” estimates were made starting almost a century ago to estimate how quickly our body temperature will lower at a given air temperature with a given wind speed.

There’s no absolute wind chill measurement but is a general agreement on the effect: for example, a 10 mph wind when its 20 degrees Fahrenheit has the same effect as a 0 mph wind when the air is at 10 degrees. This is particularly important for estimating the danger of frostbite, permanent damage to skin from prolonged cold.

But wind chill doesn’t take another factor into effect: Relative humidity. The amount of moisture in the air can affect how quickly heat is carried away from the body, so it is used to estimate a “feels like” temperature.

Again, this isn’t an absolute figure but it gives a general sense of how you should prepare when heading outdoors. You might be fine in shirt sleeves at 40 degrees when it’s still and dry, but if breeziness and dampness make it “feel like” 20, you’re going to regret leaving your coat in the mud room.

All these figures are the flip side of the heat index, which also factors relative humidity and wind speed when estimating the effect of high temperatures on the body. The heat index is basically a “feels like” estimate for high temperatures.

One other point: Wind chill is only relevant for warm-blooded animals that lose the heat they generate internally. It doesn’t matter to inanimate objects like your car engine. If it’s 40 degrees outside, water won’t freeze even if the wind chill is zero.

And what about the old saying that half our body heat is released from our head? It’s not literally true: heat loss from your skull is roughly the same as heat loss from the surface of other parts of the body. However, faces are likely to be exposed to the cold so it’s often true that half of the body heat which you are losing at any given moment does come from your head, since everything else is covered.

And always remember the New England winter maxim: Dress in layers.