Your index finger has many uses, but I bet you didn't know a weathervane is one of them.

If you've ever seen someone lick the tip of a finger and stick it into the air, or done this yourself, this is the very reason behind this peculiar gesture. But, while you'll often see people stick their finger in the air as a weather joke, it's actually a legitimate way to estimate wind direction. So the next time you find yourself on a deserted island, Survivor style, or simply without a weather app, here's what to do:

Stand as still as possible. (If your body is moving, it will be harder for you to get an accurate wind "reading.") If you happen to know which way is north, south, east, etc., face this way -- it will make determining the final wind direction easier. Lick the ball of your index finger and point it upwards. Observe which side of your finger feels the coolest. Whichever direction the cool side of your finger is facing (north, south, east, west), that's the direction the wind is coming from.

Why it Works

The reason why your finger feels cool has to do with the speedy evaporation of the moisture on your finger as the wind's air blows across it.

You see, our bodies heat (through convection) a thin layer of air just next to our skin. (This layer of warm air helps insulate us from the surrounding cold.) But whenever the wind blows across our exposed skin, it carries this warmth away from our bodies. The faster the wind blows, the faster the heat is carried away. And in the case of your finger, which happens to be wet with saliva, the wind will lower the temperature even more quickly because moving air evaporates the moisture at a quicker rate than still air would.

Not only does this experiment teach you about evaporation, but it's also a neat way to teach kids about the wind chill and why it cools our bodies down below the air temperature during wintertime.

Don't Use Your Finger in Humid or Hot Weather

Since using your finger as a weathervane depends on evaporation taking place, it doesn't work as well at helping you estimate wind direction on humid or muggy days. When the weather is humid, it means that the air is already filled with water vapor, and so, it will carry away the additional moisture from your finger more slowly; the slower the moisture from your finger evaporates, the less you'll be able to feel the wind's cooling sensation.

This weathervane hack also won't work as well when the weather is hot, sine the warm air will dry your finger before you've had a chance to feel the evaporative cooling sensation.