What is an approximate formula for converting temperature air at a temperature of water?

Did not need to be scientific, preferably just one lap to estimate the temperature of water in the lake.

Stick a thermometer in the water. The temperature of a lake has very little correspondence with the current temperature. It takes days, even weeks of hot weather to heat a lake and the water temperature may vary depending on currents and location of springs. Your hand is about 80-90 degrees F. I water seems cool, the temperature in the 70s if it feels fresh, it's in the 60s. If your hand is numb quickly, it is within 40 or less. Get a thermometer, a bucket of warm water, add cold water and ice and learn what the different water temperatures envy. Dry your hands and let him return to normal between tests. You'll use the skills for the rest of your life.

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