There's nothing like taking a nice dip in the pool, lake, or ocean on a sweltering summer day to help cool you off. But no matter how hot you may be, it never fails that you dip your hand or foot in the water first to test it's temperature before you commit to jumping in. After all, you're in the mood for a refreshing swim -- not a Polar Plunge!
Why is water always so much cooler than the air temperature?
It's because water has what's called a higher "specific heat capacity" than air, or land. In essence, what this means is that water absorbs a lot of the sun's heat before it begins to warm up. In fact, 1 gram of water must absorb 4.186 Joules of heat energy before it's temperature will rise by 1 degree Celcius (°C). For comparison sake, it only takes 0.800 Joules of heat to raise 1 gram of dry soil 1 °C, and 1.005 Joules to do so for dry air. (If you've ever burned your feet walking on the pier or sand leading up to a body of water then you've experienced how fast heat is moved through these substances. Not so with water. It will certainly warm up, but it won't get anywhere near steaming or boiling hot.)
Water not only absorbs heat more slowly than land, it also releases it more slowly. If you've ever gone for a late night swim and been surprised at how warm the water was to be so long after sundown -- this is why!
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