When is the ocean the warmest?

Dateline article| David Murphy|

by David Murphy

Some people assume that since the Summer Solstice arrives in late June (marking the so-called longest day of the year when we see the most sunlight), the warmest ocean temperatures must occur around this time as well. Actually, on average, the water along New Jersey and Delaware beaches is usually the warmest six or seven weeks later, toward the end of summer. In fact, my favorite time to be down the shore is early September, when businesses are still open, the crowds are smaller, it's not as hot AND the ocean water is often the warmest of the season.

But why is the ocean so warm so late? Shouldn't temperatures be the hottest in late June, during the Summer Solstice?

Here's the deal: As you'd expect, the earth's surface absorbs more and more solar energy as we move through April, May and June. It's also true that by late June, after the Summer Solstice, the daylight hours begin to shorten and less energy from the sun is being sent earthward. But the earth is still holding on to most of that solar energy it received earlier in the summer and cannot radiate or lose all of it at once. It takes solid surfaces like rock, dirt and sand, several weeks to get rid of the excess heat stored up over the previous months. As a result, the hottest average air temperatures occur toward the end of July and the very beginning of August, as that extra stored-up heat is gradually released.

Now, lets move on to the ocean. The ocean takes even longer to radiate its stored heat, because water is slower to react to temperature changes than land. In other words, it takes longer for water to warm up---and it takes longer for water to cool down. As a result, the warmest ocean temperatures tend to occur at the end of August and the beginning of September, because it takes that long for the water to give back all its extra, stored-up energy.

Why is water slower to react to temperature changes than land? Here's the technical explanation: Because water molecules are not as densely packed and are more on the move than the molecules within a solid substance, the heat transfer between the molecules is not as good. As a result, it takes more energy to heat a measure of water than it takes to heat the same amount of solid material. The technical term for this characteristic of water and solids is specific heat. Water's specific heat is greater than a solid's specific heat, because it takes more energy to raise the temperature of a fixed volume of water to a given temperature reading. For those keeping score, specific heat is equal to the amount of energy needed to raise the temperature of one gram of a substance one-degree centigrade.

Summing this up, water has a higher specific heat than a solid, meaning it takes more energy to raise water's temperature. All things being equal, a body of water will warm at a slower rate than an adjacent body of land. And it will also cool more gradually. This is why the ocean temperature usually stays cooler through June, even after we've started to see highs in the 80s and 90s. It also explains why the ocean stays warmer, even into September when the daily highs are slipping back into the 70s.

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