MIKE MOSS SAYS: Jim, That's a good question, and the answer has to do with a lag time that occurs largely because of the heat storage (and release) capacity of water. During the warmest months, with the longest days and most direct incoming solar radiation, the oceans store up tremendous amounts of heat energy. To a lesser degree, and over a shorter lag time, so do the upper layers of ground surfaces. In both cases, but especially with regards to ocean (and large lake) water, this heat is then released slowly as the days become shorter and the sun's angle declines approaching the winter solstice, partially offsetting the cooling that would otherwise occur more rapidly. Since the increase in day length and sun angle following the solstice is fairly gradual, there is a period of around a month or so in which the net outgoing heat energy lost continues to exceed the net incoming heat energy, especially at higher latitudes where some of our major cold outbreaks originate. During this month, the excess stored energy from the oceans and earth is gradually depleted and we reach a point that is typically our coldest of the year. Beyond that point, the incoming energy from the sun outpaces the energy being lost and we start to heat back up. Then, the oceans, which are now relatively cool compared to land areas, begin to act in the opposite direction, as it takes a lot of heat to warm them back up and they become a cooling agent by absorbing some of the incoming heat. The combination of continued excess incoming solar heat over outgoing radiated heat with gradually warming water bodies then results in the year's warmest average temperatures about a month or so after the summer solstice. In effect, the water bodies act as giant insulators and create something akin to a heating and cooling "inertia" for the earth.
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