Answer to "seasonal temperatures" question

SEASONAL (AND DAILY) TEMPERATURE VARIATIONS -- Jared Pratt of Idaho Falls, ID wanted to know. "Why is the weather hotter after summer solstice?" -- 7/3/00

To answer this, one has to look at the daily temperature cycle, as well as the seasonal one.

Consider the following. You've just put a covered pot of water on the stove and turned on the burner to HIGH HEAT. The water temperature warms. Now you turn the burner down (not off), but still allow the pot to sit on the stove. What happens to the temperature of the water?

It will likely still go up, although not as quickly, because there is still some amount of heat being added by the hot burner (even with flame or electricity turned down). Eventually some type of equilibrium will be established where heat added equals heat leaves. If the burner is now turned down still lower, the pot will start to cool, even though some heat is being added.

The same holds true for the atmosphere (and the oceans). The Earth (air and ocean) system is constantly losing energy to space via radiation. Yet, energy gain is linked to daylight hours only (sunshine). Early in the morning and late in the afternoon, more energy leaves than is replaced by solar heating. So, temperatures near the ground fall. By an hour or so after sunrise, the balance is tipped in favor of more incoming energy than outgoing. Hence, the temperature rises. This rise in temperature occurs even during the afternoon, for several hours AFTER the sun is highest in the sky.

Seasonally, things work the same way. Now, however, one has to consider the heat balance over the course of a full 24 hour period (on the average), instead of just during daylight hours. At the summer solstice, the sun reaches its highest position in the sky. However, the length of daylight compared to darkness, coupled with the amount of energy (sun angle) received vs lost over 24 hours, remains positive. In fact, this relationship lasts for about 6 weeks after the summer solstice for the atmosphere, and about twice as long for the oceans. Thus, just as the pot on the stove, the heat loss is not stopped immediately upon a certain time, but merely slows down before eventually reversing. Meteorologists refer to this a temperature lag.

For air temperatures, this means the hottest part of the year is often late July - early August. For hurricanes (largely linked to ocean temperatures), peak storm period is well into September.

Obviously, a similar lag holds true for the winter solstice in late December. The coldest weather often occurs in late January - early February.

Of course, all of this an "average". El Nino's, La Nina's, and other unusual weather patterns can affect temperatures in any given year.

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