Hi folks, here’s one which always surprises people: in the Northern Hemisphere, during our winter, we are about 3 million miles closer to the Sun than during our summer (91 million vs. 94 million miles). So why in the world isn’t it warmer in January than July? Well, this is because the Earth orbits the sun on a tilted axis, and during our winter, we are tilted away from the Sun, whereas in summer we are tilted toward the Sun. So it isn’t distance, it’s the intensity of the Sun’s rays which govern our seasons.
Speaking of seasons, here is another question. If December 21st and June 21st are our shortest and longest days respectively, why are they not the times of our average coldest and warmest temperatures?
The answer lies in something called thermal lag, and it is very easy to visualize. If you put a pot of water on the stove and turn on the burner and put your hand in the water, it will be a little while before the water gets hot to the point where you have to pull your hand out. This is because it takes the water a while to heat up. The same happens on Earth, which is 70 percent water. The most intense rays from the Sun occur on June 21st (Summer Solstice), but it takes a little while for the Earth system to respond, so average high temperatures are typically in mid- to late-July.
The same concept holds true for winter. Turn that burner off and it will still be a while before you can put your hand back in the water. So even though the least direct rays strike the Northern Hemisphere on December 21st (Winter Solstice), the Earth system takes a while to fully cool off, thus our average coldest temperature occurs in mid- to late-January.
Ted Shapiro holds the Broadcast Seal of Approval from both the American Meteorological Society and the National Weather Association. An Alexandria, Va. native, he has been Chief Meteorologist at WAGM-TV since 2006. Email him at tshapiro@wagmtv.com.
