Published July 7, 2010
It doesn't seem to make sense.
The eastern United States is broiling in a dangerous heat wave, with temperatures in some cities shooting above 100 degrees Fahrenheit (38 degrees Celsius). Yet this week Earth is farther away from the sun than the planet will be at any other time during 2010.
How can that be?
The answer, astronomers say, is that the distance between our planet and the sun has little do with Earth's surface temperature—and therefore almost no bearing on heat waves, blizzards, or other extreme weather.
Word of the Day: Aphelion
Tuesday was Earth's aphelion, when our planet's elliptical orbit took it approximately 94.5 million miles (152 million kilometers) from the sun.
By contrast, at perihelion—when Earth is closet to the sun—our planet will be about 91.4 million miles (147 million kilometers) away.
This difference in distance between Earth and the sun at aphelion and perihelion is relatively small, and it amounts to only about a 7 percent difference in sunlight reaching Earth, explained Ricky Patterson, an astronomer at the University of Virginia.
Heat Wave Encouraged by Earth's Tilt
"Most people think that the seasons are caused by the Earth coming closer and farther away from the sun," but that's incorrect, Patterson said.
Rather, it's Earth's tilt along its rotational axis that has the biggest effect on surface temperature. Right now the Northern Hemisphere is tilted toward the sun, making it summer.
"Sunlight is striking the Northern Hemisphere head on, but only glancingly in the Southern Hemisphere," which is currently in midwinter, Patterson said.
(Kids game: Weather Word Search.)
Overall, Earth's average global temperature is about 4 degrees Fahrenheit (2.3 degrees Celsius) higher at aphelion than at perihelion, he said.
That's because "there is more land in the Northern Hemisphere and more ocean in the Southern Hemisphere, and it is easier and quicker to heat up the land."
Real Heat Wave Culprit: Ridges in the Sky
So if the Earth-sun distance isn't responsible for the current heat wave, what is?
It usually boils down to a combination dry surface conditions, clear skies, and "ridges" of high pressure in the atmosphere, explained David Robinson, a climatologist at Rutgers University in New Jersey.
The ridges can linger in a region for several days, warming the air and clearing away clouds. Fewer clouds means more sunlight reaches the ground, and if the surface is already hot and dry, the extra heat just adds to the misery.
"At first, such a system might not be all that hot when it moves into a region," Robinson said in an email. "However, with each successive day, the July sun warms the air mass and brings on the heat."
Heat waves end when the high-pressure ridges weaken and another weather system pushes them out, he said.
Robinson added that people should resist blaming any individual heat wave on global warming.
"We're talking weather with these systems, not climate," he said. "Any particular heat wave, or even hot summer, cannot be associated with human-induced warming."