We are in the heart of winter's frigid clutch, and many portions of the United States have endured some of the coldest temperatures in the past decade. During winter (January 3 to be exact) the Earth, tracing out its elliptical orbit, is actually closest to the Sun. The point of closest approach is called perihelion. So how it can be that our coldest temperatures in the northern hemisphere occur during a time when the Earth is receiving on average 5-7% more solar radiation?

Even though the Earth is closer to the Sun during the northern hemisphere winter, and the Earth receives a larger amount of solar radiation than during the summer (when the Earth-Sun separation is largest), there are three important factors that offset the Sun's closeness...and lead to a cold winter season.

All three are due to the Earth's axial tilt of 23.5 degrees relative to the plane of the ecliptic (the plane which defines the orbit of Earth around the Sun).

First: Because the northern hemisphere is tilted away from the ecliptic, the Sun's rays do not strike the Earth's surface in a perpendicular manner...they are quite oblique. This means the surface is warmed much less efficiently. You can see this effect yourself by going into a darkened room with a flashlight and a piece of paper laid flat on a table. Shine the flashlight beam downward perpendicular, then try shining it so that the beam glances the paper at a shallow angle. You see that the light from the beam is smeared over a larger "footprint" in the shallow angle case, and the intensity of light striking the paper is much less. Another way of expressing this concept is to simply say in the winter, the sun is low in the sky, to the south...the rays are much weaker than during the summer, when the blazing sun is directly overhead.

Second: Because the northern hemisphere is tilted away from the ecliptic, the Sun's rays will pass through a thicker layer of atmosphere, than if the rays came in perpendicular. More atmospheric mass increases the scattering and absorption of solar radiation, and less is transmitted to the surface.

Third: Because the northern hemisphere is tilted away from the ecliptic, the length of daylight is considerably shorter in winter than in the summer. For instance, in Vancouver, Canada, daylight lasts only about 9 hours on December 22, but stretches to over 16 hours on June 22.

The combination of a low sun angle, reduced daylight hours available for surface heating, and a thicker atmospheric path for the solar beam conspire to more than offset the fact that the sun is closer to the Earth in the dead of winter...and every year the big chill wins out!