Expert: Jayendra Upadhye - 6/21/2004

Question
I live in co.usa. I understand the earth is tilted on its axis 23 degrees and we make an eliptical orbit around the sun.Why does the temp.rise in the summer if the earth is actually  
further away from the sun duringthe summer months and the sun is hitting the earth at the same angle all year round? Thank you

Answer
Hi,
Grove, I was under the impression that the earth's orbit is almost a circle! (eccentricity is to be expected, but it is not so pronounced in the case of the earth.
The sun's radiation in "our area" happens to average at 1.4 kilowatt per square meter!
This is also known as the "solar constant" if i remember right.
But you may check me out on this.
What is important to know, is that:
Summer or Winter is percieved depending on what angle the rays hit your locality on an average during 24 hours!
You may find this definition revolutionary or plain enough depending on your outlook.
You see, the earth's 23.5 deg tilt actually makes the sun to "traverse" 47 degrees in totality between the two tropics.(with respect to a fixed point on it's surface,
The sun's rays hit the earth at the same angle, but the axis of the earth presents a different angle to the oncoming rays and hence different areas of the earth recieve varying quantities of "average heat" per 24 day at different times of the year, and thus DO NOT recieve the sun's rays at the same angle all year long)
when it is on the southern tropic (capricorn), its rays hit the northern areas most glancingly over the whole of 24 hours, imparting an average "heating" that is not sufficient to compensate for radiation losses to space over each night.
Without the greenhouse gases, the earth would be a very cold place and in winter every night the earth "tries to reach THAT very cool temperature, and the glancing rays of the sun try to revive it "feebly".This results in what we call as winter.
In summer, the directly overhead blazing sun (at noon), and the generally good angle for recieving direct rays from the sun "throughout the day", help us "beat" the heat loss overnight, resulting in a "summer".
Hope you agree with the above.
In the end it is a simple game of averages.
This is also the reason why in ports, the overall variation is much less than inland as "humid air" tends to retain heat and acts as a "heat storage".
Another way to look at it is to say that the average "daily dose" of heat from the sun is same throughout the year, but the "local dose" at any given area which is part of the constant whole, varies seasonally to produce winter and summer in such a way that summer in north is winter in south! (a zero sum game).
The tilted axis manages this manipulation.
Jayen