The sun is 5 million kilometres nearer; no wonder it feels so hot

By MUNGAI KIHANYA

The Sunday Nation

Nairobi,

25 February 2006

It has been so hot the last two moths that many people are wondering whether the sun has dropped closer to the Earth. Actually, it has! The sun is closer to the Earth in December, January and February than it is in June July and August.

The reason for this is that the orbit of the Earth is not a circle but an ellipse. Thus sometimes it is near the sun (147.5 million kilometres on January 4, 2006) and other times it is farther away (152.6 million km on July 3, 2006). The difference in the two distances is over five million kilometres! However, the resultant change in the intensity of the sun’s radiation is only 7 percent. That doesn’t sound like a lot, does it?

Can an increase of only 7 percent in solar radiation have such a large effect on temperature felt? Probably. Our skin is quite sensitive to heat. If you touch the body of a person with a fever, it feels very hot. Yet a fever raises the temperature by only about two to three degrees (from the normal 37 to 39 or 40 degrees celcius).

When radiation strikes the skin, it is absorbed and heats up a thin layer near the surface. On an average cool day, the skin maintains a temperature of about 37 degrees celcius but it gets warmer when the sun’s radiation increases. A 7 percent increase in solar intensity will raise the surface temperature by about three or four degrees. That will feel quite hot.

Interestingly, the temperatures reported by the meteorological department are not as high as one would expect. Nairobi is peaking at around 27 / 28 degrees celcius, yet it feels as hot as an oven. The reason for this apparent discrepancy is that the weather people measure the air temperature, but what you feel when you go out in the sun is the effect of direct solar radiation on your body. Sometimes it feels as if you are being pierced with needles. But when you go under a shade it feels nice and cool. That’s because the air is not hot.

But the closeness of the sun is not the only factor (and also not the most important one) responsible for the hot weather. The composition of the atmosphere plays a major role in determining how hot the sun will feel. Clouds and moisture (humidity) easily block the radiation thereby drastically reducing the heating effect of the sun.

The amount of land surface area (compared to water) is also a very important factor. Indeed, right now the sun is in the southern hemisphere where there is more water than land. As a result, the average global temperature is actually lower now (when we are closer to the sun) than it will be in July (when we will be further away).

One final thought: The amount of solar radiation you absorbed depends on the colour of your skin. Africans soak up between 70 to 75 percent sunlight while Europeans take in about 50 to 55 percent.  So why did God put the black people in the sunnier parts of the planet? Well that’s a story for another day.