Hi,
I have a dispute with my son. I feel the sun burn on my head much more when it’s directly above me than when I look straight into the sun at night. He says that the illuminated surface is then larger and the radiation is distributed over a larger surface. But that refers to the temperature of the Earth’s surface. But in the evening the sun shines straight into my face and there is no dilution of the radiation by a large surface. I think it depends on the distance the sunlight travels through the atmosphere: it is greater at night. More absorption then takes place. True or False? Or does my face cool down more during the radiation because the environment is cooler and I feel the radiation less?
Answer
Dear Harry,
The fact that you “feel the sun burn much less” in the evening is largely related to the fact that the sun is a lot lower then (than in the middle of the day for example). As a result, the incoming amount of radiation (from the sun) per surface is smaller. If we assume a standard situation (without clouds, no absorption by the atmosphere…etc.) then the amount of energy that reaches us is equal to: 1368 W/m^2 * sine (height of the sun). A few examples:
- If the sun is low on the horizon (rise/sunset), e.g. 10° (an angle of 10° between the horizon and the sun, from our perspective): 1368 * sine(10°) = 238 W/m^2 (238 Watts per square meter)
- When the sun is at its highest in Belgium (approx. 63° at noon around June 20-22): 1368 * sine (63°) = 1219 W/m^2 (more than 5x as much as in the first example!)
- If the sun is directly overhead (only possible in the tropics, between the two tropics, at a specific time of the year) (height of the sun is then 90°): 1368 * sine(90°) = 1368 W/m ^2
A large part of the explanation therefore lies in the height of the sun, but other factors (such as the cooling you mentioned due to the environment or, for example, the wind strength) will of course also influence your “sense of warmth”.
I hope this answered your question!
Kind regards,
Harry
Answered by
dr. Harry Zekollari
Glaciology and Climatology
Avenue de la Plein 2 1050 Ixelles
http://www.vub.ac.be/
.