With the sun directly overhead and clear skies, earth’s surface receives about 1,000 watts per meter² shortwave radiation during mid-latitude summer. If you could convert that into electricity with 100% efficiency, that would power ten 100 watt light bulbs at noon. During winter, that is reduced to 300 watts per meter² on a clear day at noon. A rooftop covered with solar panels can potentially generate a lot of electricity – when conditions are optimal.
Using the AER RRTM model, I calculated the amount of longwave radiation reaching the ground during mid-latitude summer, is 349 watts per meter². About 97% of that is due to water vapor, and 3% is due to CO2.
During mid-latitude winter, the amount of downwelling longwave radiation is 224 watts per meter² – about 89% due to water vapor and 11% due to CO2. CO2 has more effect during winter, because there is less H2O in the atmosphere competing for the same wavelengths of longwave radiation.
During winter, the sun is relatively less important and the greenhouse effect is relatively more important.
Now consider the Antarctic winter. There is no shortwave radiation (blue) and very little H2O (red) in the atmosphere to emit longwave radiation. Thus there is little downwelling radiation – and extremely cold temperatures.
