Expert: Jayendra Upadhye - 12/23/2005

Question
Why does the heat generated by the sun through a window glass seem more intense in the winter?

Answer
Hi,
That is thanks to the green-house effect.
Radiant energy coming in thru the glass (higher than infra red frequencies) is reflected as a lower frequency "heat" radiation by most surfaces recieving sunlight. [all surfaces radiate thermal or infra-red radiation of various frequencies depending on their temperature. Higher the temperature, higher the frequency. At "red heat" this frequency is high enough to shift out of the infra-red entirely and is percieved as a dull red glow by humans. From thereon, it moves up through red to yellow to blue to white as evident in the stars of various surface temperatures].

Since glass is opaque for infra-red rays, this heat remains trapped in glassed in rooms.

That makes you feel that the heat generated by the sun through a window glass seem more intense in the winter.
hope that suffices.
Jayen