Heat during the Day: Answers to Your Questions

[Lukeblackhill]

Hi!

My question is: I understand that at noon-day it’s hotter because of the angle with which sun rays enter our atmosphere. But at the same time, I was wondering that the band of radiation responsible for heating things is infrared, and that at noon basically all bands of lower frequency than blue light are greatly scattered, mainly most of the infrared radiation. So, at sun-set for example, when this infrared radiation is no longer scattered, it wasn’t suppose to be also hot?

 

[russ_watters]

But at the same time, I was wondering that the band of radiation responsible for heating things is infrared...

It's not. This is a common misconception brought on by the common usage of the term. In reality, ir is just the lowest frequency that will transfer heat at common ambient temperatures. All frequencies of EM radiation, when absorbed, convert to thermal energy. Indeed, black body radiation is often called thermal radiation.

...and that at noon basically all bands of lower frequency than blue light are greatly scattered, mainly most of the infrared radiation. So, at sun-set for example, when this infrared radiation is no longer scattered, it wasn’t suppose to be also hot?

In light of what I said above this is largely moot, but I'm still not really following; blue light is what gets scattered by the atmosphere. About half of ir gets absorbed, depending on the humidity.

 

[klimatos]

Luke,

1. Atmospheric temperatures will increase as long as incoming radiation (from both the Earth and the Sun) exceeds outgoing radiation. This mean that daily atmospheric temperatures normally reach a peak in the middle of the afternoon, not at noon [see the meteorology text of your choice].

2. Most of the atmosphere's heat comes from the surface of the Earth, not directly from the Sun [Kiehl & Trenberth, "Earth's Annual Global Mean Energy Budget", Bulletin of the American Meteorological Society, February 1977]. This is why it gets colder as you get farther from the surface.

3. The energy content of a photon, whether solar or terrestrial, is unrelated to the time of day. Incoming solar radiation is less near sunset, because of the lessened average angle of incidence, hence my point #1 above.

4. What you think of as scattering is actually molecular absorption and emission. Only certain wavelengths (the atmospheric "windows") pass through the atmosphere without being absorbed.

5. I suspect a good meteorological textbook will answer many of your questions and clear up some of your confusion.

 

[sophiecentaur]

I understand that at noon-day it’s hotter because of the angle with which sun rays enter our atmosphere.

Not just the angle through the atmosphere but also the angle projected on the ground. There is a Cosine factor which means that the power density of radiation reaching the ground (and, to some extent, the atmosphere) is proportional to the {EDIT: Cosine of the} angle, referenced to the vertical. For an incidence angle of 60° the Power per metre squared is half (Cos 60°) of what it would be at midday near the equator. Most of the whole solar spectrum hitting the ground can be absorbed and re-radiated as near black body radiation with a peak in the infra red. Wien's Law says that the peak wavelength of black body radiation is inversely proportional to the temperature (K). So the peak wavelength of the re-radiated spectrum from the Earth's surface at 300K would be 20 times the peak wavelength (480nm) of 6K sunlight. Well into the Infra red.
But there are so many variables which contribute to the temperatures in the atmosphere.

 

[Chestermiller]

Luke,2. Most of the atmosphere's heat comes from the surface of the Earth, not directly from the Sun [Kiehl & Trenberth, "Earth's Annual Global Mean Energy Budget", Bulletin of the American Meteorological Society, February 1977]. This is why it gets colder as you get farther from the surface.

I think what you mean here is that visible and UV solar radiation from the sun is absorbed at the Earth's surface and is re-emitted as IR radiation. You are not referring to the heat flow from the Earth's core, since that is only a tiny contribution to the heat coming from the surface of the earth.

 

[nasu]

Hi!
So, at sun-set for example, when this infrared radiation is no longer scattered, it wasn’t suppose to be also hot?

Why do you think that the infrared is no longer scattered at sunset?