# why the sun is hotter at mid day

Discussion in 'Physics' started by samjesse, Oct 19, 2009.

1. ### samjesse Thread Starter Active Member

Sep 14, 2008
202
0
Hi

Why is the sun hotter in the middle of the day than the morning or the evening?

I have a problem buying the argument about the distance difference, which says that the sun is closer to the earth surface that is at 90 degrees to its rays than the surface which is at less degrees. why? because the difference is negligible considering how far the earth from the sun.

I have a problem understanding the other argument that says the rays cut the atmosphere at a 90 degree and thus is less resistive to heat that another part of the atmosphere which is cut at a less angle. why? because the atmosphere is a gas and the gas does not have a axis to reference a degree of 90 from.

thx

2. ### steveb Senior Member

Jul 3, 2008
2,433
469
Basically, the atmosphere (in particular the greenhouse gas component) is very good at absorbing infrared waves that we feel as heat. Check out the attached plot. Heat-felt infrared is between the optical window and the radio window. The steep angle of the sun at midday allows the maximum amount of infrared to reach you, and you percieve this as heat. In the morning and the evening, the light must pass through a much greater distance of atmosphere, hence more heat is absorbed in the atmosphere, which never reaches you.

By the way, this is the reason for the greenhouse effect. Not only the infrared energy is absorbed. Also, visible light rays pass through the atmosphere and hit the earth. The earth absorbs these rays and reemits infrared light back out toward space. However, the atmosphere then absorbs much of this energy. I added another plot which shows the contribution from the individual greenhouse gases.

File size:
18.3 KB
Views:
52
• ###### greengas.gif
File size:
11.1 KB
Views:
60
Last edited: Oct 19, 2009
3. ### samjesse Thread Starter Active Member

Sep 14, 2008
202
0
So a cold mirror will make the object "behind it" heat up with out transmitting much light, and the object "in front of it" visible with out heating it.
A hot Mirror will do the opposite.

So, to light up my workshop with out getting it too hot in the summer. If I install hot mirrors on the roof, that will give lots of natural light and keep the staff cool.

4. ### beenthere Retired Moderator

Apr 20, 2004
15,815
283
Could you expand on -
- especially as to the definition of "hot mirror' and "cold mirror"?

5. ### KL7AJ AAC Fanatic!

Nov 4, 2008
2,047
295
It's not the distance to the horizon that matters, as you stated the difference in distance is negligible in the big picture. However, the distance the radiation has to travel through the admosphere is vastly different for vertical incidence than it is for glancing angles!

Eric

6. ### steveb Senior Member

Jul 3, 2008
2,433
469
I'm afraid I'm not following you. I'm not sure how we got from atmospheric gasses to hot and cold mirrors. Also, I don't understand what physical situation you are describing.

What I can say is that if you use a special material that passes optical light and reflects infrared (heat) waves, and install these on your roof, you will have natural light and will be cooler than if you didn't reflect the infrared away. However, if you have no ventilation and good insulation, it's still going to get mighty hot in your building. The light rays will be absorbed in the room and infrared will be reemitted off the floors and tables. The absorbed power is heat that heats the room and the reemitted infrared waves can't get out of the room because your roof reflects them back in. Ultimately just about all of the incoming light power is converted to heat your room up. The temperature rises to the point where the temperature differential between the inside and the outside creates enough heat flow out of the building to compensate for the incoming optical power. Your staff will then sue you for maltreatment.

7. ### Mike2545 Active Member

Mar 26, 2009
116
3
The sun does not change temperature, we feel the radiation more when it does not have to cut through as much atmosphere, so the shortest distance from you to outer space is directly up from where you are.

You should also investigate why there are red sunsets and sunrises, if you don't believe the whole distance/atmosphere conspiracy

8. ### studiot AAC Fanatic!

Nov 9, 2007
5,005
515
It's not distance it's area that counts.

Consider any tube of light energy.

If it fall squarely onto any surface it forms a light circle of area A.

If it falls obliquely onto any surface it forms an light ellipse of area greater than A.
The more the obliquity the greater the excess area.

Since there is constant energy flux passing through the tube, the energy density ( = constant / area) is maximum when the sun is overhead and decreases the more the rays slant.

Draw a sketch.

9. ### steveb Senior Member

Jul 3, 2008
2,433
469
This is a good point that has been left out of the discussion so far. However, it may not be fair to say that it's not distance that counts. I think that both are important.

This point reveals two possible interpretations of the original question. The OP asks why the sun is hotter at mid-day. We could interpret that to mean two possible things as follows.

1. Why does it feel hotter. (which is how I originally interpreted it)
2. Why does it heat things up more effectively.

If we take the first case, then distance is the more critical part of the answer because, unless we are laying down, our standing or sitting bodies face more surface area toward the rising or setting sun, than at midday. Yet, we somehow feel that the sun is cooler when it strikes us. This is because the greater distance of atmosphere absorbs more of the infrared light that we feel as heat.

If we take the second case, then area is more critical because any flat laying object, most notably the ground, faces less area toward the sun, and hence less energy is absorbed locally. This also relates to solar cells. If solar cells do not face directly at the sun, the effective power absorbed is proportional to the cosine of the angle of incidence.

10. ### smanches New Member

Mar 27, 2009
4
0
Don't forget the refraction of the rays due to the high angle of incidence during early and late daylight. This also reduces the amount of light hitting the surface during these periods.

11. ### KL7AJ AAC Fanatic!

Nov 4, 2008
2,047
295
It just takes him that long to get warmed up!

12. ### semiopticalcat New Member

Jun 21, 2009
1
0
Refraction actually bend the ray downward and make the sun appear higher for observers on the ground. There is no reflection due to atmosphere because there is no well defined boundary. Therefore it increase the amount of light hitting the surface slightly.

13. ### Robin Mitchell Well-Known Member

Oct 25, 2009
738
207

Think about it as a concentration.
Here, each light beam is the same size and strength. the light beam in area A and C covers a larger area than B. So, the energy has to be spread in a larger area, and thus each square area (like 1 by 1 inch) has less energy.

14. ### Forrest M. Mims III New Member

Aug 31, 2007
4
1
You can better understand why sunlight produces more heat at solar noon by
measuring direct sunlight with a solar cell over a day. Every 15-30 minutes, record the current (not the voltage) produced by the cell when it faces the sun. If you plot the results, you will get a bell-shaped curve if no clouds were near the sun during each measurement. The peak of the curve will be solar noon.

Solar noon is when the sun is at its highest point in the sky. If the sun is straight overhead at the zenith (only possible within the tropics), its rays pass through one air mass (or m, the vertical thickness of the atmosphere). When the sun is away from the zenith, the air mass through which its rays pass increases. The approximate formula is air mass (m ) = 1/sin of the sun's angle over the horizon. The air mass formula must be corrected for refraction and other effects as the sunlight passes through more than about m = 3.

Various wavelengths of sunlight are strongly absorbed by water vapor and oxygen. Ozone absorbs both UV-B and some of the visible spectrum (look up Chappuis band). Particulate matter (aerosols) scatter and absorb sunlight. These include smoke, haze, dust, pollen, etc.

I hope this helps.

Forrest M. Mims III
www.forrestmims.org