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Sun's heat on Earth in summer and winter

  1. Sep 30, 2014 #1
    Not sure if this is the right place or if I can even frame this question sensibly. But here goes.

    I notice when driving in my car that the sensation of heat on my skin from the sun shining through my windsreen seems to be different between the seasons. Much hotter in summer versus cooler in winter. This may be my imagination, but it certainly feels less hot standing in direct sunlight in winter than in summer (assuming a cloudless sky).

    My question is, how can this be? Is it my imagination or is there a difference? I am not talking the sensation of ambient temperature, more the effect of the sun on my skin. Logically, or so it seems to me, the sun's rays should feel equally as hot regardless of season.
     
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  3. Sep 30, 2014 #2

    Bandersnatch

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    Providing everything else is equal - i.e., the temperature of air in the car is the same, the sky is equally clear etc. - the heating of your skin will depend on the height of the Sun above the horizon. In Winter, the Sun doesn't go as high as in Summer, and for any given hour it's lower(assuming northern hemisphere above the tropic of cancer).

    The reason for the "lower=less energy" relationship is explained here:
    http://en.wikipedia.org/wiki/Insolation#Projection_effect
     
  4. Sep 30, 2014 #3
    I guess I can't be certain all other things are equal, but I was considering the angle of elevation. It seems to me that for a given elevation, the sun feels stronger, or fiercer, in summer when felt from inside a car. It may be my imagination, but the sun actually LOOKS weaker in winter than in summer.

    I don't think this is the projection effect you describe above, that's considering the effect of the sun's light falling on a surface at different angles. If the sun is at 45 degrees elevation, then it falls on my skin at the same angle regardless of the time of year.

    I was wondering if indeed there IS a difference in solar intensity for a given angle of elevation at different times of the year, or if I am imagining things.
     
  5. Sep 30, 2014 #4

    Bandersnatch

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    Well, do you know if the angle was the same?
     
  6. Sep 30, 2014 #5

    SteamKing

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    You do realize that the earth's axis of rotation is tilted at about 23.5 degrees from the vertical? And that it is this axial tilt which gives rise to the seasons we experience? In other words, in summertime, in the northern hemisphere, the sun's rays are striking the ground at an almost perpendicular angle, and in the wintertime, the angle which these rays make with the earth's surface is something less than perpendicular.
     
  7. Sep 30, 2014 #6

    Bandersnatch

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    Here, have a look at this calculator:
    http://www.esrl.noaa.gov/gmd/grad/solcalc/azel.html

    Try and compare the elevation of the Sun at your location in Winter and Summer months.

    For example, at 55N lattitude, the Sun shines as strongly at noon in December, as it does at 7PM in June.
     
  8. Sep 30, 2014 #7

    olivermsun

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    There are several factors that can influence the intensity of solar radiation you receive on your skin. For example, atmospheric conditions can be a bit different in winter and summer. However, I'm guessing that a major difference in what you feel is due to the ambient conditions. If the you're driving along on a bright sunny day in 85°F weather, then you're feeling the radiation from the sun, the warmth from the surrounded air, IR coming from the windows, etc., etc. The additional feeling of heat from the direct summer is going to make you feel very warm.

    If on the other hand it's 60°F in your car and 25°F outside, you're going to be losing heat to the environment a lot faster, and your face is never going to feel as "heated" as it would in the summer case.

    I don't think so either. You can turn your face directly toward the Sun no matter what elevation it's at.
     
  9. Sep 30, 2014 #8

    Bandersnatch

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    Elevation may not play a role if you start tracking the Sun with the receiver surface, negating the projection effect. But that's no longer "all other things being equal". Still, I can see how it weakens the argument.
    It still does play a role in the sense that the light gets scattered by the increasingly thick column of atmosphere. The Sun at noon and an hour before sunset are two different beasts, all due to the elevation.

    But, if the elevation was in both cases equal, then I'd suspect some atmospheric effects related to lower air temperature or regional climate specifics. But that's something I know very little about.
     
  10. Sep 30, 2014 #9
    I know that overall my question seems remarkably fuzzy, but it's an effect I've observed and often wondered about. I agree that there may be many contributing factors in terms of local conditions and it is likely that it is these that produce the effect. However, I wondered if the sunlight's intensity does differ and if there is an explanation. From what's been said, it seems likely that it doesn't. I am definitely not talking about the axial tilt or how that produces seasons etc.

    Of course this effect I am describing has not been rigorously observed, but I am reasonably confident that the sensation is noticeable with the sun at comparable elevations. There can be little difference in the thickness of the atmosphere if the sun is at the same (ish) elevation regardless of season.

    I wondered about atmospheric conditions as Bandersnatch suggests - for example more humidity in winter or vice versa. But on the whole, I don't think that could be the cause as it's something that is quite consistent whereas atmospheric conditions may not be that consistent.

    But you all must have noticed that the sun just looks and feels weaker in winter than in summer, regardless of where it is in the sky. A sun barely above the horizon in high summer is very hot on the face, a sun at 45 degrees in winter is not. Why? I think that logically it cannot be different and it must be a perception thing related to the points that olivermsun suggests above. But that is quite an effect just the same...
     
  11. Oct 3, 2014 #10
    Since the Earth is closer to the Sun in January than it is in July, a device for measuring solar intensity should show a higher reading during January than during the summer if all other factors are equal. These factors include angle of incidence, atmospheric absorption, atmospheric reflection, etc..

    You are probably feeling radiant heat from other sources--especially the windscreen. Our physical perceptions are easily deceived.
     
  12. Oct 12, 2014 #11

    D H

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    For people who live in the southern hemisphere, this means that the summer sun is a bit more intense than is the winter sun. However, this is a rather small effect, one that is easily overwhelmed by those "other factors." That the winter solstice is in December in the northern hemisphere gives credence to those "other factors" being dominant.


    As a mentor, I can cheat and see that you live in the southern hemisphere, so for you there is a small effect that does indeed make the summertime sun a tiny bit more intense than the wintertime sun. This is a small effect, however. You are more or less imagining things.

    The difference between winter and summer is not that much if you live in the tropics. If you don't live in the tropics, (which you don't; I can cheat), you can't see the Sun at an elevation of 45 degrees in winter. The elevation angle of the Sun at local noon is 90° - latitude + solar declination angle. The solar declination angle varies between -23.44° to +23.44° over the course of a year.

    People who live within the tropics can see the Sun directly overhead. Ancient tropical civilizations such as the Mayans marked the occasions when the Sun was directly overhead with lots of fanfare. Those who live outside the tropics will never see the Sun directly overhead. They will see the Sun reach some maximal elevation at the summer solstice, late June in the northern hemisphere, late December in the southern. Ancient civilizations tended to mark the solstices and equinoxes with lots of fanfare.

    For those who live outside the tropics, solar intensity at local noon at the winter solstice is equivalent to the summer solstice solar intensity at late afternoon to very late evening, depending on latitude.
     
  13. Oct 13, 2014 #12

    psparky

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    The angle of the sun to the earth is why we have summer and winter. In the summer the angle of the sun to the earth in that particular hemisphere is nearly perpendicular.....it absorbs a lot of sun, therefore warmer temperature and high UV on your skin....making the sun feel extremely warm.

    In the winter the angle is large and is anything but perpendicular. The angle of the suns light almost skims across the earth in a nearly horizontal angle, making it very cold....and very low UV on your skin....making the sun not feel...not so warm.

    Shadows are very short in the summer. Shadows are very long in the winter.

    Also, if its winter in the northern hemisphere, it is summer in the southern hemisphere....and visa versa.

    The equator has a nice angle where it is basically almost always summer.
     
  14. Oct 13, 2014 #13

    Integral

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    Here is an experiment for you. Take a square of cardboard and push a pencil through the center of it, now while keeping the pencil normal to the surface of the cardboard. Now point the pencil at the sun and orient until the shadow of the pencil is laying on the pencil only, that is no, or minimal, shadow on the card board. Now observe the shadow of the cardboard on the ground. Compare the size of the shadow in different seasons. As long as the size of the cardboard does not change you are intercepting the same amount of solar energy, the size of the shadow on the ground is the area that amount of energy is distributed over.
     
  15. Oct 13, 2014 #14
    Much as I hate to disagree with such a distinguished and experienced contributor, I am afraid that I must.

    If we keep the angle of incidence the same, the incoming solar radiation is still going to vary from time to time at a given area and from place to place for a given angle of incidence. There are three major reasons for this: 1) Variations in the intensity of solar radiation. From aphelion to perihelion, the intensity of solar radiation at the outside of the atmosphere will vary an average of 6.7%, or some 91 watts per square meter. 2) Variations in atmospheric absorbency of solar radiation. The 24.8% average value represents tremendous variation in cloud cover and density, humidity, and ozone levels. All of these factors vary considerably from time to time and from place to place. 3) Variations in atmospheric scattering and reflection by clouds and other aerosols. The 22.5% average value for these processes also represents substantial variation in atmospheric albedo from place to place and from time to time.

    Hence, even the cardboard's constant angle of incidence does not keep the ground area's incident solar energy the same for all seasons and for all places. If it did, it would make the study of climatology so much simpler.
     
  16. Oct 14, 2014 #15
    Some interesting responses here, with many mistaking my question for the matter of angle of incidence on a surface. My question was nothing to do with that - I understand how the angle of the sun on a surface means that insolation is spread over more or less area.

    I was posing a far simpler question. For the same angle of incidence, why does the sun seem weaker in winter than in summer. Now, this is highly subjective. To test my perception, we'd need some device that measures the intensity of the sun's rays at different times of year with the angle of incidence identical and with weather conditions as near to identical.

    klimatos is observing that the effect I describe may be real. Logic tells me that the intensity of the sun's rays on the earth's surface must be the same regardless of season, but perhaps it is not, as klimatos suggests. I wonder whether there is data that supports that contention?

    That said, I suspect the commenters who suggest ambient conditions contribute to the perception are probably right. If it is cold, my body is giving up heat and so the feeling of warmth offered by the sun is subjectively lessened...

    Integral, my question in terms of your experiment is that the areal coverage remains the same but does the intensity of the energy differ from season to season.
     
  17. Oct 14, 2014 #16

    russ_watters

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    The problem with klimatos's post for the OP is that most relevant factors make the sun's rays more intense in winter than in summer: the earth and sun are closer together in winter than in summer (edit: oops: northern hemisphere bias...) and the sky tends to be much clearer in winter than in summer because the air is drier.

    Furthermore, the angle of the sun makes for more sun exposure for a person at any given time of day when the sun is up in winter than in summer, not less. Why? People are vertical, not horizontal. They expose more area to the sun in winter than in summer.

    The effect the OP is detecting is almost certainly due to differences in ambient temperature and clothing (similar to what oliver said). If it is colder and you are wearing more clothing, you won't feel the sun as hot on your skin than when it is hot outside and your skin is exposed.
     
  18. Oct 14, 2014 #17

    russ_watters

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    Well you can calculate the geometry yourself. To add the influence of weather, you need something like this:
    http://rredc.nrel.gov/solar/old_data/nsrdb/
     
  19. Oct 14, 2014 #18

    NTW

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    I have often wondered about this... It is evident that, for the same altitude of the sun, the perception of solar radiant heat, in the face, for example, is very different in summer and in winter. In sunny winter days, the atmosphere is often much more transparent than in most summer days, but the fact is that the sun on the face is welcome in winter, and usually avoided in summer days. For the same altitude of our star...

    The explanation is probably related with the perception of heat by the skin, that may be strongly influenced by the surrounding air temperature...
     
  20. Oct 14, 2014 #19
    russ_watters, I must not be expressing myself well enough. Geometry has nothing to do with it, as far I can see. Forget that. I am talking of an instant in time, sun same place in the sky. The difference is in the time of year. NTW gets what I am saying. NTW, I particularly noticed this while inside a car, in which case the ambient conditions are WARMER in winter and colder in summer which would lend credibility to the likelihood it's a perception thing, although this would mean that the perception is based in a real physical effect...
     
  21. Oct 14, 2014 #20
    Winter is wamer than summer??

    Anyways, here's what I think
    There is a phychological, physiological, and physical aspect to what you describe.
    1. Physical - already discussed in that your skin will radiate and conduct heat to the outside world at a rate dependant upon the temperature differences.
    2. Phychological - if you are already feeling hot, any increase in heat directed towards your skin will make you more uncomfortable. You, or your brain, could interpret that as the heat source being hotter than it actual is. The converse, when you are feeling cold, you may welcome heat input.
    3. Physiological - your heat sensors do not actually register absolute temperature per se, but change in temperature.

    see,
    https://www.physicsforums.com/threa...why-do-fans-cool-you-off.754579/#post-4752839
    which also adds in another feature of interpretation of heat by your brain, which is the amount of skin exposure and number of sensors sending signals to the brain. In summer, less clothing; winter, more skin covered up.

    Hopefully this adds another dimension to your query for discussion.
     
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