Sun's heat on Earth in summer and winter

[Graeme M]

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.

 

[Bandersnatch]

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

 

[Graeme M]

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.

 

[Bandersnatch]

Well, do you know if the angle was the same?

 

[SteamKing]

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.

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.

 

[Bandersnatch]

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.

 

[olivermsun]

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.

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 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 don't think so either. You can turn your face directly toward the Sun no matter what elevation it's at.

 

[Bandersnatch]

I don't think so either. You can turn your face directly toward the Sun no matter what elevation it's at.

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.

 

[Graeme M]

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...

 

[klimatos]

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.

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.

 

[D H]

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.

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.

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.

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.

 

[psparky]

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.

 

[Integral]

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.

 

[klimatos]

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.

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.

 

[Graeme M]

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.

 

[russ_watters]

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.

 

[russ_watters]

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?

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/

 

[NTW]

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...

 

[Graeme M]

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...

 

[256bits]

... inside a car, in which case the ambient conditions are WARMER in winter and colder in summer , ..

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.

 

[russ_watters]

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...

The link I provided has data for the US that shows for a given sun elevation, the sun is more intense in winter than in summer due to the lower humidity. But since you are likely wearing more clothing in the winter and the air temperature in your car and the surfaces in your car are lower, you don't feel it as much.

 

[psparky]

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..

Not buying this. I mean, yes I understand the angle, but the sun feels week in late fall, winter and early spring.
What evidence do I have to support this? Well, the average UV index in the summer is around 8 or 9 during peak times...
And in the winter the average UV index is around 1 or 2 during peak times of the day as well.

Also, one can lay on a blanket in summer and get "flat on" diret rays.
Are you saying one can get more of a sun tan in the winter by standing outside for a half hour in 32 degree weather with a 1 UV index?
Or would I get more of a suntan laying on a blanket on a 90 degree day with full 9 or so on UV index?

Now I do agree you can get sun burn from snow skiing. Especially with the snows reflective properties. But the UV rays are way down.
Again, compare a tan in the winter slopes...compared to a hot day on the water while in a boat.
People nearly die from sun stroke on boats all the time.

 

[psparky]

Heres a link supporting UV index.

http://www.cpc.ncep.noaa.gov/products/stratosphere/uv_index/gif_files/msy_13.png

And that's Louisiana...In Ohio for example, that curve gets much steeper.

 

[russ_watters]

Not buying this. I mean, yes I understand the angle, but the sun feels week in late fall, winter and early spring.
What evidence do I have to support this? Well, the average UV index in the summer is around 8 or 9 during peak times...
And in the winter the average UV index is around 1 or 2 during peak times of the day.

UV index is based on the very geometry issue we are discussing. Sunburn is a huge issue for skiers, despite a UV index of only 20% of summer's.

. Are you saying one can get more of a sun tan in the winter by standing outside for a halfhour in 32 degree weather with a 1 UV index?

I don't know the mechanism for sunburn: I don't know if temperature and sweat matter.

 

[psparky]

UV index is based on the very geometry issue we are discussing. Sunburn is a huge issue for skiers, despite a UV index of only 20% of summer's.

Correct. Inexperienced skiers get sunburned because they think they can't get sun rays in winter. So they put no sunscreen on and stay out on a sunny day for 8 hours. Plus the doubling up of the reflective snow. 8 hours at 1 or 2 UV on reflective snow is going to burn someones face...just like a an hour in the sun in the summer in the high UV's.

 

[Graeme M]

That's interesting about the UV data etc, I haven't had a chance to look at your link russ_watters but hope to make time tonight. 256bits, the reason I said it is warmer in winter and colder in summer in my car is because in winter my heater is on and in summer my aircon is on cold. I am pretty sure I prefer the interior colder in summer than I prefer it to be in winter. So my skin surface may be warmer in winter than it is in summer when driving.

 

[psparky]

That's interesting about the UV data etc, I haven't had a chance to look at your link russ_watters but hope to make time tonight. 256bits, the reason I said it is warmer in winter and colder in summer in my car is because in winter my heater is on and in summer my aircon is on cold. I am pretty sure I prefer the interior colder in summer than I prefer it to be in winter. So my skin surface may be warmer in winter than it is in summer when driving.

How about when you leave your car in the sun with the windows closed on a sunny hot day...90 degrees hot and humid.

Is it warm? Yes, about 130 degrees or more inside when you first enter. Glass has a super magnifiying effect.

Heck, a police dog just died in my area when the officer left him inside the car on a 70 degree day that was mostly overcast. The dog was inside the hot car for 2 hours...dead from heat exhaustion. Just imagine when you leave a child in a car on even a 60 degree day.

Also, the distance from Earth to the sun has little to do with nothing. The distance to the sun is so far...that the slightly closer effect doesn't mean anything since it is freezing in the winter. However, the angle of the suns rays to the Earth's surface is everything.

 

[NTW]

Forgetting the car, because it introduces undesirable and complicated variables, the fact is that -for the same sun altitude- the heat sensation in the face is far stronger in summer than in winter. And, that can't be explained but as an effect of the air temperature in contact with the skin, since -in clear days- air is more transparent, and more dry than in the summer, and -IIRC- air humidity absorbs strongly in the infrared. But this higher transparency is more than compensated by the air temperature, sensation-wise, I mean...

 

[psparky]

Forgetting the car, because it introduces undesirable and complicated variables, the fact is that -for the same sun altitude- the heat sensation in the face is far stronger in summer than in winter. And, that can't be explained but as an effect of the air temperature in contact with the skin, since -in clear days- air is more transparent, and more dry than in the summer, and -IIRC- air humidity absorbs strongly in the infrared. But this higher transparency is more than compensated by the air temperature, sensation-wise, I mean...

Agreed.

 

[klimatos]

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.

Russ,

1) Most of the northern hemisphere is cloudier in the winter than it is in the summer. The same thing is true for the land areas of the southern hemisphere. I have thousands of climatic data charts at my disposal, and a quick scan shows that cloud cover is higher during the low-sun period than during the high-sun period for most of them. The exception is in the wet-and-dry tropics with their high-sun rainfall. In contrast, the Mediterranean Subtropics (where I live) is notorious for its "winter rain and summer drought".

2) Verticality is certainly a factor in personal comfort in intense sunshine, but if you are equally vertical for equal angles of incidence of solar radiation, then the incidence per unit area of skin surface will not change. I don't think people are more vertical in the winter than they are in the summer. And the total duration of low sun angles does not significantly change from one season to another. The amount of time it takes the sun to move through a give angle of arc is essentially a constant for any particular latitude.

3) As to exposing more skin to the sun in the winter than in the summer, I follow the cowboy rule. I don't expose any more skin to the sun than is absolutely necessary at any time of the year. The old joke is that a real cowboy will put on his hat just to look out the window! I used to live in the Near East. I noticed that I never saw a local in a short-sleeved shirt. The hotter it is, the more skin area they cover. And they have lived in that desert climate for a long, long time.

 

[psparky]

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...

So the sun is in the same place in the sky, but at different times of the year.

That is impossible. The Earth orbits around the sun. The sun is not in the "same place" from day to day and certainly changes drastically from season to season. The angle of the sun's light to the Earth has everything to do with it.

Please name an instance where the sun is in the same place in the sky at different times of the year.

Please, I beg of you.

 

[NTW]

So the sun is in the same place in the sky, but at different times of the year.

That is impossible. The Earth orbits around the sun. The sun is not in the "same place" from day to day and certainly changes drastically from season to season. The angle of the sun's light to the Earth has everything to do with it.

Please name an instance where the sun is in the same place in the sky at different times of the year.

Please, I beg of you.

Well, -for example, and forgetting refraction- for an observer at -say- 40º N latitude, the apparent altitude of the highest point of the celestial equator is 90º - 40º = 50º. Now, the declination of the sun varies, along the year, between +23,5º and -23,5º. Thus, at its lowest possible declination, on Winter's solstice, the sun reaches an maximum apparent altitude of 50º - 23,5º = 26,5º. It is obvious that -in that day- the sun can be observed, at a given moment, at any altitude between zero and 26,5º. Well, in fact, two times that day for any given altitude comprised between 0º and 26,5º

In the other 364 days of the year, the sun can also be observed, and twice a day, at any altitude reached by the sun in Winter's solstice.

 

[Baluncore]

I believe it comes down to the length of the path through the atmosphere. In winter and at dawn or dusk the sun is close to the horizon and more energy is absorbed over the longer path through the atmosphere. In summer at noon the path can be much shorter and so less scattering or absorption takes place. I notice that moving a few thousand feet above sea-level makes a noticeable difference to the intensity, also crops that mature in late summer often do better at higher altitudes. That is consistent with atmospheric path length determining intensity.

 

[DaveC426913]

...the ambient conditions are WARMER in winter and colder in summer...

You sure about that? What would you say is the actual temperature in your car, even with the heater on, in the winter? You think it's going to be 70F in your car in the winter? You'd be tearing your clothes off and cracking the window.

What about in the summer, with the AC on? You think it'll be colder than your car is in the winter?
BTW, note that there is more at-play than conduction with the ambient air. The windows of your car are radiating huge amounts of energy at you in the summer (yes, even with the AC on). In the winter, they are effectively radiating cold (actually, drawing heat off your body that is not being returned). Touch the window - or even get near it - in winter and again in summer. You will feel the diff.

Also, do not dismiss humidity. Dry air in the winter means insulation from cold. Humid air in the summer means more conduction of heat from air to skin.

So, my point is: it is virtually impossible to set "all other factors equal", as everyone keeps saying.

 

[NTW]

I believe it comes down to the length of the path through the atmosphere. In winter and at dawn or dusk the sun is close to the horizon and more energy is absorbed over the longer path through the atmosphere. In summer at noon the path can be much shorter and so less scattering or absorption takes place. I notice that moving a few thousand feet above sea-level makes a noticeable difference to the intensity, also crops that mature in late summer often do better at higher altitudes. That is consistent with atmospheric path length determining intensity.

But that path is the same with the same elevation, and any elevation lower than that of the sun at noon in the winter's solstice is reached twice a day in any other day of the year, if at different hours, of course... There are only two places on the Earth from which, within the year, you never observe the sun with the same elevation, and those places are the north and the south poles... Starting from a pole, and if you begin moving your observatory to the equator, you will have a growing number of days of the year with the sun reaching twice in that day the same altitude. And, from the artic circles to the equator, that number of days is 365...

 

[Graeme M]

I am surprised about the number of comments discussing the elevation of the sun in this question. It seems perfectly simple to me. Of course at noon in summer and noon in winter the sun is at a different elevation, and indeed even a different position relative to a fixed point at my location.

That wasn't what I meant, nor did I mean anything especially exact. I was simply saying that for an apparently similar elevation, say midday in winter vs maybe 9 AM in summer (again, don't take that as an exact statement) the sun feels weaker in winter than in summer. It even LOOKS weaker.

I am not saying that is actually the case, I was musing whether it is or is not, and what is the explanation for the perception if it is not.

I agree that there are many variables, even more than I'd have thought of.

Dave C, my point about conditions in the vehicle are again not to be taken so literally. In terms of my skin, in a car in summer with the A/C on strongly, my skin will be cool to the touch. In winter, with the heater on, it will be warm to the touch. I think then that similar amounts of insolation will 'feel' different - my colder skin will I assume absorb more energy and heat up more, than my already warm skin will in summer.

I do take your point about the windows though, I definitely hadn't thought of that.

 

[olivermsun]

The windows of your car are radiating huge amounts of energy at you in the summer (yes, even with the AC on). In the winter, they are effectively radiating cold (actually, drawing heat off your body that is not being returned).

I do take your point about the windows though, I definitely hadn't thought of that.

;) See some of the early replies in the thread:

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.

You are probably feeling radiant heat from other sources--especially the windscreen.

 

[klimatos]

So the sun is in the same place in the sky, but at different times of the year.

That is impossible. The Earth orbits around the sun. The sun is not in the "same place" from day to day and certainly changes drastically from season to season. The angle of the sun's light to the Earth has everything to do with it.

Please name an instance where the sun is in the same place in the sky at different times of the year.

Please, I beg of you.

On the equator, the Sun rises directly in the East and sets directly in the West every day of the year.

 

[NTW]

On the equator, the Sun rises directly in the East and sets directly in the West every day of the year.

Well, no... Not every day... Only two days...

For an observer on the equator, the sun rises exactly in the East and sets exactly in the West only in two days of the year, precisely in the equinoxes of spring and fall. Why? Because, in the equinoxes, the declination of the sun is precisely zero, and its apparent trajectory coincides with the celestial equator. In the rest of the year, the sun rises and sets a bit to the North or to the South, but not much, as the magnitude of that 'deviation from the east/west' reaches a maximum of 23,5º on the days of the solstices. In one of them (summer solstice for the northern hemisphere) the sun rises at a point 23,5º to the north (measured from the east), and sets at a point 23,5 to the north (measured from the west). On the other solstice, a similar thing same happens, but with the sun rising at a point 23,5º to the south (measured from the east) and setting at a point 23,5º to the south (measured from the west).

It sounds complicated in words, but is really very easy: the sun wanders in the south celestial hemisphere during one-half of the year (when its declination is negative) and in the north celestial hemisphere during the other half, when its declination is positive...

 

[Graeme M]

Fair point Olivermsun, I did tend to just scan some of those earlier responses and missed those particular statements. :)

 

[psparky]

I believe it comes down to the length of the path through the atmosphere. In winter and at dawn or dusk the sun is close to the horizon and more energy is absorbed over the longer path through the atmosphere. In summer at noon the path can be much shorter and so less scattering or absorption takes place. I notice that moving a few thousand feet above sea-level makes a noticeable difference to the intensity, also crops that mature in late summer often do better at higher altitudes. That is consistent with atmospheric path length determining intensity.

What Baluncore has posted here is the remaining piece of the puzzle. The angle of refraction is signifigant, but the "length of path through the atmosphere" is the biggest contributor. The sun just loses UV rays when it takes a long time to penetrate atmoshere. In summer, "the length of the atmosphere is much shorter due to the almost perpendicular penetration".

 

[DaveC426913]

Yes, I had intended to bring that up, but that is all part of the "sun's angle" issue - which has been addressed and factored out.

If the OP is claiming that he's comparing two events (at different times of the year), when the sun is at the same position in the sky, then it will be (ipso facto) traversing the same path length through the atmo

 

[psparky]

Yes, I had intended to bring that up, but that is all part of the "sun's angle" issue - which has been addressed and factored out.

If the OP is claiming that he's comparing two events (at different times of the year), when the sun is at the same position in the sky, then it will be (ipso facto) traversing the same path length through the atmo

So are you saying that there is some truth to the OP's claim...original question?

Give me the dates of the possible scenario you have above? The sun is at the same angle and same atmoshpere traveling length at two different times of the year?

What are the exact dates please?

 

[DaveC426913]

So are you saying that there is some truth to the OP's claim...original question?

Give me the dates of the possible scenario you have above? The sun is at the same angle and same atmoshpere traveling length at two different times of the year?

What are the exact dates please?

You seem to have misread what I wrote. I said "if same angle (irrespective of time of year) then same path length". So I am in agreement with you.

The longer version:
Several members suggested that a lower incident angle would make for less insolation.
The OP said 'thanks I get that, and I am ruling out sun angle as a factor because reasons'.
So, if the OP is ruling out incident angle as a factor in his hypothesis (for whatever reasons), then he is implicitly ruling out any loss in isolation due to length of path through the atmo. (because same angle means same path length).

Early on, I was going to suggest 'differing path length due to differing angle of Sun' - but that was before the OP said 'same angle'. So my suggestion became moot.

 

[psparky]

No, you misunderstand.

Several members suggested that a lower incident angle would make for less insolation.
The OP said 'thanks I get that, and I am ruling it out as a factor because reasons'.
So, if the OP is ruling out incident angle as a factor in his hypothesis (for whatever reasons), then he is implicitly ruling out any loss in isolation due to length of path through the atmo.


You also seem to have misread what I wrote. I said "if same angle then same path length". So I am in agreement with you.

Ok, but I am confused. If we neglect the incident angle and path thru atmosphere...what was the point of the original question?

That's like playing a football game without a football...it just doesn't make any sense.

 

[DaveC426913]

Ok, but I am confused. If we neglect the incident angle and path thru atmosphere...what was the point of the original question?

The OP is looking for alternate causes for a discrepancy in the feeling of warmth on his face.

At first analysis, we all agree that sun angle would be the most obvious cause for discrepancy, but the OP is ruling that out and asking for other causes. (He's ruling it out based on his experiments that suggest that, even when the Sun is in the same position in the sky (yet different times of day) between winter and summer, he feels this discrepancy).

Some of us have proposed factors - such as ambient air temp, radiation off windows, humidity, etc. - that would throw a wrench in his perception of the sun's insolation.

 

[klimatos]

Well, no... Not every day... Only two days...

For an observer on the equator, the sun rises exactly in the East and sets exactly in the West only in two days of the year, precisely in the equinoxes of spring and fall. Why? Because, in the equinoxes, the declination of the sun is precisely zero, and its apparent trajectory coincides with the celestial equator. In the rest of the year, the sun rises and sets a bit to the North or to the South, but not much, as the magnitude of that 'deviation from the east/west' reaches a maximum of 23,5º on the days of the solstices. In one of them (summer solstice for the northern hemisphere) the sun rises at a point 23,5º to the north (measured from the east), and sets at a point 23,5 to the north (measured from the west). On the other solstice, a similar thing same happens, but with the sun rising at a point 23,5º to the south (measured from the east) and setting at a point 23,5º to the south (measured from the west).

It sounds complicated in words, but is really very easy: the sun wanders in the south celestial hemisphere during one-half of the year (when its declination is negative) and in the north celestial hemisphere during the other half, when its declination is positive...

Sorry, NTW, but you are in error. The Equator is a "great circle" . The Sun's path through the sky (including the path below the horizon) is also a great circle. Two great circles that intersect always cut one another in half. This is evidenced by the fact that every day on the equator has exactly the same period of daylight (sunrise to sunset) as every other day. It is slightly more than twelve hours (between twelve and thirteen minutes) because the sun appears as a disc rather than a point, but it does not vary from day to day. If the sun were a point rather than a disc, this period would be exactly twelve hours--half a day. In order for this to be true, the sun must rise directly in the east and set directly in the west. From the June Solstice to the December solstice, its path through the sky will be in the northern half of the sky as seen from the Equator. In the other half of the year, its path will be in the southern half of the sky. Twice a year, on the equinoxes, people on the equator will see the sun directly overhead at local noon (cloud cover permitting).

I speak as a professional climatologist, but I do not want you to take my word for it. Check with any professional astronomer who is conversant with Earth-Sun relationships.

 

[NTW]

Sorry, NTW, but you are in error. The Equator is a "great circle" . The Sun's path through the sky (including the path below the horizon) is also a great circle. Two great circles that intersect always cut one another in half. This is evidenced by the fact that every day on the equator has exactly the same period of daylight (sunrise to sunset) as every other day. It is slightly more than twelve hours (between twelve and thirteen minutes) because the sun appears as a disc rather than a point, but it does not vary from day to day. If the sun were a point rather than a disc, this period would be exactly twelve hours--half a day. In order for this to be true, the sun must rise directly in the east and set directly in the west. From the June Solstice to the December solstice, its path through the sky will be in the northern half of the sky as seen from the Equator. In the other half of the year, its path will be in the southern half of the sky. Twice a year, on the equinoxes, people on the equator will see the sun directly overhead at local noon (cloud cover permitting).

I speak as a professional climatologist, but I do not want you to take my word for it. Check with any professional astronomer who is conversant with Earth-Sun relationships.

No. I'm not in error. I ratify myself in everything that I have written in this thread.

Just to comment a single point of what you say, please note that the apparent path of the sun through the sky is not a great circle. It's easier to see that if one positions himself precisely on one of the poles and forget refraction. The path of the sun will be seen as an spiral, starting at the summer solstice with an elevation of 23,5º, and that elevation diminishing with every passing day, till, at the equinox day, with an elevation of zero, the sun disk is exactly parted in two by the horizon. (That day, and only that day, the 360º path of the sun comes close to a great circle. The rest of the days of the year, the path, always tracing a 360º arc of spiral every day, will approximate a small circle, smallest at the solstices). After that equinox, with each passing day, the sun, invisible for the observer, continues its spiral path below the horizon, till -at the winter solstice- it reaches a depression (or negative elevation, if you wish) of -23,5º.

The path of the sun, with its continuously varying declination, is always an spiral, independently of the latitude of the observer. If I have put the example of an observer sited at the pole, it's just because it's easier to visualize.

Yes, twice a year, people sited on the equator can observe the sun at the zenith. But that's not a privilege of 'equatorials'. Any observer sited in the tropical zone -between the tropics- can observe the sun twice a year at the zenith, at local noon... The separation of those two dates, minimum for an observer based precisely on one of the tropics, (where they become just one day), grows gradually, as the latitude of the observer diminishes, reaching a maximum of six months for the equator-based observer.

I'm editing to add a comment: just think, klimatos, in the apparent path of a star with any declination different from zero. It's the case of the sun at any time except in the equinoxes. You'll easily see that that stellar path is not a great circle. Take Polaris, for example, that is a little off the north celestial pole. Its apparent path is a very small circle... Isn't it..?

 

[klimatos]

No. I'm not in error. I ratify myself in everything that I have written in this thread.

Just to comment a single point of what you say, please note that the apparent path of the sun through the sky is not a great circle. It's easier to see that if one positions himself precisely on one of the poles and forget refraction. The path of the sun will be seen as an spiral, starting at the summer solstice with an elevation of 23,5º, and that elevation diminishing with every passing day, till, at the equinox day, with an elevation of zero, the sun disk is exactly parted in two by the horizon. (That day, and only that day, the 360º path of the sun comes close to a great circle. The rest of the days of the year, the path, always tracing a 360º arc of spiral every day, will approximate a small circle, smallest at the solstices). After that equinox, with each passing day, the sun, invisible for the observer, continues its spiral path below the horizon, till -at the winter solstice- it reaches a depression (or negative elevation, if you wish) of -23,5º.

The path of the sun, with its continuously varying declination, is always an spiral, independently of the latitude of the observer. If I have put the example of an observer sited at the pole, it's just because it's easier to visualize.

Yes, twice a year, people sited on the equator can observe the sun at the zenith. But that's not a privilege of 'equatorials'. Any observer sited in the tropical zone -between the tropics- can observe the sun twice a year at the zenith, at local noon... The separation of those two dates, minimum for an observer based precisely on one of the tropics, (where they become just one day), grows gradually, as the latitude of the observer diminishes, reaching a maximum of six months for the equator-based observer.

I'm editing to add a comment: just think, klimatos, in the apparent path of a star with any declination different from zero. It's the case of the sun at any time except in the equinoxes. You'll easily see that that stellar path is not a great circle. Take Polaris, for example, that is a little off the north celestial pole. Its apparent path is a very small circle... Isn't it..?

Virtually every textbook on astronomy speaks of the Sun's apparent path through the sky as a circle--360 degrees. Part of this circle is below the horizon and part is above. How much of each depends upon your latitude and the time of year. You are absolutely correct in that this circle is not a great circle. Properly speaking, it is not a circle (nor a spiral) but is a double-helix.

The circle of illumination, however, is a great circle. It is this great circle that always cuts the Equator in half.

From the March equinox to the September equinox, every place in the northern hemisphere (outside of the Arctic) will see the Sun rise to the south of directly east and set to the south of directly west. How much south depends upon your latitude and the time of year. Every place in the southern hemisphere (outside of the Antarctic) will see the Sun rise to the north of directly east and set to the north of directly west. The same thing is true (in reverse) for the other half of the year. A moment's thought will show that, halfway between the two hemispheres (that is, on the Equator) the Sun will always rise directly east and set directly west.

This phenomenon, along with the brevity of twilight, has been commented on by generations of Equatorial travelers.

I believe that we have digressed substantially from the OP.

 

[NTW]

Facts are stubborn, klimatos...

Think it again... In the equator (and elsewhere outside the artic zones) the sun rises exactly in the east, and sets exactly in the west ONLY in the equinoxes... Twice a year... In the rest of the days of the year, the sun rises and sets somewhat due north or south, depending on its declination, that varies from a maximum of +23,5 degrees to a minimum of -23,5 degrees. The rising and setting point of the sun, for an observer on the equator, will reach a maximum deviation of 23,5 degrees, either to the north or to the south, on the days of the solstices.

That deviation of the rising and setting points from the east and west grows if the observer increases his latitude, growing through the temperate zone, and reaching a maximum of 90 degrees at any point of the arctic circle, where in the summer solstice the sun does not set, but is parted in half by the horizon exactly due north, and in the day of the winter solstice the sun does not rise, but is parted in half by the horizon exactly due south...