Naked Science Forum

Life Sciences => The Environment => Topic started by: Petrochemicals on 20/09/2019 05:36:19

Title: Why are the suns rays hot some days and not others?
Post by: Petrochemicals on 20/09/2019 05:36:19
Why is the sunhine varied in temperature to such a great deal ? Usually during the summer the temperature of the suns rays during clear days is really noticable, whilst in the winter sunshine under the same conditions is not noticable. This is all the more perplexing as the the water  content in the  air is far higher during the summer than the winter, yet this does not filter the sunshine of its impact? Is it due to emmisivity as the temperature difference between skin and surrounding has an effect upon emmisivity ?
Title: Re: Why are the suns rays hot some days and not others?
Post by: Colin2B on 20/09/2019 10:12:22
If you consider the elevation of the sun, rays have a lot more atmosphere to pass through in winter, hence greater absorption of IR.
Also, if you consider the energy/m2 in a plane perpendicular to the rays then consider the angle of ground relative to the rays you will see that the energy is spread out over a greater area in winter reducing the heating, and hence air temperature, next to the ground.
Obviously, if you are standing then you are closer to the ray perpendicular, in which case the distance through atmosphere has greater effect.
Title: Re: Why are the suns rays hot some days and not others?
Post by: Petrochemicals on 04/10/2019 21:25:01
Thank you colin, in the days of continued sun during even elevation, such as june july august, the rays also become stronger the longer the continued sun happens. I am wondering whether this is the greenhouse effect of higher water vapour content increacing the radiation felt.
Title: Re: Why are the suns rays hot some days and not others?
Post by: Hayseed on 05/10/2019 00:01:51
The intensity of light should be limited by two basic lengths.  The length of source and the length of atmosphere.  Both lengths increase in the northern hemisphere at winter time.
Title: Re: Why are the suns rays hot some days and not others?
Post by: Petrochemicals on 05/10/2019 00:47:25
The intensity of light should be limited by two basic lengths.  The length of source and the length of atmosphere.  Both lengths increase in the northern hemisphere at winter time.
That sounds plausable but I often find the sun hottest in the mid afternoon not noon. Also in the mountains it is easy to get alot more solar radiation and get burned by the ultraviolet, yet it does not feel as intense  ? Einsteins old photoelectric effect at a guess. But why does the sun feel more fierce at lower altitudes ?
Title: Re: Why are the suns rays hot some days and not others?
Post by: Hayseed on 05/10/2019 01:08:46
You are confusing two different effects with two different conditions.  The length of atmosphere is detrimental for both IR and UV.   IR is heat.  UV is sunburn.  There is another component of the atmospheric length, and that is density.  If we add altitude to the condition......then density must be accounted for.

The density is more detrimental to UV.  A high mountain anywhere should burn you.  If you can expose yourself.
Title: Re: Why are the suns rays hot some days and not others?
Post by: Petrochemicals on 05/10/2019 01:51:04
You are confusing two different effects with two different conditions.  The length of atmosphere is detrimental for both IR and UV.   IR is heat.  UV is sunburn.  There is another component of the atmospheric length, and that is density.  If we add altitude to the condition......then density must be accounted for.

The density is more detrimental to UV.  A high mountain anywhere should burn you.  If you can expose yourself.
But thats not an explanation of why at greater depth the suns rays feel more warm ? It is contrary.
Title: Re: Why are the suns rays hot some days and not others?
Post by: Hayseed on 05/10/2019 02:13:41
I'm sorry that I confused you.  I shall retire and maybe someone can help you.  Good luck on your studies.
Title: Re: Why are the suns rays hot some days and not others?
Post by: Petrochemicals on 05/10/2019 21:34:38
I'm sorry that I confused you.  I shall retire and maybe someone can help you.  Good luck on your studies.
Touchť
Title: Re: Why are the suns rays hot some days and not others?
Post by: Colin2B on 10/10/2019 09:08:09
Thank you colin, in the days of continued sun during even elevation, such as june july august, the rays also become stronger the longer the continued sun happens. I am wondering whether this is the greenhouse effect of higher water vapour content increacing the radiation felt.
There are a lot of different factors at work here.
Firstly, the elevation of the sun will increase during the day, hence less atmosphere to pass through at midday compared to early morning.
Also, the temperature of the air around us will have an effect on how we feel the effect of direct sunlight, certainly water vapour in the air will affect air temperature close to the surface due to reradiated surface heat.
Interesting question - if the air around you is cold do you feel the sunís rays as warmer than if the air is warm?
Title: Re: Why are the suns rays hot some days and not others?
Post by: Petrochemicals on 10/10/2019 20:46:03

Interesting question - if the air around you is cold do you feel the sunís rays as warmer than if the air is warm?

Thats a good insight. If the air is cool the sun does not feel as warm as when if the air has been warm for a number of days, yet same sun at the same elevation. Even at noon the suns heat does not seem the most intense, usually about 2 or 3 in the afternoon, the evening sun is also far more powerful than the morning. Morning sun is usually quite pleasant evening sun can be unbearable. Thats my opinion.

 Over the 3 months of summer the suns angle does not change much, the day lengths are a sine wave of change, very quickly changing during the middle sector.
Title: Re: Why are the suns rays hot some days and not others?
Post by: Petrochemicals on 14/12/2019 22:45:55
Does this have anything to do with the infrared greenhouse effect,  water vapour content and the like. As more moisture is present in the atmosphere a greater ammount of IR is bouncing around the sun adds to it and feels more intense.
Title: Re: Why are the suns rays hot some days and not others?
Post by: alancalverd on 15/12/2019 12:00:28
Wet air (near the ground) has a much higher specific heat capacity than dry air (at altitude). Hence the "bitter cold" of the UK (all below 3000 ft and rarely below 0 deg C or 80% humidity) compared with the "bracing cold" of a continental winter. Also the "oppressive heat" of Malaysia compared with the "burning heat" of  the Australian desert.

Not that the human body is a reliable thermometer. We are sensitive to changes (turn the aircon up or down 1 degree and you'll notice it) and differentials (put one hand in water at 20C and the other at 21 C and you'll notice the difference). And we get acclimatised fairly quickly: I recall a school swimming pool being unbearably cold one week, and pleasantly warm then next. No change in the pool temperature but the ambient air temperature had dropped by about 10 degrees.  (That was in the 1950s - compulsory Latin, and if you couldn't walk across the pool you had to swim across, every Thursday. Funny that what passed for a good education and a bit of fun in my youth would probably be considered unacceptable cruelty nowadays. Unless, of course, you have paid a fortune for a sauna and ice bath.)
Title: Re: Why are the suns rays hot some days and not others?
Post by: Bored chemist on 15/12/2019 17:42:48
Wet air (near the ground) has a much higher specific heat capacity than dry air (at altitude).
Even saturated steam only has roughly twice the heat capacity of air.
The few percent (at most) of water vapour in air barely affects the heat capacity.
Title: Re: Why are the suns rays hot some days and not others?
Post by: Petrochemicals on 13/08/2020 14:37:34
Coming back to this,

Over the last few days the weather in the uk has been hot, yet on the hottest day which was at the beginning of the wave, it seemed far cooler and the sun far more fierce than on the next days.  Did anyone else think so in the uk
Title: Re: Why are the suns rays hot some days and not others?
Post by: alancalverd on 13/08/2020 15:28:12
Same question. The first few days, after the cold front moved westward over the UK, were very low humidity with a dry east/north east wind from Europe, so you could easily lose body heat by evaporation. As the wind direction shifted westerly, the incoming air had a longer fetch over the sea and became steadily more humid, plus evaporation from the warming soil, so the rate of evaporation from exposed skin decreased.

The classic "fair weather" has a clear blue sky with occasional cumulus clouds and a dry northeast wind. Lack of cloud and atmospheric moisture allows maximum infrared to reach the surface, hence a high recorded shade temperature but it feels comfortable. High level cloud with a wet westerly breeze reduces direct infrared radiation but the incoming air is warm, damp and uncomfortable.