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I have this big window in my room that the Sun shines through every morning. When I wake up I usually notice that the Sunlight coming through my window feels hot. Much hotter than it normally does when you're standing in it outside. I know if the window were a magnifying glass that it would feel hotter because it is focusing the Sun's rays, but I'm pretty sure that my window doesn't focus the rays, otherwise things outside would appear distorted.

So my question is, why does Sunlight always feel hotter when it shines on you through a window than when it shines on you outside? I thought it might simply be a matter of convection, but anecdotal evidence would seem to say it still feels hotter even if you had a fan blowing on you. Am I just crazy?

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+1 This question has the potential to be very useful to visitors to the site from a random google search –  Jim Jun 12 '13 at 13:54

2 Answers 2

up vote 6 down vote accepted

You were right with your original though. This is an effect purely due to a lack of convective cooling. When you are inside, there is normally very little to no airflow, which means when you stand in the sunlight and heat up due to it, the air around you will be warmed by you but isn't moving enough to be replaced with cool air and carry the heat away. Don't misunderstand, the air is moving, which is why you won't continue to build up more and more heat, your temperature will plateau. It's just that the air isn't moving enough to prevent an increase in skin temperature. When you step out of the direct sunlight, it feels cool again because now you are not being heated as much and what convective flow exists is enough to cool you to normal levels. Additionally, there is a component that is attributed to the air being directly heated by the sunlight as well as re-radiation from heated objects (like a table or chair), but most is due to a lack of convective cooling.

As for your anecdotal evidence, I cannot comment on why it might have felt just as hot with a strong fan on you, but I can try to convince you that my answer is true. If you go outside one morning when there is little to no wind/airflow and stand in the sunlight, it will feel just as hot. Even in the winter, the sunlight will feel hot, however the air around you will have slightly more cooling power so it may still feel cold.

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Regarding the anecdotal evidence: I think that air humidity plays a role here. When it's warm, our bodies remove heat by evaporation, which is more efficient if the air is dry. And the humidity outside is usually lower than in the room. –  gigacyan Jun 12 '13 at 14:11
    
@gigacyan good point. You're probably right about that. I'm going to leave it out of my answer though, no offense, just in case Dan lives in a rainforest –  Jim Jun 12 '13 at 14:15

This is due to the greenhouse effect (that how a normal greenhouse works). Glass has a low thermal conductivity, but is also transparent. So the sun light enters though the transparent window, turns in to heat, and then cannot leave outside. Trapped, the heat accumulates at the air close to the windows, and that is what you feel.

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But I don't have to stand close to the window to feel the heat. Also, does that mean it's hotter due to lack of convective cooling, or does the Sunlight actually get turned into heat rays somehow? –  Dan II Jun 12 '13 at 12:47
    
This isn't quite the case. The Greenhouse effect refers to when sunlight heats an object, which then releases the heat through convection or long-wave radiation, which is then trapped by the glass and thus heats the inside area. This process works for heating the entire inside area as the re-radiated heat is diffuse. However, Dan's phenomenon is heating only within the direct sunlight. Once you step outside of the sunlight, you no longer feel the heat. Were this due to greenhouse effect, it would still feel warm, if not hot. –  Jim Jun 12 '13 at 13:06
    
@Dan Here is a small calculator to estimate the heat produced from various objects (better append this to the answer to make it more whole, edits are welcome). –  GuySoft Jun 12 '13 at 13:08
    
@Jim, that is also true, but I dont think its what Dan was talking about (or was he?) –  GuySoft Jun 12 '13 at 13:12
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That should be due to less convective flow. I guess we can test it by placing a thermometer outside and inside with ventilation, the temperature should be the same. –  GuySoft Jun 12 '13 at 13:31

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