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Old 05-21-2004, 03:39 PM
Wesley Clark Wesley Clark is offline
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Why is it cooler under a tree

its 85 F, 91 with the heat index. Why does it feel so much cooler when you are standing under a tree?

I would assume the tree blocking the sun would play a role but i figure other factors are at play. For example, does the fact that a tree absorbs photons and sunlight make being under it cooler than being under an inanimate object that just blocks and reflects them like an umbrella?

Does a tree absorb moisture in the air, making the humidity lower near a tree, lowering the heat index?
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Old 05-21-2004, 03:44 PM
Sat on Cookie Sat on Cookie is offline
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All I can say is, Thank You.
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Old 05-21-2004, 03:48 PM
Lobsang Lobsang is offline
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Funnily enough I was wondering something similar the other day - why is it so much cooler in the shade? Surely the heated air would fill the space.


And how do warm nights happen? (the sun being on the other side of the planet, surely it should be freezing cold every night, as cold as the poles)
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Old 05-21-2004, 04:14 PM
Jophiel Jophiel is offline
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Can't answer the tree part, but AFAIK it stays warm at night because the atmosphere holds the warmth. I believe its mainly a facotr of humidity; hence you often have stuffy hot summer nights, but not dry, hot summer nights. Also why it gets cold at night in the desert (no humidity to trap the heat). Without an atmosphere, it'd be like the dark side of the moon at night.
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Old 05-21-2004, 04:17 PM
tremorviolet tremorviolet is offline
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Evidently, beyond just the shade, trees provide active evaporation.

Quote:
65% of heat generated in full sunlight on a tree is dissipated by active evaporation from leaf surfaces.
source page

Not really finding a whole lot of refernces tho'...
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Old 05-21-2004, 05:15 PM
barbitu8 barbitu8 is offline
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85 degrees F is the temp in the shade, like in the shade of a tree. It is hotter in the sun, and in the summertime can be as much as 35 degrees hotter, as the link above notes. So, what you are feeling in the shade is 85 degrees, but it's probably over 100 degrees in the sun a few feet away. Take a thermometer and test it.
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Old 05-21-2004, 06:24 PM
lektrikpuke lektrikpuke is offline
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When you're in the sunlight, it's radiant energy is striking you and is absorbed by your skin, hair, and clothes. This raises your outward temperature above the ambient, and although your body tries to regulate 98.6 degrees, you still feel hotter.
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Old 05-21-2004, 06:33 PM
Mangetout Mangetout is online now
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Quote:
Originally Posted by Lobsang
And how do warm nights happen? (the sun being on the other side of the planet, surely it should be freezing cold every night, as cold as the poles)
It takes a while for things to cool down; the air itself stores heat, but so does the ground and other objects, then at night, they radiate it out again, keeping the air warm.
Try standing next to a south-facing wall in the evening after a hot sunny day; you can feel the heat coming out of it.
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Old 05-21-2004, 07:18 PM
Excalibre Excalibre is offline
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Quote:
Originally Posted by tremorviolet
Evidently, beyond just the shade, trees provide active evaporation.

source page

Not really finding a whole lot of refernces tho'...
Always been my understanding. Trees, in large enough numbers, cool an area far more than most sources of shade. They certainly evaporate water; that evaporation cools the leaves and, presumably, the area around the tree. That's most noticeable in the shade underneith. Basically, trees sweat like anyone else, and they sweat enough that the whole area cools off.
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Old 05-21-2004, 07:56 PM
barbitu8 barbitu8 is offline
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Quote:
Originally Posted by Excalibre
Always been my understanding. Trees, in large enough numbers, cool an area far more than most sources of shade. They certainly evaporate water; that evaporation cools the leaves and, presumably, the area around the tree. That's most noticeable in the shade underneith. Basically, trees sweat like anyone else, and they sweat enough that the whole area cools off.
The evaporation may cool the leaves, but how could it cool any other object? When I sweat, evaporation cools me, but doesn't cool off my running buddy (his own will do that) nor does it cool off a dog which may be running with me, even if the dog is running in my shade.
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Old 05-21-2004, 08:02 PM
Excalibre Excalibre is offline
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They're big. Big, and they have LOTS of surface area to evaporate from. Trees evaporate large, large quantities of water, from a very large area - allowing water to evaporate much faster, proportionate to weight, than it does from a human.

If the tree cools off enough, the surrounding air cools off a little too. When you run, you're not evaporating away nearly as much water, and your increased metabolism means you're producing more heat besides.
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Old 05-21-2004, 08:16 PM
barbitu8 barbitu8 is offline
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Other than under boiling conditions, evaporation is caused by some of the molecules of water on the leaves moving fast enough to be bumped off into the atmosphere. I have difficulty in visualizing how that would equate to evaporation from my body under its shade. In fact, there would be more evaporation if I were in the sun, as I would be producing more sweat and the temps would be higher. In addition, if all those big leaves were producing evaporating water, the RH nearby would be higher, inhibiting evaporation from nearby objects.

I've noticed that when I run on the grass on a hot morning, it is decidely more muggier than if I were to run on a solid surface. I attribute that to the evaporation of the dew, raising the nearby RH. I would think this would be applicable, in some sense, to a tree also.
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