Why is the shade of a tree cooler than an artificial shade?

Question

Sitting under a tree is cooler than sitting under a sheet of cloth, for example. Why should that be?

This question was actually already asked 2 weeks ago, but I wasn't happy with the answer. The answer suggested then was that wind can blow easier in the tree shade, which I did not find convincing. I have two ideas:

(1) The tree shade is thicker, there are lots of leaves, so it insulates better

(2) The tree has to keep itself relatively cool to continue photosynthesis, so it cannot transfer as much heat downwards.

I think that explanation 2 is correct. Explanation 1 would only delay the time it takes for the heat to be transferred down, not eliminate it altogether.

I would be grateful for any alternative explanations, or supporting evidence for the explanations I thought of.

Thanks! Eyal

Since people have agreed that the tree cools itself by evaporation, thereby letting the shade under it to be cooler as well, I was wondering - what is the "operating temperature" of a tree - Is it as constant as a mammal;s, or does it vary more? I also know that at a certain temperature the trees stop photosynthesis and shut down cooling evaporation if they are short on water, and I wonder if they can then withstand a much higher temperature.

Answer 1

A perceptive question.

There are two reasons.

First; most trees and plants evaporate and exhale water from pores in their leaves during the day, while they are photosynthesizing. This evaporation cools the leaves and the air around the tree, just like a wet cloth, or a "swamp cooler."

Large moisture loving trees, like willows or poplars(some of my favorite to stand under) can evaporate dozens to hundreds of gallons of water a day.

You've probably noticed that sidewalks and pavement are rather hot on a sunny day, but grass, even though it is dark colored, is cool. You've probably also noticed that the air feels a little more humid around trees.

The second reason is a little more subtle.

Most plants strongly absorb visible light(except green,) but they *reflect* almost 100% of infrared light. This means that most of the thermal infrared from the sun gets reflected away, back into space.
http://www.henniker.org.uk/images/places/local_a/ed_sth/radical_rd21ir.JPG
(© Dave Henniker)
In this infrared photo, the roads and rocks are dark gray, but the trees and plants are snow white(a nice effect...)

Rocks, concrete, and pavement tend to absorb and re-emit radiant heat from the sun, so even if you are out of the direct sunlight, you still have to deal with the nearby hot surfaces, but underneath and around trees and plants, you are cool.

Hope that makes sense,
~W.O.M.B.A.T.

Additional:
Plants don't actively regulate their temperature, (except in rare cases.) Doing so takes far too much energy, and as long as it's above freezing, they don't really need to. The amount of light, water, CO2 and available nutrients are a much greater controlling factor on the rate of photosynthesis, then is temperature.

In relatively dry conditions, plants and trees adapt in few different ways. Some close their pores during the heat of the day, only opening them in the morning and evening. This limits photosynthesis, but saves water when they are at the most risk of drying out. Some plants use a system of photosynthesis that is less efficient, but doesn't require their pores to be open all the time.

Succulent plants and cacti, that live in extreme environments, are able to store a limited amount of light energy, in a sort of rechargeable chemical battery, called "Crassulacean acid." they use this stored energy to to make sugars from water and CO2, only during the coolest time of night; when they lose the least water. http://dwb.unl.edu/Teacher/NSF/C11/C11Links/www.bl.rhbnc.ac.uk/plant/cam.html

Answer 2

Plants use more CO2 than it produces. Thats how it grows. During the day, it converts Co2 into plant matter. We are technically in a CO2 defecit (If you look at our CO2 over millinia.) I've seen charts, on CO2 level and there is little change. These date may be bias though, since the source tries to dispel the greenhouse affect. It is known that the plants are growing faster because we are making more CO2 available. The question is if the plants are absorbing enough CO2. The other question is, does these plants permanently solve the excess CO2. Most of the increase growth in plant matter is made in food production. If we eat the food, most of the Carbon stored in the plant is transformed back into co2. Also if we cut down the tree and burn it, the carbon is mostly returned into the atmosphere. If we bury it, some of the plant matterial will decay and return into the atmosphere as well, but some will stay underground. I think it helps, but the question is how much. Even if you burn the tree, some of the carbon would stay out of the atmospere. My oceanagraphy teacher told me the best way to remove CO2 is to encourage coral growth. coral produces calcium carbonate. This would eventually turn into beach sand, which is obvious won't burn.

Answer 3

Your option 2 is more correct.
Trees actually release moisture into the air as they transfer oxygen and carbon dioxide through the leaves.
Even though the moisture isn't enough to form a mist or fog, its enough to lower the air temperature around the tree by a few degrees. The leaves overhead also serve to keep that slightly cooler moister air from dispersing too quickly.
But in the sunlight even close to the tree the sun will evaporate those tiny molecules of water very quickly and we wouldn't notice much.
But if you had a very accurate thermometer near a tree in the sunlight it would be a tiny fraction of a degree cooler than at a distance from the tree (all other things being equal).

Answer 4

I think you are missing the point. Shade under a tree feels cooler because planted trees are typically separated enough to allow good airflow around them, they are often larger than other sources of shade, and they are often planted in areas that are cooler than other areas in an urban location.

For example, shade from buildings, by definition, are close to buildings which often block the wind. Shade from a patio umbrella is typically much smaller, so the surrounding area is heated more by the sun. And walking down a street with an asphalt surface is bound to feel warmer than walking under a tree in a lawn that was watered the previous night.

I would suggest that there is little to no difference in the actual temperature under trees than under normal shade all things being equal. The problem is, not all things are equal. Just ask anyone who has walked in a rain forest, with a canopy of thick, interlocking trees--they will tell you the air is MUCH more oppressively hot than in the open.

If anything, psychology plays as much, if not more of a role in the perception you are describing than anything else.

Answer 5

Evaporation! If you sat under a damp sheet or blanket, you might find that cooler than sitting under a tree.

The leaves of the tree are actually giving off moisture and that has a cooling effect.

Here in the arid southwest many homes and building use evaporative coolers instead of air conditioners.

Answer 6

we feel more relaxed when we sit under the shade of a tree due to two reasons.
1.the tree gives out oxygen which is very essential for a body asking for rest.
2.transpiration(evaporation of water from the surface of the leaves and other aerial parts) takes place and evaporation always cools a body.

Answer 7

The transpiration of plants keeps them cool in the sun. Transpiration is the continual loss of water through the leaves--think of it as sweating. On a brutal summer day, if you walk on the sidewalk, it's scorching. A lush green lawn is cool. It's not until a lawn is scorched and brown, i.e. dead, that it, too, begins to heat up in the sun. Similarly, a cloth hung in the sun gives you shade, but also gets hot. whereas the shade of plants is cooler.

Answer 8

shade of a tree cooler than an artificial shade because a tree releases certain gases after photosynthesis which r volatine gases & causes us 2 feel intense cool....(this thng is smiliar 2 a cooling by a refrigerator)

Take Care
bye

Answer 9

Wouldn't the tree absorb more heat (sunlight) than the cloth would, if it was a light-colored cloth? Also, cloths generally have holes and are thinner than trees, letting more sunlight through.

Answer 10

Thanks to lindajune. I want to add ,ordinary shades filter the sunlight. Foliage of trees consumes the sunlight and heat for photosynthesis . Oxygen released from foliage is cooler.