Weather Control

I live in a house with lots of big old trees. There are 60 to 80 year old walnuts, oaks and elms, which tower over our house. On a hot summer day, there is plenty of shade in the yard. In fact, there is little sunlight, as there is no good place to put a garden. I've always thought that the yard feels a good 10 to 15 degrees cooler than the sunnier area outside of the shade trees.

I've often wondered if it really is cooler under the trees. I've heard many advertisements and advice pieces that say that you can lower your air conditioning bills by planting trees around your house. This begs the obvious question: Do trees cause actual cooling?

Being a Mechanical Engineer who studied Thermodynamics in college, I tried to reason this out. I remember that Skylab's failed solar panels left inadequate power to cool the space station, and the problem was solved by putting a high tech beach umbrella on the station, putting it in the shade and immediately dropping the temperature to a reasonable level. The difference between that and any situation on the surface of the planet is that we have air. There are three ways heat can move or flow 1) conduction (through a solid) 2) convection (through a fluid: air or water, for example) or 3) radiation (directly out of the body independent of what kind of surroundings the body is in). In space, it's easy. The sun hits the umbrella, and radiates back out. If you make the umbrella silvery on one side or if it has very little mass, you insure that most of the heat striking the shade does not get into the thing it is shading. On a planet, objects heat up in the sunlight (receiving the energy via radiation) then re-radiate it as well as having it leave by convection and sometimes conduction. So a leaf on a tree that is struck by the sun then re-radiates the heat into the surrounding air, as well as having some of it taken away by the breezes.

I heard about an Americorps project in the Bronx where people painted rooftops of buildings white. The theory is that a white roof will re-radiate a significant amount of heat rather than absorbing it. I heard that there was a scientist that calculated that if we painted all the roofs of all the houses in the world white, that we would re-radiate enough heat into space that it would offset what we are doing that is causing global warming.

If you imagine an invisible dome over your yard, that would be what we call a "control volume". This just means an invisible boundry that you consider when analyzing something. What we know about a control volume is that if it is in equilibrium, meaning that the temperature is not rising in the volume, that the amount of heat going in equals the amount leaving. If the temperature is changing, that means that the heat is flowing toward the rising temperature zone and away from the lowering temperature zone. So you look at the cool yard and the hot open field down the street and you observe that the same amount of heat hits both areas, but you feel hotter in the direct sunlight. Even if you have a little sunshade directly over just you, you are going to be hotter in the open field.

It's pretty clear that what is happening is that you are picking up heat that is re-radiating from the ground around you when you are out in the open. In the shaded yard, a portion of the heat that would have hit your or the ground surrounding you is being re-radiated upward into space. From a convection standpoint, the leaves are being cooled by whatever breeze is coming by, and I have to assume that there are layers of air, and the air at the ground does not mix thoroughly with the air above, so there is a blanket of hotter air above you, and the air near the ground stays cooler. In the absense of a breeze, the air makes its own currents. The heated air expands and rises, moving away from the ground.

The part of the equation we moved away from is conduction. When I was attending Heat Transfer classes at K-State, it was 1984. That is still close in time to the oil embargo and the energy crisis of the late 70's. Laws had been put in place that gave builders incentives to construct energy efficient homes. You got a tax credit for installing solar panels. Back then, they were not very often panels that generated electricity, but mostly black panels that you ran water through and got free hot water out of. We learned how to design what was called passive solar buildings. These buildings blocked the sun in the summer with wide eaves, then in the winter, when the sun was lower in the sky, the south facing side of the house was able to get direct sunlight under the eaves. The best way to take advantage of this was to put a lot of glass on that wall, and put massive objects like stone, concrete, or water tanks just inside the glass (say a hallway's width away. You make the glass well insulated from convection, air tight as possible, and at best, you put a double pane glass with an evacuated space in between to prevent heat from conducting through the air and glass to escape the house. The radiant heat from the sun goes through the glass, then strikes the massive objects, which conduct heat into their centers and store it, to later be re-radiated back into the house at night. Ideally, you would have shades that you would draw after the sun goes down, preventing radiant heat loss and further insulating your windows from convection and conduction loss.

So there's no doubt that a tree conducts some of the heat into itself, but it should re-radiate this heat, making it neutral over the course of a day.

There is one difference. The sunlight that hits the tree gets used. The tree isn't just a series of canvas umbrellas, it's a living thing with cells that are using the sunlight through photosynthesis to make hydrocarbons. Some of it is sugar, some cellulose, some proteins, but the energy from the sun gets bound up in chemical bonds in the tree.

My question is: does a tree capture and use enough energy to lower the temperature? In some quick google based research, I have not yet found the answer. I'm sure they use energy, but is it like an air conditioner where they are sucking in so much solar energy that the temperature of the air drops? It certainly goes up when you release that energy by burning the logs the tree makes.

If you were a scientist with a nice research budget, you could probably model a tree as a static series of little shades, a close copy of a real tree, except yours is not alive, and you could probably measure the temperature around the real and fake tree and compare the readings, determining if there is absorption of substantial energy by the tree.

Since the method that the tree converts sunlight into sugar or cellulose is a chemical equation, you could probably calculate energy rate theoretically. Assuming sugar production has a fixed caloric rate, that might have a thermal equivalent. I believe that the work has been done before, I just haven't found it.