Having shade trees around a house can decrease the cost of cooling the house with air conditioning. A house not shaded from the sun absorbs some of the light from the sun and heats up the outside surface of the house. If the house is poorly insulated, some of this heat will penetrate into the house, heating up the inside. The air conditioner will use more energy to remove this added heat.
Properly designed roof overhangs can significantly decrease the heating and cooling costs of a house. Because the earth’s axis is tilted, the sun is lower in the winter in the northern hemisphere. In the summer, the sun is higher in the sky. A properly designed roof overhang allows sunlight in the winter to shine through windows and warm the furnishings in the rooms that receive the direct sunlight. This reduces the heating cost in the winter. In the summer, the overhang blocks the sunlight from shining into the window and heating the furnishings. This reduces the cooling cost in the summer.
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Materials
- Two large, glass jars
- Shady tree
- Water
- Thermometer (optional)
Procedure
You will want to do this experiment on a warm, sunny day. Fill both large, glass jars nearly full with water. Place one of the jars outdoors in a spot where sunlight will strike it for several hours. Place the other jar outdoors under a shady tree.
After three hours, feel the water in the jar that was left in the sun. Next feel the water in the jar kept under the shady tree. If you have a thermometer, check the temperature of the water in each jar. Also check the air temperature with the thermometer.
Observations
Does the water in the jar left in the sun feel warmer or cooler than the water in the jar left under the shady tree? If you are using a thermometer, what is the temperature of the air and of the water in each jar? What is the temperature difference of the water in the two jars?
Discussion
The water in the jar that was left in the sun should feel much warmer than the water in the jar left under the shady tree. The temperature difference of the water in the two jars will vary depending on the air temperature and wind conditions. On a hot, sunny day you may find that the temperature of the water in the jar left in the sun is over 18° F (10° C) warmer than the air temperature and the temperature of the water in the jar kept in the shade.
The jar of water left in the sun is constantly bathed with light energy from the sun. Some of this light energy is absorbed by the glass jar and the water in the jar. The light energy that is absorbed by the glass jar and the water is converted into heat energy. This is why the jar of water becomes warm after being in the sun for several hours.
The tree shades the other jar of water from the sun. The jar of water under the tree does not absorb light energy directly from the sun as the other jar did. Instead, much of the light energy from the sun is absorbed and used by the tree.
Since the jar of water under the tree does not absorb light energy directly from the sun, it remains cooler than the jar of water kept in the sun. You may even find that the temperature of the water in the jar shaded from the sun is a few degrees cooler than the air temperature. This is caused by water evaporating from the jar. Heat energy is removed when a liquid evaporates, and the liquid becomes slightly cooler.
Other Things to Try
Repeat this experiment at different times of the day. Do you get similar results? Repeat this experiment when it is cloudy. How does the difference in the temperatures of the jars of water compare on sunny and cloudy days.
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