Effect of Radiation on Temperature Changes of Land and Sea

Many of the climatic features in various regions of the earth can be directly or indirectly attributed to the differences in thermal properties between land and sea. In the middle of a continent, the summer temperatures are higher, the winter temperatures are lower than they are at the same latitude near or over the ocean. Island nations have much more moderate climates where winter temperatures are rarely very cold. Monsoon winds as well as land and sea breezes are also a consequence of the differences.

The differences between land and water insofar as they affect the heating can be summarized as follows:

a) It takes about three times as much heat to raise the temperature of a kilogram of water by one degree than of a kilogram of soil;

b) radiation is absorbed by a thin top layer of soil, whereas radiation penetrates the water to considerable depth; thus over a unit area a much smaller volume of soil is involved in absorbing radiation, than of water;

c) water is mobile so that wind and water currents stir it up, whereas the soil is immobile and

d) some of the radiation absorbed by the water is used to evaporate some of it, whereas all the radiation absorbed by dry soil is used to warm up the soil.

Cut the bottom portion of the holders of two identical thermometers so the bulbs and about 1/4" of the glass neck of the thermometer tubes are free. Be extremely careful not to break the thermometers or to disturb the mounting of the capillaries to the scales. If using paper backed thermometers, you will probably need to make sure they read the same when immersed in warm water.

drawing of petrie dishes and lightbulbObtain two identical dishes 4" diameter of larger and one inch deep. Cake pans for layer cakes are ideal but you will need to seal the rivets. Other similar dishes can be used, but both should be of the same material and size.  Fill one with water, the other one with fine, sifted, dry soil. The dishes should be filled to within 1/8 inch from the top. Let them stand for a while so their temperatures will be approximately the same. Insert one thermometer bulb in the center of the can with water, so that the bulb is just covered with water. Insert the other thermometer into the center of the can with soil, so that the bulb is just barely covered with soil. The thermometers should rest on the rim of the cans as in the diagram and be supported at the upper end.

Set up a piece of graph paper with temperature on the vertical axis and time on the horizontal axis. As readings will be every minutes or so, one tick on the horizontal axis can represent 1 minute. The vertical axis must be determined by experiment but the most change will probably be of the order of 10 degrees.

Place a 150 to 200 watt bulb about 10" above the middle between the two cans so that both receive the same amount of radiation. Turn the light on and record temperatures every minute or so for about 10 minutes. Turn light off and continue reading, until the temperatures are approximately what they were at the beginning. Direct sun light can be used but you will need to shade the cans before and after exposure with a piece of cardboard held not too near the cans.

Now push the thermometers into the soil and water so that the bulbs rest on the bottom in the middle of the cans, and repeat the experiment.

Place the thermometer bulbs into their original positions, just barely under the surfaces of the soil and water. Sprinkle water on the soil without soaking it and repeat the experiment.

Material: Two pans or cans, at least 4" diameter, 1" high

Two identical thermometers

One 150 to 200 watt light bulb

Water and fine, dry, sifted soil.