Optional Laboratory:
Comparing Soil Temperatures In Sun And Shade

Shade is one of the most recognizable characteristics of a tree. On a hot summer day, the outline of a shady area is probably more noticed than the tree itself. What are the effects of shade, other than cool comfort, as one escapes from the direct sun?

The shade from a forest tree creates a micro-climate suitable for many species of plants and animals to survive and flourish. Many small plants have adapted to the understory of a forest, and need protection from the direct rays of the sun. The forest soil is cool and moist, which is good for plants. The moist forest soil is also a good place for microorganisms to survive. Many of these microorganisms such as soil bacteria and fungi are beneficial to forest plants.

Soil temperature is a critical part of the survival of many organisms both big and small. Shade not only cools the subsurface, but also the air temperature above the ground which helps to stabilize the entire area. A city street lined with trees has sidewalks that are much cooler than a city street without trees. Many people suffer from heat related illnesses. People are more likely to show signs of heat stress in a city where there are few trees and shade. In this exercise, measuring soil temperature differences will illustrate the effects of shade from a tree.


  1. You probably have guessed that there will be a difference in soil temperature between sun and shade, but will there be a difference of 10 degrees or more?

     Students should make their own before continuing.

     Soil thermometers
     Data sheet


  1. If a long range study is planned, the school grounds are a good place to locate two permanent sites so that daily readings can made in a routine fashion.
  2. The site in the sun should be well away from any structure because the structure might radiate heat onto the soil where the thermometer is located. The thermometer can be concealed and will not affect the reading because the tip of the probe is detecting the soil heat, not the round gauge on top of the probe.
  3. The site in the shade should not be too close to a tree trunk.
  4. Leave probes in the ground for at least a half hour to adjust to the soil temperature.
  5. (Optional: if time permits)

  6. Use thermometers to record air temperatures.
  7. Use wind gauges to record wind velocity.
  8. Use sling psycrometers to record humidity data.


  1. Urban heat islands are isolated pockets of increased temperatures located over city and urban areas. The heat pockets are greater because of the increased number of building structures and paved areas. Structures and pavements absorb and reradiate direct solar radiation. Find two sunny sites to place the soil thermometer probes where one is in the lawn and the other is next to a building wall. A south facing wall will provide the best results.

Fig. 1. Generalized cross-section of a typical urban heat island.
(Boundary Layer Climates. Oke, T.C., New York Methuen. p. 435. 1987.)

Soil Temperature Comparison: Sun / Shade

NAME: _____________________________

Location of Soil Thermometers: _________________________

. Soil Temp.
Day # Air
Windy/Calm % Humidity Cloudy/Clear Sun Shade
. . . . . . .
. . . . . . .
. . . . . . .
. . . . . . .
. . . . . . .
. . . . . . .
. . . . . . .
. . . . . . .
. . . . . . .

Background Information
The quest for shade is one of the ultimate experiences in the summer. Escaping the direct rays of the sun is probably deeply rooted in our primal past, so that seeking shade has become a subconscious act. The shade tree in your backyard or your favorite park is a place of welcome shelter and enjoyment. The human benefit of shade is quite evident, but what about the more subtle attributes of forest trees and shade?

Shade helps to create a more favorable understory environment. Many herbs, shrubs and saplings require a cool environment to grow successfully. Shade also reduces the rate of soil moisture evaporation. Many animals such as small insects and worms depend on a high soil moisture content to survive. These small animals are food for other larger animals, which therefore become indirectly dependent upon the shade trees provide. Since shade promotes the growth of grasses, herbs and shrubs, shade also then can be linked to soil stabilization, because the roots of plants reduce soil erosion. One might look at a timber area and say that the tract is helping to control erosion. But if the understory vegetation is being grazed by livestock the forest tract is actually contributing to erosion. Too often in rural areas, livestock are allowed to graze in woodlands, particularly along watershed drainages. An estimated 66 percent of erosion occurs because of heavy grazing. The resulting disturbance of forest duff and understory increases erosion and sedimentaiton into adjacent waterways.1

Moderating solar radiation for a city or urban environment can have extensive benefits other than the immediate escape from the sun. A concept that forest researchers have used is the "Urban Heat Islands". Urban areas are often warmed by the heat generated by sources of reradiation such as paved parking lots, stone buildings and roads. It has been found that city parks within a community can be 2-6 degrees cooler than the rest of the urban heat island. Urban heat islands have also been blamed for increased health risks such as cardiovascular diseases and other heat-aggravated illnesses. Trees in city parks and streets have been attributed with reducing air conditioning costs. Shade that trees provide is a complex phenomenon, but often overlooked when discussing the benefits of planting and maintaining trees.

Target Group
     Sixth through eighth grade.

This activity can be a daily reading of the soil thermometers which takes only 10 minutes to read and record. Aquiring additional data such as humidity and air temperature might require more time. These can also be found in a newspaper, by calling a local weather recording, or they can be measured in the field.

Student Learning Objectives
The student will be able to:

  1. Relate many shade benefits trees provide.
  2. Read and record from a soil thermometer.
  3. Calculate the difference between soil temperature in the shade and in the sun.
  4. Relate other climatic conditions to the difference between soil temperature in the sun and shade.

Setting up this data comparison seems quite simple, and it is! However, there are a number of interesting variations that could be used based upon imagination and time availabilty. This particular study is best suited for a long term study, so that interpretations can be made with reference to the other climatic conditions. However, the results from just one day of comparison can be very dramatic as the students see the big difference a shade tree(s) can make on soil temperature.

Select two spots on your grounds; one in the full sun and the other in the shade of a tree. One to two hours is enough time to take a reading and record a difference of 15 degrees. You can choose to install permanent thermometers for reading on the hour, or take them with you on your daily trips.

Mulch around the base of a tree is designed to trap rain and moisture and insulate the soil immediately below from direct radiation. In the full sun, place one soil thermometer in the grass and another through the mulch around a tree. Read in an hour.

insulation: a barrier creating dead air space thereby reducing the tendency of cool or warm air to move from a high concentration to a low concentration (diffusion). Mulch is a type of insulation; so is a tree canopy.


  1. Chicago's Evolving Urban Forest. McPhearson E. C., and Nowak. USDA Forest Service, Northeastern Forest Experiment Station. 1992.
  2. A Long Range Plan for Illinois Forest Resources. Illinois Council on Forestry Development, Sept 1990.
  3. Boundary Layer Climates. Oke, T.C. New York Methuen. p. 435. 1987.

1 A Long Range Plan for Illinois Forest Resources. Illinois Council on Forestry Development, Sept 1990.

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