Black Rock Forest Student Investigations

Solstice versus Season

Insights to be discovered:

• When exposed to the same heat source, some materials heat up faster (temperature rises faster) than other materials.

• Conversely, some materials cool down faster (experience a drop in temperature) than others.

• Materials that heat up fastest in the presence of a heat source are the same materials that cool down fastest in a cool environment.

• Specifically, soil heats up and cools down more slowly than air.

• And water heats up and cools down more slowly than either air or soil.

• Over the course of the year, the maximum input of solar radiation occurs at the time of the summer solstice, late June.

• Over the course of the year, the minimum input of solar radiation occurs around the time of the winter solstice, late December.

• The hottest time of the year (air temperature) does not usually coincide with the period of maximum solar radiation input. Usually, the hottest time of the year is later than the summer solstice, in July and August.

• The coldest time of the year (air temperature) does not usually coincide with the period of minimum solar radiation input. Usually, the coldest time of the year is later than the winter solstice, in January and February.

• Soil temperature (and water temperature) in the earth lag behind the warming of air temperature in the spring and the cooling of air temperature in the fall.

Procedure:

• Laboratory hands-on investigation of heat capacity.
  • Students have three flasks, each with a thermometer inserted through a hole in the stopper.
  • One flask contains air; a second flask contains water; the third contains soil.
  • Students expose all three flasks to a heat source, for example an electric space heater.
  • At regular intervals, students observe, record and plot on a graph, the temperature of each flask.
  • Experiment is repeated with a second set of flasks, this time cooled (as by an ice bath) rather than heated.
  • Students observe:
    1. some materials heat up faster than others,
    2. the materials that heat up fast also cool down fast, and vice versa,
    3. soil is slower to heat up and cool down than air,
    4. water is slower to heat up and cool down than either soil or air,

• Seasons versus solstice.
  • Teacher asks students when is longest day of year and shortest day of year.
  • Students examine data showing solar radiation input over the course of one or more years.
  • Students note that solar radiation input has maximum around longest day of year, and minimum around shortest day of year.
  • (Optional) Teacher reviews what controls variation in solar radiation input over the course of a year: length of day plus angle of incidence (plus cloud cover day-to-day) (See Cycles within Cycles investigation.)
  • Teacher asks students when is coldest time of year. Typically answer will be January/February. When is hottest time of year? Typically answer will be July/August.
  • Students examine air temperature data over the course of one or more years, and verify their subjective impression that hottest time of year is typically July/August and coldest time is typically January/February. (Clearest way to see this is to download data and create a 7 or 9 point running average before plotting data.)
  • Teacher poses paradox: why isn't the hottest time of year at the time of the maximum solar radiation input? why isn't the coldest time of the year at the time of minimum solar radiation input?

• Generating & Testing hypothesis.
  • Thinking back over investigation with heating/cooling flasks, and with teacher guidance as needed, students generate hypotheses about why air temperature lags solar input.
  • One hypothesis: Solar energy is "spent" warming up soil and water as well as air. It takes more energy input to warm up soil or water by a given amount than it does to warm up air by the same amount. Therefore, soil (and water) should warm up more slowly than air in the spring, and cool down more slowly than air in the fall. Contact with still-cool soil and water keeps the air cooler than it would otherwise have been in the spring. Contact with still-warm soil keeps the air warmer than it would otherwise have been in the fall.
  • Students can test this hypothesis by comparing air temperature versus soil (and water) temperatures in the spring and fall.

Data sets needed, and their status:

• One year (or more) of air temperature data

• Solar radiation data, covering the same time interval as air temperature data, from the same site as air temperature.

• Soil temperature data, 10 cm subsurface, covering the same time interval as air temperature data, from the same site as air temperature

• Soil temperature data, 100 cm subsurface, covering the same time interval as air temperature data, from the same site as air temperature

• (optional) Water temperature data, covering the same time interval as air temperature data, from a lake or pond or ocean near the air & soil measurements

• Suitable air and soil data are available for Black Rock Forest Open Lowland Site. Appropriate pond water temperature measurements are planned for the future.

Curriculum tie-ins:

Pertinent Web resources:

Follow-up activities:

• Use Stella to build a box model of heat flux among reservoirs of air, water, and soil. Try to match the temperature profiles observed in the natural data.

• Monsoons are seasonal winds that are caused by the differing heat capacity of oceans and continents. Explore the origin of monsoons using CD-ROM "ODP: From Mountains to Monsoons."

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