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Comparing Infrared and Air Temperature

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Author: Randall Frost, Ph.D.

Grade Level: Middle School, High School; Type: Physical Science, Earth Science

Objective:

To formulate and test a hypothesis about the relationship of infrared and air temperatures throughout the day.

Research Questions:

  • How do the infrared temperatures of reflective objects vary throughout the day?
  • How do air temperature and infrared temperature correlate at different times of the day?

The surface of the earth absorbs radiation from the sun, and reflects much of it back into the atmosphere as infrared radiation--that is, non-visible, thermal energy.

The air in the lower atmosphere is usually not heated directly by the sun. It is heated rather by the re-radiation of heat from the earth’s surface, by air molecules coming in contact with warmer objects on the ground, and by air molecules coming into contact with each other. Dry air consists of about 78 percent nitrogen and 21percent oxygen. Nitrogen and oxygen molecules do not absorb infrared radiation, but water vapor and carbon dioxide do. These gases then re-emit energy back to the earth’s surface.

On any given day, sunlight is most intense when the sun is highest in the sky. Daytime air temperatures usually lag a few hours behind peak sunlight intensity. This is because after the sunlight warms the ground, the transfer of energy from the ground to the air molecules is not instantaneous.

A conventional thermometer measures the average speed of air molecules surrounding the thermometer. Air temperature is higher when the air molecules are on average moving faster, and cooler when they are on average moving slowly.

An infrared thermometer measures the infrared radiation emitted by an object, and uses that measurement to calculate the temperature at the surface of the object. The infrared thermometer does not come into contact with the object whose temperature it measures.

Materials:

Experimental Procedure:

  1. Read about heat exchange between objects at the earth’s surface and air molecules in the lower atmosphere. Also read about the reflection of energy from sunlight as infrared radiation. Then formulate a hypothesis that predicts how the temperatures of objects reflecting infrared radiation will correlate with air temperatures throughout the day.
  2. Starting in the early morning, suspend the thermometer to be used for air temperature measurements in the shade, in direct line with the air to be measured. Keep it away from buildings and surfaces that reflect heat.
  3. Measure and record the air temperature.
  4. Measure and record the infrared temperature of objects in the sun that reflect infrared radiation. These might include concrete or asphalt surfaces, or walls made of stone or brick.
  5. Note the time of day.
  6. Repeat these measurements at one hour intervals throughout the day.
  7. Tabulate and plot your data.
  8. Evaluate your hypothesis using your data. Revise it if necessary, and propose additional experiments.

Time

Air temperature (deg C)

Infrared temperature (deg C)

Object measured

 

 

 

Concrete surface

 

Brick wall

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

Terms/Concepts: Infrared radiation; Atmosphere; Air molecules; Air temperature; Sunlight; Infrared temperature

References: NASA's "Sunlight and the Earth" page; NASA page on Infrared Energy; "Lab Handout: Infrared and Visible Light Radiation"

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