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Name: John
Status: other
Grade: other
Location: FL
Country: USA
Date: N/A 


Question:
How would I go about calculating the change in temperature of the ground surface over time, for example 10 square meters of asphalt, as it is heated by the sun? I understand that I can use asphalt's emissivity to calculate the energy transfer at a certain temperature, but how would I calculate (estimate) the actual change in temperature over time? (assuming the sun is directly overhead, and ignoring weather effects)



Replies:
Gregory,

Capturing useful energy is actually very easy. The hard part is capturing it in a cost-effective way. There are lots of sources of useful energy around us -- the problem is that it costs to much to use them. (electricity and gasoline are just too darn cheap!).

Using an optical measurement is certainly not the simplest approach. It is full of possible experimental difficulties. A simpler approach would be to use a thermocouple, or resistance thermometer buried a few centimeters beneath the surface of the ground. This would be accurate to (+/-) 0.1 to (+/-) 0.05 C. This would give a direct reading and with a simple switching system you could use more than one probe -- maybe one on the surface, one maybe about 2 cm beneath the surface, and another maybe 10 cm below the surface. For a few more dollars you could set up a timer system to take readings continuously or at variable time intervals. This would provide you with data that you could correlate with air temperature, rain fall, cloudiness, and more data. A problem you should anticipate is being buried by the amount data you will gather. Careful planning is a bigger issue. The 'hardware' will probably be the least of your problems. I suspect you would be able to find temperature recording systems fairly reasonably, since multiple site temperature measuring devices are a common laboratory issue.

Vince Calder


John

How would you calculate the temperature change "over time" assuming the sun is directly overhead? The sun would be directly overhead only for a very short period of time, depending on your latitude. Perhaps your question needs further definition.

The temperature change would depend a great deal on where your 10 square meters (3.162277 meters on a side) of asphalt laid and during which parts of the day you are taking your measurements. Asphalt is a lot colder in Alaska than in Florida and the temperature change is not as much near sunrise/sunset than it is during the mid-day hours.

So for your local problem, lay a thermometer on the asphalt and take readings at whatever time interval you choose and chart it to derive temperature change rates.

Most long range temperature measurements (like satellite measurements of earth's surface) is done using infrared instruments.

Sincere regards,
Mike Stewart



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