MUSE Physics Group June 2013 . The physics of Earth's temperature Sally Shafer and Joe Straley. “Point and shoot” thermometer. When does the thermometer correctly measure the temperature? How do you think the thermometer works? How do your observations support your theory?.
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The physics of Earth's temperature
Sally Shafer and Joe Straley
All objects emit light.
The total power per unit area radiated increases with the temperature (proportional to T4, where T is the absolute temperature).
The color of the light emitted also depends on the temperature. Hotter objects emit more blue light. Cool objects (below 500 C) only emit infrared light.
The point & shoot thermometer is detecting the infrared light emitted by objects.
How fast did the box warm up, and what was the final temperature
Explain your observations.
Why does it take a while for the box to reach the final temperature?
Here is a graph showing the temperature in Lexington, KY during the week starting September 14, 2012.
How is this related to the experiment with the box?
Explain the shape of the “temperature waves.”
During the daylight hours, power arrives from the sun.
All the time power leaves in the form of infrared radiation.
The temperature variation is determined by the changes in the amount of energy accumulated
Power in = 1 – ((time-12)/6)^2
=1 kilowatt/m2 at noon
= 0 at 6 AM and 6 PM
Power out = constant (= average of Power in)
Is this a good model?
Does it agree with the data?
The average temperature of a satellite orbiting the earth is below freezing, but the average temperature of the earth’s surface is about 50 F.
We can explain this using the “greenhouse” model that explained the box activity.
But what corresponds to the plastic lid?
The claim is that burning coal, oil, and gas adds CO2 to the atmosphere, making it more opaque to infrared radiation and therefore increasing the greenhouse effect.
The prediction is that the earth should be getting warmer.
Let’s discuss how to verify this.
Weather Underground http://wunderground.com
Has a section “Weather history” which will give you data for a day, a week, a month, or a year, for many locations around the world.
There are other sites that will give similar information.
Weather Warehouse http://weather-warehouse.com
We used it to construct an Excel page that gives the average temperature for Lexington, KY, each year since 1900.
(at http://www.pa.uky.edu/sciworks/climatechange/MUSE )
US temperature history