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?

Radiation of light

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.

Spectrum of radiated light

Infrared thermometer

The point & shoot thermometer is detecting the infrared light emitted by objects.

• Plastic and glass are opaque to this far infrared light, so the thermometer can’t “see” through it

• Shiny materials (aluminum foil) reflect light, so you are more likely to measure the temperature of an object reflected in the foil than the temperature of the foil itself.

“Point and shoot” outside

• What interesting observations did you make?

Solar heating

How fast did the box warm up, and what was the final temperature

• With white paper in it?• With black paper in it?• With the plastic cover?Explain your observations.Why does it take a while for the box to reach the

final temperature?

Why does it get cold at night?

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.”

A model for the daily temperature variation

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

The model in Excel

Power in = 1 – ((time-12)/6)^2 =1 kilowatt/m2 at noon = 0 at 6 AM and 6 PMPower out = constant (= average of Power in)

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solar inputradiation

The model in Excel

Result:

Is this a good model? Does it agree with the data?

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solar inputradiationtemperature

Average temperature of the earth

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?

Energy balance of the earth

Greenhouse effect

Radiation input and output

The global warming scenario

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.

Getting temperature data

Weather Underground http://wunderground.comHas 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.

Getting temperature data

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 )

Lexington and US history

Lexington temperature history

US temperature history1880 1900 1920 1940 1960 1980 2000 2020

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5-yr avg T (F) Series13