Hands On Physics


There are several ways to analyze your data to find the extraterrestrial or ET constant, all of which should yield the same result.
After you find your instrument's ET constant, you will insert it in a formula that will give you a standardized measurement of haze known as aerosol optical thickness (AOT).
When you made your Langley measurements for a range of Sun angles, each time the angle changed the thickness of the atmosphere (the air mass) between you and the Sun changed

Figure P2
Air Mass

When the Sun is straight overhead, its light passes through one thickness of atmosphere or one air mass. The air mass (m) increases as the Sun moves closer to the horizon. The m for each of your measurements is 1 divided by the sine of the Sun's angle

If you make a graph of the natural logarithm (ln) of the signal you measured versus m when you made the measurement, the data points should fall along a straight line out to an m of 10 or so, assuming the sky conditions were stable during your measurement period. If you extend the straight line, the ET constant will be where the line crosses the air mass axis at m = 0. When m = 0, there is no air mass, which means this is the signal the TERC VHS-1 will give if you could take it to the top of the atmosphere.

Several methods are available for extracting the ET constant from your Langley test data. They should all give similar or nearly identical results. All three methods perform what is called a linear regression on your data. Briefly, a linear regression fits the equation for a straight line to your data. In this case, ln of signal = a + (b x air mass).
You can use this equation to find the signal for m = 0.

Graph Paper Method

Scientific Calculator Method

Computer Spreadsheet Method

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