I. Mean
III. Angstrom Exponents
IV. Interpolation of Aerosol Optical Depth at 500nm
V. Limitations
I. Calculation of Mean: Back to Top
** All averages are calculated using the time in GMT for the solar day.
** The GMT solar day may overlap two calendar days for some AERONET sites.
"Day" is defined as the GMT solar day.
Daily
A daily mean is computed when more than three observations were available in a day.
Monthly
A monthly mean is computed using the daily averages available in the month.
Annual
An annual mean is computed using the monthly averages when each month contained an average.
Multi-Annual Monthly
A multi-annual monthly mean is computed from the daily averages for each month over a multi-annual period.
Multi-Annual Annual
A multi-annual annual mean is computed using the multi-annual monthly averages when each month contained an average.
The following formula is used to compute the averages:
NOTE: The aerosol optical thickness (AOT) is calculated for the following channels: 1020, 870, 670, 500, 440, 380, 340, and 500 (not interpolated). The "500" channel includes interpolated and measured data at 500nm. The "500 (not interpolated)" category contains only AOT values derived directly from 500nm measurements made by the sun photometer.
II. Calculation of Standard Deviation: Back to Top
Standard deviation is calculated under the same conditions as the mean. The following formula is used:
III. Calculation of the Angstrom Exponents (alpha) Back to Top
Angstrom Exponents
For each measurement in a day, aerosol optical depth and wavelength logarithms are calculated for each non-polarized wavelength between two channels: 340-440, 380-500, 440-675, 440-870, 500-870, and polar 440-675 (i.e., only non-polarized channels 440 and 675 for the polarized instrument). These numbers are used to form a least squares regression fit to the data. The negative slope of this line to the x-axis is the Angstrom exponent.**
Least Squares Regression Fit
Linear Relationship:
,
Angstrom Exponent (Slope):
Offset:
**Reference: Angstrom, A., On the atmospheric transmission of Sun radiation and on dust in the air, Geogr. Ann., 12, 130-159, 1929.
IV. Interpolation of Aerosol Optical Depth at 500 nm: Back to Top
If a wavelength does not exist, the aerosol optical depth for this wavelength is estimated by determining the slope of the two closest wavelengths to predict the value of tau at the given wavelength.
,
where slope and offset are calculated from the least squares regression fit of the 440-870 Angstrom exponent in III.
V. Limitations: Back to Top
** Yearly averages are not calculated when at least one monthly average is missing.
** Multi-year yearly averages are not calculated when at least one multi-year monthly average is missing.
** Limits are not placed on the number of days for the monthly averages.
** Statistics are updated once daily at 2AM ET.
** Precipitatable water measurements are only raised to Level 1.5.
Last Updated: 24 October 2003