Site-specific Langley voltage intercepts are generated monthly using the theory briefly described in the In-Situ Langley Calibration under Data Processing. Bias and cosine corrected direct normal voltages, expressed as ln(mV), are regressed on airmass over a specific airmass range to determine the Langley voltage intercept value (V0). To ensure stable V0 values are obtained, variations in measured voltages due to clouds or atmosphere anomalies are removed using a two standard deviation test. V0 values are then taken as the x-intercept (airmass=0) of the final regressions performed using data points that pass all criteria(cloud-free); the slope of the line is the average optical depth. Gaussian process regression with estimated dynamic input uncertainty is used to provide the smoothed daily V0.
The data is downloaded in CSV format. Each set of data has a header. The header lines begin wth the # sign, and contains a description of location and data, along with possible data issues. The nighttime data is set to -9999 and data that are not currently available or associated with instrument errors are set to -9998.
The langley voltage offset data is processed monthly. The current month's data will be available at the beginning of the next month.