Ambient Data Interpolation
The temperature is the only variable for which hourly data is available in addition to monthly averages. So for the temperature, a double interpolation is performed to get a temperature profile for every time in a year depending on the date and the hour. For other data, the interpolation is based on date only. This interpolation provides:
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The annual fluctuation of the dew-point temperature, the relative humidity, the wind speed, and the direct and diffuse solar irradiances
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The annual along with the diurnal fluctuation of the temperature
In all cases, the interpolation is of second order, with continuous first-order derivative.
When the Filter diurnal variation checkbox is selected, the diurnal temperature fluctuation is smoothed out, and only the annual fluctuation remains. As relative humidity depends on temperature, it will display the same fluctuation.