Clear sky global shortwave irradiance [W m^-2] is calculated using the model detailed in Appendix D of the ASCE Standardized Reference Evapotranspiration Equation.

Clear sky ultraviolet-A irradiance (UV-A), photosynthetically active radiation (PAR), and extended photosynthetically active radiation (ePAR) are calculated from clear sky global shortwave irradiance using ratios of UV-A to shortwave, PAR to shortwave, and ePAR to shortwave calculated from reference solar spectra. The UV-A to shortwave, PAR to shortwave, and ePAR to shortwave ratios are dependent on atmospheric air mass and vapor pressure and vary as a function of these variables in the calculator.

PAR and ePAR are converted to units of micromoles of photons per square meter per second [µmol m^-2 s^-1] using constants that represent the average energy content of photons in the PAR and ePAR wavelength ranges, 400-700 nm and 400-750 nm, respectively.

Estimated PPFD

Estimated Photosynthetic Photon Flux Density incident on horizontal plane for clear sky conditions

Measured PPFD

Measured value of Photosynthetic Photon Flux Density incident on horizontal plane for clear sky conditions

Difference from Model

Difference between the measured and estimated values expressed as a percentage

Latitude

Latitude of the measurement site measured in degrees. Insert as a negative number for the southern hemisphere.

Longitude

Longitude of the measurement site measured in degrees. Expressed as positive degrees west of the standard meridian.

Timezone

Timezone of the measurement site

Elevation

Elevation of the measurement site measured in either meters or feet

Air Temperature

Air temperature at the time of measurement in either celcius or fahrenheit

Relaitve Humidity

Relative humidity at the time of measurement expressed as a percentage

Date

Date at the time of measurement

Time

Time of day of measurement

Daylight Savings

Checked box indicates that it was daylight savings at the time of measurement