Transmittivity for area solar radiation tool

[ShirleyLi]

I'm using area solar radiation tool but I don't know how to determine the transmissivity parameter. The documentation says "Transmittivity is a property of the atmosphere that is expressed as the ratio of the energy (averaged over all wavelengths) reaching the earth's surface to that which is received at the upper limit of the atmosphere (extraterrestrial)". How can I obtain the values for my study area in the US? Thank you!

[DanPatterson]

Area Solar Radiation (Spatial Analyst)—ArcGIS Pro | Documentation

The amount of solar radiation received by the surface is only a portion of what would be received outside the atmosphere. Transmittivity is a property of the atmosphere that is expressed as the ratio of the energy (averaged overall wavelengths) reaching the earth's surface to that which is received at the upper limit of the atmosphere (extraterrestrial). Values range from 0 (no transmission) to 1 (complete transmission). Typically observed values are 0.6 or 0.7 for very clear sky conditions and 0.5 for only a generally clear sky.

[ShirleyLi]

Thank you! Is this measured by sensors? If so where can I download it? Or do I have to estimate the values? 

[DanPatterson · Accepted solution]

It is something that is derived usually over long term measurements by comparing actual measured insolation relative to the theoretical (calculated) values based on the latitude (actual/theoretical = transmissivity)

There is no downloading it, You would have consult the closest major meteorological station that measures incoming solar radiation to see if they have a value for their location.

For example, for Canada

High-Resolution Solar Radiation Datasets (nrcan.gc.ca)

You could do your own estimate by doing the comparison of measured to theoretical insolation if you wanted, but 0.65 is a pretty good value to use unless your airspace is extremely polluted.

There are lots of academic papers and reports out their  (Dr Google .... insolation transmissivity

A simple model for estimating the transmittance of direct solar radiation through clear atmospheres ...

[ShirleyLi]

Thank you so much! It is very helpful!