Hillshade values for winter and summer

Question

I'm working with summer and winter hillshade values that were calculated in ArcGIS9.3 (but I'm now viewing them/working with them in ArcGIS10.2).

Hillshade values were calculated using the hillshade tool with 30m DEMs. Logically, I would assume that if two sample locations had the exact same winter hillshade values, they should also have the exact same summer hillshade values. However, the data doesn't match this thought process.

Can anyone offer a reason/situation where sites may have different summer hillshade values when their winter values are identical?

Answer 1

hillshade is based on the angle between the normal to the slope face and the sun rays. This angle is in a 3D space, so different combination of slope and aspect give the same value (like a cone with the sun ray for axis). In winter and in summer, both the sun elevation and the sun azimuth can be different at the same hour of the day. So you can have different angles in summer when the angles were the same in winter.

Note : even in 2 D, you would possibly have the same sun/object angle for different slopes (maximum 2, while in 3D there is an infinity). In the example below you can see 2 slopes (vertical and horizontal faces) with that would have the same hillshade value in winter (with the sun at 45degree) but different hilshade values in summer.