The photosynthetically active radiation (PAR) varies by time of day, season, and the presence -- or absence -- of clouds. If shade is imposed, the PAR is reduced for the duration of the shaded period.

This is what afternoon shade looks like, when shade that blocks 80% of PAR is imposed from 2 hours after solar noon until sunset.

For more about PAR and shade, see these posts:

• Simulated shade at Corvallis and Ithaca
• PAR and temperatures at Corvallis and Ithaca
• This is what PAR looks like
• This is what morning shade looks like