The photosynthetically active radiation (PAR) changes through the day and through the year. The PAR is measured instantaneously for a duration of 1 second as the photosynthetic photon flux density (PPFD), and by adding up the PPFD for all the seconds in the day, one gets the daily total of PAR, which is called the daily light integral (DLI).
These charts show the average PPFD on an hour by hour basis. With a look at a chart like this, one can see:
- how length of the day affects PAR, by looking at what time in the morning and what time in the evening the PPFD goes to 0.
- how time of the day affects PAR, by looking at the change in PPFD hour by hour through the day.
- how day of the year, and consequently sun angle, affects the PAR, by looking at the maximum values of PPFD at midday and seeing how they change through the year.
- how clouds reduce the PAR, by comparing PPFD on sunny hours or days to PPFD on hours or days that don't have full sun. For more about sun and clouds and time of year, see these descriptive slides with data from 4 days in Tokyo this year: a sunny summer day, a very cloudy summer day, a sunny autumn day, and a partly sunny autumn day.
This chart shows, for every hour of this year through last Friday, the average PPFD for that hour at Tokyo (red) and at Watkinsville (blue). Each panel of the chart is a single day, and the DLI in units of mol m-2 d-1 is written on each panel, in red for Tokyo and in blue for Watkinsville.
There have been 296 days this year, through October 23. On one of these days, February 10, there were erroneous data at Watkinsville, so I don't have a DLI. That leaves 295 days with a DLI for both Tokyo and Watkinsville. These locations have similar temperatures, and similar latitudes. How do they compare for photosynthetically active radiation? There have been 115 days with a higher DLI at Tokyo than at Watkinsville, and 180 days with a higher DLI at Watkinsville than at Tokyo.
I've made a couple other similar charts. This one shows the average PPFD at Tokyo hour by hour this year through October 12. Because the chart shows data for only one location, I've used color to indicate the month.
And the next one is the same location and dates as the above, with the addition of the DLI written on each panel.