Hanging around 1750 all summer

I looked at the photosynthetically active radiation (PAR) in Corvallis and Ithaca for each day of 2015, and there was something strange in the data. I didn't think anyone would notice, but someone caught it right away:

The station -- I've used data from the U.S. Climate Reference Network -- latitudes at Corvallis (44.4°N) and Ithaca (42.4°N) are similar, so I expect the maximum radiation to be similar. In fact, on average during summer I'd expect Corvallis to receive more radiation, and Ithaca less, because of the climate differences between those two locations.

What's going on? Are my calculations wrong? Is it sensor error? Were there forest fires producing smoke over Corvallis through the summer of 2015? Was it exceptionally sunny in Ithaca, and cloudy in Corvallis? I decided to look at more years, getting data from every year since 2007 for comparison. I looked at the daily totals.


Something strange happened with the data for Corvallis in 2015. In all the previous years, there were some days with global solar radiation above 27 megajoules per square meter per day (27 MJ m-2 d-1). In 2015, nothing. At first I'd thought that the Ithaca data were abnormally high. But in looking at the data from 2007 to 2015, it seems that Ithaca is within a normal range of global solar radiation, but Corvallis data are abnormally low.

I counted the days with global solar radiation greater than or equal to 27 MJ m-2.


In the 8 years prior to 2015, Corvallis had 4 years with more than 40 days above 27 MJ m-2, and only 1 year (2011) with less than 20 days reaching that level. Then in 2015, 0 days.

I'm guessing this was an undetected sensor problem or other data error. The photosynthetic photon flux density (PPFD) in Corvallis probably should not be hanging around 1750 all summer. It was in 2015, because of the data I was working with. But after a closer look, those data seem abnormally low. I'd expect the PPFD in Corvallis to be blasting through to 2000 in early summer. Can anyone with a quantum meter in the Willamette Valley confirm this?

A rule of thumb for cloud effect on PAR

I've shared some charts of photosynthetically active radiation (PAR) as it changes through the day and through the year at different locations. For example, this is the PAR at Corvallis for each day of 2015.

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For each day, as a function of latitude, and for each time within the day, as a function of both latitude and longitude, there is a maximum possible quantity of PAR. Then, in areas with no tree or structural shade, the PAR will be close to the maximum possible quantity, unless there are clouds.

But how much do the clouds reduce PAR? If you carry a PPFD meter around with you all the time, you can check it yourself. If you don't carry a PPFD meter, I suggest this rule of thumb to get some idea of just how much the clouds are reducing PAR.


I explained this at the Asian Turfgrass Field Day last week, when it was conveniently partly cloudy, with clouds of varying thickness, and I also conveniently had a meter for measuring the PPFD.

If you don't have a meter, but pay attention to your shadow, clouds, and the sun location, you can get a pretty accurate estimate of PPFD. Here's the rule of thumb, for four possible levels of cloud shade:

  1. If there are no clouds, or there are no clouds between you and the sun, then you can expect the cloud shade to be negligible, and the PAR should be close to the maximum possible. This app calculates the maximum possible for any location.
  2. If you can see your shadow, but there are clouds between you and the sun, then expect the PAR to be reduced by about 25%.
  3. If the clouds block so much light that you cannot see your shadow, but you can still tell which part of the sky the sun is in, expect the PAR to be reduced by about 50%.
  4. If you cannot see your shadow, and at the same time the clouds are so thick that you cannot tell where the sun is in the sky, then expect the PAR to be reduced by about 75%.

When there are no clouds, then one will see the maximum possible PAR, as shown in this chart for Bangkok and Tokyo. They are different because Bangkok is about 13 degrees north of the equator, and Tokyo is 35 degrees north.

image from c2.staticflickr.comOne can also look at measurements of solar radiation and express them in units of PAR. Now the cloud effects are taken into account. For example, this is Ithaca, NY, and you can see the effect of clouds on PAR.

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PAR data for Corvallis and Ithaca are converted from the global solar radiation measurement of the U.S. Climate Reference Network sub-hourly data. I usually use a transmittance value of 0.75 on a clear day to estimate the portion of extraterrestrial solar radiation (Ra) that reaches the surface as global solar radiation (Rs). That's the basis for the max possible (blue lines) I've shown on these charts. But these ones use different transmittance values. I used 0.68 for Corvallis, and 0.8 for Ithaca, because those are what made a good match for the USCRN data for those locations. I'm not sure why the measurements are different on clear days -- Ithaca has a higher Rs than does Corvallis but only a small difference in latitude. That's something I'll study some more.

Animated charts showing photosynthetically active radiation for a year

I spoke at the Sustainable Turfgrass Management in Asia 2016 conference about light at different locations. The presentations slides can be viewed here, or embedded below. For more about the conference, which saw 278 delegates from 24 countries and 5 continents travel to Pattaya this year, see this post at the Asian Turf Seminar site.

Light is important. Without enough light, grass won't grow well. I suggested that "no-problem" daily light integral (DLI) values for putting greens of bermudagrass, seashore paspalum, and zoysiagrass, may be about 40, 30, and 20 respectively. And I showed what PAR is, and how PAR is measured in one second as the photosynthetic photon flux density (PPFD), and then how all the PPFD over the course of a day are added together to make up the DLI.

I showed charts for one day, and also animated charts that show PPFD and DLI for every day of the year. This chart shows the maximum expected PPFD by time of day, and maximum possible DLI by day of the year, at Tokyo and Bangkok if there were no clouds. You may need to click the browser's "refresh" button to play these animations.


I wanted to visualize how these maximum possible values, on days when the sky is clear and about 75% of the extraterrestrial radiation reaches the earth's surface. To do that, I looked up the global solar radiation for Tokyo for every hour of 2015, converted those values to PAR units, and plotted them together with the maximum possible values assuming 75% transmittance of extraterrestrial radiation. That is plotted here.


I also explained that the global solar radiation has a large influence on the evapotranspiration (ET). I demonstrated this ET calculator that uses the Hargreaves equation to estimate the ET based on global solar radiation.

December and January DLI in Everglades City, Florida

I've been reading about the rains and clouds in South Florida and how extraordinary the past couple months have been. I saw these charts from Travis Shaddox, and I wondered what the light would be in photosynthetic units.

I downloaded monthly summary data since March 2007 for Everglades City from the NOAA. I use these data because they include global solar radiation, and I converted from energy units of MJ/m2 to photosynthetic units of mol/m2 using the 2.04 conversion factor of Meek et al.

This shows the average daily light integral (DLI) each month. One can see the seasonal changes, and one can also see that December 2015 had the lowest DLI of any December and that January 2016 had the lowest DLI of any January. I plotted all the data I could get, which is since 2007; I don't know what the values would have been before that. In the past decade, though, these were the lowest.

image from farm2.staticflickr.comLooking just at December and January year by year, January 2016 really stands out for having a low DLI. Blue triangles are December DLIs and red circles are January DLIs; the vertical dashed lines (blue for December, red for January) show the averages prior to Dec 2015 and Jan 2016.

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In a normal year at Everglades City, January would have more photosynthetic light than December. For seven out of the past eight years, the month of December had a lower DLI than January.  Only 2014 had a lower DLI in January than in December. But January 2016 is a big outlier; not only does January 2016 have the lowest DLI of any of the previous Januaries, but it also has a lower DLI than any of the previous Decembers.

My handouts for the Northern Green Expo

I'm giving five presentations at the Northern Green Expo.

  • The (New) Fundamentals of Turfgrass Nutrition
  • Nutrient Use by the Grass and Nutrient Supply by the Soil
  • Calculating the Fertilizer Requirement for any Turfgrass, Anywhere
  • Soil Water Management: Amount, Timing, and Syringing
  • Instead of Shade, Let's Talk About Light

I combined the handouts for each presentation into this single document for easy reading. I especially like this part:

Selection_060 Selection_061

A summary of photosynthetically active radiation for 1 year at Tokyo

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This chart (download it here) shows the average photosynthetic photon flux density (PPFD) for each hour of 2015 at Tokyo. The daily light integral (DLI) is the number written in black at the top right corner of each facet in this chart.

One can get some idea of how the DLI changes seasonally and with cloudy weather; one can also see how the PPFD changes from sunny to cloudy days at different times of the year.

40, 30, & 20

I spoke about light -- photosynthetically active radiation, to be specific -- in this presentation at the Japan Turf Show.

I was asked what daily light integral (DLI) is required for different grasses. My answer was, for warm-season grasses on putting greens, I'd look at the moving average of DLI, and I think good numbers are 40 for bermudagrass, 30 for seashore paspalum, and 20 for korai (Zoysia matrella).

If the DLI is above 40, bermuda won't have any light problems. If the DLI is less than 40, it will be a challenge. For seashore paspalum, I'd estimate that value of no problem above, and challenge below, to be 30. For korai, I'd put the number at 20. And for cool-season grasses, I guess the number is about 20 also. I base my guesses on observations of turfgrass performance in locations with varying DLI. Fortunately there is some ongoing research in this area that should give more accurate values than my guesses.

DLI values aren't always available; sunshine hours data are around -- at least the average sunshine hours data are available for a lot of places around the world. And to make a rough estimate of DLI from sunshine hours, one can estimate each hour of sunshine will give 5 moles of photons per square meter. Thus, on a day with 5 sunshine hours, one could estimate the DLI to be 25.

The climate charts at this website have normal sunshine hours data for a lot of places. To look at numbers in tabular format, the climate information on the Hong Kong Observatory website has sunshine data in an easy to view format.

On the charts I've made, I sometimes showed the average daily hours of sunshine, and sometimes the average monthly total. If daily, multiply by 5 to get an estimate of DLI. If monthly, 100 hours of sunshine gives a DLI of about 16 moles of photons per day; 200 hours is a DLI of about 33 per day; and in a month with 300 sunshine hours the average DLI would be about 49 each day.

One could, for example, look at locations such as Atlanta and Ishigaki and plot the sunshine hours for an entire year. I like to look at the combination of temperature and sunshine for each day, to see the area encompassed on the chart.

One could also display the sunshine hours on their own, with time on the x-axis.

Either way, one can see that more than half the year the sunshine hours in Atlanta are more than 200 per month, and for more than half the year in Ishigaki the sunshine hours are less than 200 per month. Looking even more carefully, it seems like Atlanta has about half the year at 230 or above, and Ishigaki has about half the year at 150 or above. From a chart like this, and a conversion of those sunshine hours to estimate DLI, one can get an idea if there is enough photosynthetically active radiation to easily manage a certain species, or if such management will be a challenge.