How Much do Clouds Affect Photosynthetic Irradiance?
Measures of Light at Thailand, Hong Kong, Vietnam, andJapan
Micah Woods*

ABSTRACTThe main factors inﬂuencing turfgrass growth arenitrogen, water, temperature, and light. Turfgrassmanagers are able to modify the nitrogen and water supply to the grass, but are limited in theamount of change they can make to the temperature and the light. Light, more speciﬁcally the instantaneous photosynthetic irradiance, was measured at multiple locations in East and Southeast
Asia during July, August, and September of 2012.
Because of typical cloud cover in these regions during this season, the average photosynthetic irradiance at midday was 30% lower than in full sun,with photosynthetic irradiance reduced by 50 to
75% on cloudy days. A daily light integral was calculated based on these measurements. In 3 days ofmeasurements at Hong Kong, 17 days at Thailand,
2 days at Vietnam, and 33 days in Japan, the average daily light integral (DLI) was 38.4 mol m−2 d−1
at Hong Kong, 35 at Thailand, 38 in the Ryukyu Islands, 48.1 in Central and West Japan, and 22.4 inVietnam. This report provides details of these measurements and gives suggestions on how these meters can have practical use for turfgrass managers.

T
he importance of light in determining which
turfgrass species grow well and how they can best

be managed cannot be overstated. This report discusses data on photosynthetic irradiance collectedat Thailand, Hong Kong, Vietnam, and Japan, withthe locations of data collection shown in Figure 1.
The light that is available for plants to use
is termed photosynthetic irradiance and is expressed on an instantaeous basis in units of micromoles of photons per square meter per second(µmol m−2 s−1 ) or on a daily basis as a daily lightintegral (DLI) which is in units of moles of photons
per square meter per day (mol m−2 d−1 ). Anotherbroad term for photosynthetic irradiance is photosynthetically active radiation, or PAR. And the photosynthetic irradiance, speciﬁcally that measurementof the photons per square meter per unit of time,is also called photosynthetic photon ﬂux density, or* Chief Scientist, Asian Turfgrass Center,
http://www.asianturfgrass.com

1

Figure 1: This map shows the locations, in blue,
where irradiance measurements were made at
Thailand, Hong Kong, Vietnam, and Japan
PPFD. In this article, I’ll generally use the terms
photosynthetic irradiance and daily light integral
rather than PAR and PPFD.
Temperature data are readily available and theresponse of turfgrasses to variations in temperature are well-established. The response of turfgrass
to shade is also well-established, but irradiancedata are less readily available. Recent review articles on shade discuss reduced light environments
related to trees, shrubs, and buildings, but do notconsider the reduction in photosynthetic irradiancecaused by clouds (Beard, 1997; Stier and Gardner,2008; Dudeck and Peacock, 1992). Wiecko (2000)
noted the effect of clouds on turfgrass performanceand explained how bermudagrass did not perform
well in Guam and other parts of tropical Asia due
to restricted light from clouds.
Based on the relative performance of Cynodon
and Zoysia species in Asia, speciﬁcally in reference
to their performance as a regularly-mown turfgrass in areas unaffected by tree, shrub, or building shade,1 there is widespread empirical evidence
1 All the measurements of irradiance reported in this document have been in what we would term “full sun”, meaningthat there was no building or tree shade blocking the light. Any
reduction in midday irradiance in the data presented here is theWWW.ASIANTURFGRASS.COM

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Figure 2: Shown here near Naha, Japan, the quantum light meter from Spectrum Technologies was used
to make hundreds of measures of instantaneous photosynthetic irradiancethat shade from clouds has a profound effect onturfgrass performance in Asia.

Normal Ranges of Irradiance
On a sunny day, in the middle of the day, theinstantaneous photosynthetic irradiance will be
about 2000 µmol m−2 s−1 . Irradiance sensors (Figure 2) are available that measure just the photosynthetic irradiance, and in this project the Field
Scout Quantum Light Meter from Spectrum Technologies2 was used to collect the irradiance data.
When the sun is high in the sky, the photosynthetic irradiance is at its maximum; at night thephotosynthetic irradiance will be 0; when the sun is
low in the sky, the irradiance will be at a value morethan 0 and less than 2000 µmol m−2 s−1 . In measurements made approximately hourly during the dayfor more than two months at Thailand, Hong Kong,Vietnam, and Japan, the time at which maximum
irradiance occurs is from about 09:00 to 15:00.

Measured Irradiance and the Cloud
EffectFigure 3 shows the instantaneous photosynthetic
irradiance measured on 4 August 2012 at Ishigaki,
result of cloud shade.
2 Spectrum Technologies, Inc., Plainﬁeld, IL, USA, http://www.specmeters.com, Field Scout Quantum Meter Item #3415F

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Japan. It was a mostly sunny day with brief rain
showers in the morning and in the afternoon. The
photosynthetic irradiance dropped by more thanhalf during those rain events because of the associated clouds that blocked much of the irradiance.
On many days, however, there is not bright sunshine, and the clouds block a portion of the irradiance throughout the day. Figure 4 shows theaverage photosynthetic irradiance measured over
17 days at Thailand. Figure 5 shows the average
photosynthetic irradiance measured over 3 days atHong Kong, Figure 7 shows the average photosynthetic irradiance for 2 days at Hanoi, and Figure 6
shows the average photosynthetic irradiance measured over 33 days in Japan. The blue line on eachchart traces the average value of photosynthetic irradiance by time of day based on data collected atthat location.
At each location, midday cloud cover reduced
photosynthetic irradiance by 50 to 75%. This isa meaningful amount of shade, enough to cause
a decrease in turfgrass performance, especiallyfor species with a high light requirement suchas bermudagrass. Bunnell et al. (2005) demonstrated a continuous decline in Tifeagle bermudagrass quality as the daily light integral went from42 down to 22 mol m−2 d−1 . A 55% reduction in irradiance, similar to the effect of clouds, resulted inunacceptable quality of Champion bermudagrass(Baldwin and Liu, 2010). And after eight continuous weeks of a 65% reduction in irradiance,
Baldwin and Liu (2008) found that only Diamond

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Photosynthetic Irradiance

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Figure 5: Photosynthetic irradiance measured atHong Kong

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Figure 3: Photosynthetic irradiance on a relatively
sunny day at Ishigaki, Japan; drops in irradianceat about 9:00 and just after 15:00 were during briefrain showers

zoysiagrass3 maintained acceptable turfgrass quality; Sea Isle 2000 seashore paspalum and Celebration and Tifway 419 bermudagrass did not maintain acceptable quality with such a reduction in irradiance.

Implications of Reduced Irradiance
The data4 shown in Figures 3 to 7 demonstrate thatcloud cover at midday can reduce the photosynthetic irradiance by more than 75%. These results
also show that when averaged across multiple daysof varying cloudiness, the midday irradiance, in areas not affected by tree or building shade, was only
about 70% of full irradiance, due to the affect ofclouds. This varies throughout the year, obviously,
by location, so during cloudier times the reduction
may be more, and during sunnier times – January
in Bangkok comes to mind – there is expected to bealmost full irradiance at midday, every day.
There are a number of steps that can be taken toimprove turfgrass performance under conditionsof reduced irradiance.

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Figure 4: Photosynthetic irradiance measured atThailand

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1. Choose a grass that is tolerant of reduced irradiance (Figure 8). In Southeast Asia, manilagrass (Zoysia matrella) and broadleaf carpetgrass (Axonopus compressus) are the most tolerant of reduced irradiance. At low mowing
heights of 5 mm or less, such as found on
3 Diamond is a type of Zoysia matrella (manilagrass), similar
to the types found in tropical Southeast Asia.
4 This report was prepared on September 17, 2012. The datain this report are updated as more observations are made; theﬁrst version of this report was generated on August 16, 2012.
This report was written in RStudio (http://www.rstudio.
org/) and used the knitr package (Xie, 2012) to compile the ﬁnal
version which was typeset in XELATEX. Charts were made using
the ggplot2 (Wickham, 2009) package in R (R Development CoreTeam, 2012).

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golf course putting greens, seashore paspalum(Paspalum vaginatum) also shows excellent tolerance of reduced photosynthetic irradiance.
2. Most types of bermudagrass perform worse
than do the previously-mentioned grasses under reduced irradiance.

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3. To improve the tolerance of bermudagrass orother species to reduced irradiance, the mowing height should be increased, trinexapacethyl should be applied, and the nitrogen application rate should be reduced.

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How Can This Information be Used?
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Figure 6: Photosynthetic irradiance measured inJapan

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Figure 7: Photosynthetic irradiance measured at
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Photosynthetic irradiance sounds complicated, but
it is really quite simple. At midday, in full sun,the instantaneous photosynthetic irradiance willbe 2000 µmol m−2 s−1 . If you go under a tree thisvalue may be reduced to about 200 µmol m−2 s−1 . If
there are low clouds, the irradiance may be about
500 µmol m−2 s−1 . So we might measure a 75% reduction in irradiance from clouds, and we mightmeasure a 90% reduction in irradiance from a tree.
Any reduction in irradiance will cause a decline inturfgrass quality, especially for bermudagrass.
Being able to quantify that reduction in light allows a turfgrass manager to take proactive steps
to minimize turfgrass problems. That may involve
removing the tree or structure that is causing theshade, choosing a grass species (or variety) for theshaded area that is more tolerant of reduced irradiance, and determining when to make changes inmowing height and nitrogen rate and trinexapacethyl application as a result of reduced irradiance.
Just as digital soil moisture meters are becoming
an essential tool in turfgrass management, helping
to minimize water use while at the same time reducing drought stress and improving surface consistency, light meters that measure the photosynthetic irradiance will come to be a regularly-used
tool by turfgrass managers. The four main factors
inﬂuencing turfgrass growth are nitrogen, water,temperature, and light. Turfgrass managers knowexactly how much nitrogen they apply. Turfgrassmanagers know how much it rains, how much water is applied through the irrigation system, andwith a soil moisture meter can determine exactly
how much plant-available water is in the soil. And
when it comes to temperature, turfgrass managers
almost always know the day’s high, and low, andthe soil temperature, and sometimes even the leaf
surface temperature.
Adding irradiance data gives the turfgrass manager a complete picture, expressed in data that canbe compared with benchmarks, tracked throughthe season and from year to year, identifying theareas on a course that require special attention because of reduced irradiance, and quantifying theeffect of clouds on the light available for plantgrowth.

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Figure 8: There are three species of grass here, grown under reduced irradiance and mown at putting
green height in Southeast Asia during the sunniest time of the year; a) is seashore paspalum; b) is manilagrass; c) and d) are hybrid bermudagrass
In the absence of irradiance data, the Asian Turfgrass Climate Charts website5 provides a number of charts that show daily or monthly sunshine
hours, with links to the data sources for those whowish to explore this in more detail. As a general rule, for warm season grasses, when there areless than six daily sunshine hours, or less than 180
monthly sunshine hours, the photosynthetic irradiance can be considered reduced, and the mowing,
nitrogen, and trinexapac-ethyl shade management
strategies should be implemented.

References
Christian Baldwin and Haibo Liu. Altered light
spectral qualities impact on warm-season turfgrass growth and development. USGA Turfgrass and Environmental Research Online, 7(9):1–12, 2008.
Christian Baldwin and Haibo Liu. Nitrogen management for ‘Champion’ bermudagrass underfull sun and shade. USGA Turfgrass and Environmental Research Online, 9(6):1–8, 2010.
J.B. Beard. Shade stresses and adaptation mechanisms of turfgrasses. International Turfgrass Society Research Journal, 8:1186–1195, 1997.
B. Todd Bunnell, Lambert B. McCarty, James E.
Faust, William C. Bridges, Jr., and Nihal C.

Rajapakse. Quantifying a daily light integral
requirement of a ‘Tifeagle’ bermudagrass golfgreen. Crop Science, 45:569–574, 2005.
A.E. Dudeck and C.H. Peacock. Turfgrass, chapter 8. Shade and Turfgrass Culture. ASA-CSSASSSA, 1992.
R Development Core Team. R: A Language and Environment for Statistical Computing. R Foundationfor Statistical Computing, Vienna, Austria, 2012.
URL http://www.R-project.org. ISBN 3900051-07-0.
John C. Stier and David S. Gardner. Handbook ofTurfgrass Management and Physiology, chapter 28.
Shade Stress and Management. Taylor and Francis Group, 2008.
Hadley Wickham. ggplot2: elegant graphics fordata analysis. Springer New York, 2009. ISBN
978-0-387-98140-6. URL http://had.co.nz/ggplot2/book.
Greg Wiecko. Asian golf growth taxes bermudagrass’s ﬂexibility. Golf Course Management, 68(2):69–72, February 2000.
Yihui Xie. knitr: A general-purpose package for dynamic report generation in R, 2012. URL http://CRAN.R-project.org/package=knitr. R
package version 0.5.

5 http://climate.asianturfgrass.com/

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