Mapping Light Across the United States
Historically, light has been a difficult topic to discuss with growers for two reasons: 1) everyone seems to use a different light unit – footcandles, lux, lumens, micromoles, etc., and 2) accurate light sensors are expensive. So, our first goal was to develop a method for growers to estimate the amount of light being delivered to their greenhouse crops without having to invest in a lot of equipment. The second goal was to provide common ground on which the greenhouse industry could discuss the important issue of light.
With these goals in mind and the support of the Bedding Plant Foundation Inc., we developed a series of monthly light maps of the United States. These maps provide an estimate of the sunlight delivered outdoors. The amount of sunlight actually getting to your crops inside the greenhouse can be estimated through a relatively simple process outlined later in this article.
These maps present the light data as "the number of moles of light per day." Don’t let this terminology put you off. We recognize that this is a relatively new term for many horticulturists; however, it is the most useful term to describe light that horticulturists can use. As we all become more familiar with the term "moles per day," or simply "moles," light will become an easier topic to discuss, and we will do a better job at managing our greenhouse light environment.
Instantaneous Light Measurements
Instantaneous light measurements made with footcandle meters or quantum sensors provide a measure of the light intensity during one slice in time. What good is this? Well, it’s about as good as a meteorologist trying to describe yesterday’s rainfall by stating that, "We had 5 raindrops per second yesterday." It is preferable to know the quantity of rain that fell over the entire day, e.g., "We had one inch of rain yesterday."
Similarly, if we are trying to describe how much light our greenhouse crop is receiving, stating that "we had 5,000 footcandles (or 1,000 micromoles) yesterday at 2:36 p.m." is not very helpful. But if we could measure the light intensity delivered to the greenhouse all day long, then add up all of those individual measurements, we could state "the quantity of light delivered to our crop yesterday was 14 moles. "
Integrated Light Measurements
Light sum, daily light integral and light quantity are terms used to describe the amount of light delivered over the course of one day. These are referred to as integrated measurements because integrated literally means "added up." Thus, the light sum is calculated by adding up, or integrating, all of the instantaneous measurements made during the day. The unit most often used to report the light sum is moles per day (abbreviated mol m-2 d-1).
The next question is, "How do I measure moles per day?" One answer involves purchasing more equipment. The simple answer is to use the light sum maps provided in this article to estimate the number of moles of light per day delivered to your location during any given month of the year.
Mapping Light across
the United States
The light sum contour maps presented in this article are derived from 30 years of hourly data collected at 239 locations across the country. This represents a large amount of data but is still only an average of five locations per state. Therefore, variation due to local geography and microclimate cannot be accounted for entirely. Also, remember that these light sum maps represent light levels outside the greenhouse, not inside.
The maps clearly demonstrate the large changes in light that occur throughout the year. For example, Pittsburgh, Pa. typically receives 5 to 10 moles per day in January, 25 to 30 moles in March, and 40 to 45 moles in May.
The largest changes in light sums occur during the spring and fall. For example, during February most of Florida receives 30 to 35 moles, the same light sum that Maine receives in March. Similarly, Atlanta, Ga. typically receives 30 to 35 moles per day in October, the same light sum as Michigan receives in September.
The light sums are influenced by several factors including: solar angle (the position of the sun in the sky), photoperiod, cloud cover, air humidity and elevation. Note the horizontal patterns across the United States that occur during the winter months. This general pattern is a result of lower light intensities and shorter daylengths across the northern United States. During the summer, the light intensities are still lower in the north, but the daylength is longer in the north than in the south. These effects result in no significant differences in the light levels during July along the east coast from Miami, Fla. to Portland, Maine. During the summer, the major differences occur from east to west. These differences are due to lower humidity and fewer cloudy and rainy days in the west.
Light Inside the Greenhouse
The maps provide an indicator of outdoor light levels, but how much of that light actually gets inside the greenhouse? Not as much as you may expect. The light levels measured at the bench or floor inside a greenhouse are typically 40 to 70 percent of the levels measured outside the greenhouse. Greenhouse light transmission depends on many factors including: time of year (solar angle), greenhouse orientation (north-south versus east-west), glazing material, greenhouse infrastructure, hanging baskets, shade cloth, dust on the glazing material, etc. So, there is only one way to know how much light is transmitted through your greenhouse structure. Take some measurements.
Don’t throw out that footcandle meter, yet!
Footcandle meters are a relatively poor means of measuring light, but they can work well for measuring greenhouse light transmission. Greenhouse light transmission is determined by comparing the light intensity measured inside the greenhouse to the light intensity measured outside the greenhouse.
Take a measurement inside the greenhouse, then quickly run outside and take a measurement away from any structures that might be reflecting light. Repeat this procedure several times. Repeat the measurements in different greenhouse structures and at different times of the year, since greenhouse light transmission will vary from one greenhouse structure to the next and from one month to the next.
Greenhouse light transmission measurements are best made on a hazy or uniformly cloudy day. Partly cloudy days are difficult for measuring greenhouse light transmission. Because light levels are continuously changing, it is difficult to compare the inside measurement with the outside measurements. Sunny days are also difficult because the light intensity measurement inside the greenhouse can vary tremendously due to the placement of the sensor. Placement in a sunny spot will overestimate light transmission, while placing the sensor in a shadow will underestimate light transmission.
You can estimate the number of moles of light delivered to your bench crop by multiplying the greenhouse light transmission (use 0.40 for 40 percent transmission) by the light sum number indicated on the maps for the desired location and month. For example, a Chicago, Ill. greenhouse in March with some hanging baskets overhead may have 40 percent light transmission. The maps indicate that, on average, Chicago will receive 25 to 30 moles per day of light in March. Therefore, the plants on the bench will likely receive approximately 10 to 12 moles per day during the month of March.
Apologies to the Outliers
IN 1998, we presented in GPN a series of light sum maps for the Eastern United States, and we received an overwhelming response from readers. However, most of the e-mails questioned why we didn’t include the rest of the United States. So, this time around, the maps contain the contiguous 48 states, and we offer our apologies to those of you in Hawaii and Alaska. There were insufficient data available for us to have mapped those locations, so this was the best that we could do. As for Canada, solar radiation data were not readily available from the sources that we used; however, sufficient sources do exist for Canadian light maps to be developed.
With the support of the Bedding Plant Foundation Inc., we are currently conducting experiments to let growers know how much light greenhouse crops really need. Our results will appear in an upcoming issue of GPN.
The authors acknowledge the financial support of the Bedding Plant Foundation Inc., which allowed this project to be completed.