What is outgoing longwave radiation and what is its influence on plants?

I notice that there was a paper titled, ‘The underestimated effects of Longwave Outgoing Radiation on your greenhouse crop” at the recent Protected Cropping Australia Conference. Would you please explain what this means. 

Answer RICK DONNAN

All I remember about glasshouses from science in high school was that they allowed in and captured the incoming radiation from the sun, but then didn’t transmit out the emitted heat radiation from within the glasshouse. Hence, they could maintain higher air temperatures than cold outside temperatures.

Radiation
Radiation is universal. Known as electro-magnetic radiation, it all travels at the speed of light. However, there is an infinitely wide range of different wavelengths, going from extremely short gamma radiation at a fraction of a nanometre (1 billionth of a metre = 10 – 9 metre), to radio waves with wavelengths in kilometres. Also, the shorter the wavelength, the higher the frequency of the radiation and the more energy it contains.

All solid surfaces emit heat radiation. How much they emit is influenced by several factors, especially their temperature and also the emissivity of the surface. However, the wavelength of the radiation is directly determined by the surface temperature, the higher the temperature, the shorter the wavelength and vice versa.

Solar radiation
The radiation we are most familiar with is that from our sun, known as solar radiation. This is also the major impact upon a greenhouse and the plants within it. The range of radiation frequencies from the sun is known as its spectrum. This is graphed in Figure 1. The vertical axis is the energy strength of the radiation, termed irradiance and measured in Watts per square metre (w/m2). The horizontal axis is the wavelength of the radiation given in nanometres.

Solar Radiation Spectrum

Solar Radiation Spectrum (click graph to enlarge)

The black line is a reference and is the spectrum emitted by an ideal black body at 5250ºC. The bright yellow zone is the solar spectrum before it reaches our atmosphere. The bright coloured zone is the spectrum once it has passed through the atmosphere and reached earth. Various gases in the atmosphere absorb portion of some wavelengths in the spectrum. The most important is ozone (O3), which absorbs a high proportion of the dangerous ultra violet (UV) wavelengths. This is why astronauts in space require very strong UV protection. There is also some absorption by water (H2O), and a little by oxygen (O2) and CO2.

The global spectrum is split into three bands, the UV (below 400nm), the visible (400 to 700nm), and the infrared (heat, 700 to 3000nm). About 50% to 55% of the energy contribution comes as heat. It is important to recognise that heat radiation does not heat air directly. It lands on surfaces and heats those surfaces. In a greenhouse, the heat from surfaces such as plants, floor, structure, covers, etc, heats up a boundary layer of air, which is then moved by convection to warm the air within the greenhouse.

Outgoing longwave radiation
Usually taken as wavelengths between 4,500 and 100,000 nanometre. These come from much lower temperature emitters such as plants. So, the plants are losing energy to cooler surfaces.

The loss is not usually noticeable during the day other than when incoming radiation is low, that is, cloudy winter days. However, at night when there is no incoming solar radiation the impact on plants can be severe.

Sky temperatures
As measured with an infrared thermometer, the temperature of a clear sky is around – 50ºC. In outer space the surrounding temperature is absolute zero, that is, -273ºC. With cloud cover the temperature of the cloud is usually between -20 and -30ºC.

Greenhouse cladding properties
Greenhouses basically harvest light. Consequently, high light transmission is the fundamental requirement for greenhouse cladding material. Most common claddings, whether glass or plastic, sheet or film, transmit about 85 per cent to 90 per cent of incoming radiation.

Most claddings do not transmit UV, with the exception of low iron glass. Removing the iron gives slightly higher light transmission, but also enables crops such as coloured lettuce to develop deeper colour than under normal coverings.

Most coverings do not transmit longwave radiation, with the notable exception of plain polyethylene. This means that on a clear night the plants are losing heat energy to a sky temperature of – 50ºC. An example of the potential impact came where I live on an abnormally cold clear night late in spring. There was plenty of soft spring growth in nursery pot plants and the energy loss in polyethylene tunnels was such that there were huge losses due to frost damage. The same damage can be sustained outdoors, however, in this case it was the soft indoor growth, which made those plants extra vulnerable. The addition of additives and/or coatings can change the properties of cladding materials.

Plant energy losses
The major surface to which plants lose longwave radiation energy is the greenhouse cover. The colder the cover, the greater the energy loss and consequently the lower the plant temperature will go. The extreme of frost damage is extremely rare, however, it is possible for plant temperatures to get below the greenhouse air dew point. In this case, plants can suffer direct damage, but in particular they become vulnerable to attack by fungal diseases such as botrytis and mildews.

Plant energy losses can be significantly reduced by using an energy screen, the temperature of which will be much higher than the cladding. A shade screen will help to a lesser extent, and it is essential to close it at night, especially when the sky is clear. The inner cover of a twin-skin greenhouse will be warmer and reduce plant energy losses, however the incoming light levels are also permanently reduced.  RD

PH&G August 2017 / Issue 182


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