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UV Light in Nature:  Solar Ultraviolet Light

Outside in the Sunshine - UVB recordings across the world - The effects of mesh and glass


Outside in the Sunshine
Fig. 1. Direct solar reading
Two types of recording are possible outdoors.Fig. 2. Reflected UVB
Direct readings are those where the meter is aimed directly at the sun (Figure 1). These indicate the maximum amount of UVB which a basking reptile would receive on its skin if it was sitting in full sunlight, angling its body toward the sun.

Indirect readings are those where the meter is aimed in other directions. These are more indicative of the amount of UVB a reptile receiv
es on its skin from reflected UVB. On a sunny day, UVB reflected and diffused from the sky and from the ground can reach quite high levels even in full shade. Figures 2 and 3 show readings of reflected UVB from a stone slab, and diffused UVB recorded in shade, taken within minutes of the direct solar reading shown in Figure 1.Fig. 3. Diffused UVB

To give us some idea of the sort of UVB levels found in nature, we are working with other members of the international internet group UVB_Meter_Owners to build a database of solar UVB recordings taken throughout the world at different times of day and months of the year.
Because there are considerable technical difficulties involved in the measurement of ultraviolet light, and different UVB radiometers of equivalent accuracy are known to give different readings with the same light source (18) all the recordings are collected using the same instrument, the Solarmeter Model 6.2 UVB.
This work is far from complete. Full details and updates are available from the group website, and we welcome new contributions.

When considering UVB in nature it is very important to remember that NO reptiles sit out in the sun all day. It is easiest to record maximum levels of solar UVB out in the open, under a clear sky. It is valid to gather such data since this gives a clear indication of the maximum UVB that a basking reptile could receive. It is far from valid, even dangerous, then to suggest that these levels should be provided in the vivarium. Account must be taken of the microhabitat, for every species. What shade and shelter does a wild reptile have? Does it ever see full sunlight? We are hoping to collect data from reptile microhabitats as well as readings taken under an open sky on clear days... but both types of recording are necessary.

Figure 4 (below) shows one set of "clear open skies" data from this group: solar UVB recordings from right across the world, made over the December solstice 2005.

Fig. 4: Solar UVB recordings  - December solstice 2005

The most important factor is the angle of the sun. When the sun is low on the horizon, its light must pass through a thicker layer of the atmosphere, which absorbs more UV light. Highest readings are therefore obtained around mid-day when the sun is highest in the sky or at high altitudes where the air is thinner. The highest readings of all (at sea level) will be seen under clear skies in the Tropics when the sun is directly overhead. At the equinoxes, this will be at the Equator; at the solstices, it will be along the Tropic of Cancer in June and the Tropic of Capricorn in December.
The further from the equator, the lower the UVB reading even at mid-day, because the sun will be lower in the sky. The lowest readings of all will be seen at high latitudes in either hemisphere during winter months - when the days are short and the sun does not rise high in the sky.

This effect is easily seen by examining the above chart. At the December solstice the sun is almost directly overhead at Alice Springs, in the Northern Territories, Australia. On clear days the UVB rises to above 100 microwatts per square centimetre (W/cm) by 8.00am and reaches over 450W/cm by noon. Levels above 20W/cm are seen for 11 hours or more.

The further north that recordings are made, the lower the maximum (noon) reading and of course, the shorter the day. In Florida, for example, at latitudes 25 - 30N, observers recorded maximum readings around 150 - 250 W/cm whereas further north in Colorado, 40N, readings only just topped 100W/cm. Readings here were boosted, too, by the high altitude; not much further north, in Illinois at 42N, only 70 - 90W/cm was seen.

Around 50N, the days are very short at this time of the year and UVB levels are very low indeed. The maximum reading taken in Wales (latitude 52N), despite clear sunlight, was only 29W/cm; and at Helsinki at 60N, our observer climbed onto his roof to see the sun and recorded only 7W/cm.

Readings taken over several days, from the same location, can be used to gain a clearer picture of the UVB available throughout the course of a day. Figures 5 and 6 show recordings taken in Wales over four days in March and early April 2005, and seven days in June 2005, respectively.

Fig. 5. Solar UVB : Wales, UK March- April 2005 

Fig. 6. Solar UVB : Wales, UK June 2005 

These graphs illustrate clearly the profound effect that the amount of cloud cover has upon the ambient UVB levels; however, by combining readings taken on different days, it is possible to build a picture of the maximum possible UVB as seen under clear skies, for any given time of day during that period. Comparison of the two charts also demonstrates the great increase in ultraviolet light as the days lengthen and the sun rises higher in the sky. The highest readings in June, at the time of the summer solstice, approach 400W/cm whereas in March, around the spring equinox, maximum readings are around 160W/cm.
Fig.7. The highest UVB recording on earth. Photo courtesy of Torey Lehman
One of the highest readings yet recorded on earth with a Solarmeter 6.2 was made by UVB_Meter_Owners member Torey Lehman on the summit of Haleakala, Mauii, Hawaii, latitude 21N, at 12.30pm on 25th May 2005. (Fig. 7) The sun was almost overhead and the high altitude (10,023ft) and clear sky resulted in a mid-day UVB level of 545W/cm.
Fig. 8. UVB in early September in the UK. Above: full sun. Below: small cloud in front of sun.
Clouds greatly affect the amount of UVB reaching the earth. Recordings show that even near the equator, heavy cloud and rain can reduce mid-day readings to below 100 W/cm. Figure 8 shows how in the UK, one small cloud reduced, in seconds, the UVB reaching two basking bearded dragons on a sunny day in August from 200W/cm to 66W/cm. UVB readings on overcast days in winter in the UK may be close to zero.

Outdoor UVB levels are affected by many other factors. A reptile basking in sunshine receives most of his UVB directly on his sunlit back from the solar rays, but all parts of his body receive some UVB, just as they receive light. Both ultraviolet and visible light are diffused and reflected by the sky, trees, grass; everything around him. Fig. 9. UVB on an overcast day in Essex, UK.

To investigate this further, a series of readings were taken in a garden in the UK on an overcast summer day (no direct sunlight) to examine the reflection and absorption of diffused UVB from the sky. The results are shown in Figure 9. UVB levels varied considerably. On the lawn and surrounding planted areas, levels ranged from 54W/cm (highest) to 13W/cm (lowest). Interestingly a small area to the side of the house surrounded by light concrete walls with a light stone patio area produced UVB levels of in excess of 130W/cm, demonstrating how reflection can cause UVB "hotspots" in nature.
Live plants absorb large amounts of UVB producing shade, but even on an overcast day as far north as the UK, dense vegetation did not reduce our outdoor readings to zero; even reptiles living in shade will be exposed to low levels of ultraviolet light.

The Effects of Mesh and Glass upon Solar UVB

Mesh Tests
Whilst taking readings out of doors, we examined the loss of UVB available to reptiles when mesh is used to cover a basking pen.

Direct readings from the sun taken from inside and outside of a sunlit pen covered with 2cm twillweld plastic-coated mesh indicated losses of 15-20% under the mesh.

On an overcast day, readings were taken from outdoor enclosures roofed with chicken wire, which blocked 7% of the UVB, and quarter-inch twillweld mesh, which blocked 35-37% of the UVB.Fig. 10.  50% loss of UVB through a thick flyscreen mesh

Further tests carried out on a piece of finer, 5mm zinc plated mesh revealed that it blocked out 20% of the UVB; and a type of flyscreen mesh commonly used for chameleon vivaria caused a loss of 30% of the UVB. 
The nylon mesh of an Apogee Reptarium reduced UVB by 40%; and finally, a thick flyscreen mesh used to roof a small outdoor enclosure blocked 50% of the UVB. (Figure 10.)
This demonstrates the importance of using the widest mesh possible for outdoor pens where maximum UVB exposure is desirable.

Glass and Acrylic Sheeting
We tested the transparency of various substances to ultraviolet light. 

"Crown glass" - window glass and aquarium/ vivarium glass - Fig. 11. No UVB reaches these bearded dragons through the window almost completely blocks UVB. A meter aimed directly at the sun through one double-glazed glass window recorded 2uW/cm when the unobstructed reading was 147uW/cm - over 98.5% was blocked. Through other windows and several glass vivarium panels, a 100% block was seen. Reptiles living indoors behind glass are unlikely, therefore, to benefit from natural UVB from the sun.

One of the authors was fortunate enough to visit a large private collection of European and Eurasian lizards housed in Sussex, in outdoor enclosures and converted glasshouses. Fine quality horticultural glass used in one greenhouse proved little better than ordinary glass: 97- 98% of the sun's UVB was lost. However, panels of 2mm Clear UV-Transmitting Plexiglas Acrylic Sheets and 10-year-old sheets of a twin-walled acrylic now sold as Plexiglas Alltop used to roof several others proved to have excellent UVB transmission. Losses of only around 20% were recorded with the acrylic sheeting and 30-40% with the twin-walled acrylic.

Further studies on these materials are planned. However, sheets of UV-transmitting acrylic are no longer mass-produced and are increasingly difficult to obtain. Most acrylics are now formulated with additives which block all UV transmission. We have found a very small number of stockists which sell acrylic sheets FOR USE IN SUNBEDS, which do transmit up to 80% of the UVB.

Tests on the effects of mesh upon the UVB output of lamps and tubes in the vivarium are described fully in the relevant sections of our Lighting Survey.



It's important to know the natural levels of UV light available to our reptiles across the world; but how much do they actually need? There's more on the current research into the UV requirements of reptiles in the next section:
What UV light do reptiles need?

How much UVB can be provided in the vivarium? We look at the provision of UVB using lamps and fluorescent tubes in:
UV in the vivarium

These sections are also accessible from the side navigation bar.

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