Redfoot tortoises in the shade, do they need UVB?
A common question from new Redfoot tortoises is "Why do I need a UVB light. Don't Redfoot tortoises live in the shade?"
Aside from the fact that they aren't strict deep forest dwellers, the following shows that they do get UVB even in deep shade. Here in South Carolina I have confirmed this using a Solarmeter 6.2 UVB radiometer.
Excerpts from:
DIFFUSE COMPONENT OF THE SOLAR ULTRAVIOLET RADIATION IN TREE
SHADE A.V.
Parisi, M.G. Kimlin, J.C.F. Wong, M. Wilson Centre for
Astronomy and Atmospheric Research, University of Southern
Queensland, Toowoomba, 4350, Australia.
J Photochem Photobiol B. 2000 Feb;54(2-3):116-20.
Comments added by me are in red italics.
INTRODUCTION
- UV exposure is due to sunlight
received as both direct and diffuse radiation.
- This diffuse radiation may
constitute a significant component to the UV exposure
received by eyes and skin as it is incident from all
directions and difficult to minimize with the usage of
hats, tree shade and shade structures as it can reach
surfaces shaded from the direct sun light.
- Others have measured the
spectral global and diffuse solar ultraviolet radiation.
The diffuse UV increases with decreasing wavelength
(UVB wavelengths are shorter than
UVA so UVB penetrates deeper
into the shade) due to the stronger
scattering at the shorter wavelengths.
- In tree shade, a larger
proportion of the UV exists compared to that in full sun
may be as a result of the diffuse component
(...
adding to the total UV amount from direct sunlight
areas). However, no previous research has
considered the diffuse UV irradiances in tree
shade.
- This paper presents: the results of quantitative measurements of the diffuse erythemal* and diffuse UVA (320 to 400 nm) at ground level on a horizontal plane at a Southern Hemisphere sub-tropical forest floor. This paper has provided the first set of quantitative data of diffuse erythemal UVB and UVA in tree shade at a sub-tropical Southern Hemisphere latitude.
CONCLUSIONS
- Over the summer, approximately
60% of the erythemal UV*(UVB)
radiation in the tree shade is due to the diffuse
component. Similarly, approximately 56% of
the UVA radiation in the tree shade is due to the
diffuse component.
- In the tree shade these diffuse
UV percentages are relatively constant from the morning
to noon to afternoon periods. In comparison, in full
sun, there is a decrease in the percentage diffuse UV
from morning to noon to afternoon.
- The exposures to diffuse UV on
a horizontal plane were measured in the tree
shade.
- The high diffuse UV component in the shade may result in high UV exposures to not only parts of the body on a horizontal plane that are not protected, but also, equally high UV irradiances to parts of the body, including the eyes and face, that are not UV protected.
*UV Erythemal action spectrum for humans has been employed widely for assessing the UV effect on human skin. The value of the sensitivity is normalized to unity at 298 nm (the UVB wavelength also most efficient for Vitamin D3 synthesis).
The whole paper can be viewed here
But there is more !!
Photochemistry and Photobiology
78(2):180-183. 2003
doi: 10.1562/0031-8655(2003)078<0180:SUBPSS>2.0.CO;2
Scattered UV Beneath Public Shade Structures During Winter¶
D. J. Turnbull*, A. V. Parisi, J. Sabburg
Centre for Astronomy, Solar Radiation and Climate, University of Southern Queensland, Toowoomba, QLD, Australia
102*To whom correspondence should be addressed at: Centre for Astronomy, Solar Radiation and Climate, University of Southern Queensland, Toowoomba 4350, QLD, Australia. Fax: +61-7-46311530; turnbull@usq.edu.au
Abstract
Broadband field measurements were conducted beneath three different-sized public shade structures, small, medium and large, during winter in the Southern Hemisphere. These measurements were compared with the diffuse UV to quantify the relationship of the UV under and around the shade structures to the diffuse UV. For the shade structures, a relationship between the diffuse UV and the UV in the shade has been provided for clear skies and solar zenith angles (SZA) of 49–76°. This allows the prediction of the UV in the shade of these structures if the diffuse UV is known. The ultraviolet protection factors for the three shade structures ranged from 1.5 to 5.4 for decreasing SZA. For the greater SZA of 70–76°, the erythemal UV in the shade was 65%, 59% and 51% of that in full sun for the small, medium and large structures, respectively. For the smaller SZA of 50–53° the erythemal UV in the shade was 35%, 41% and 18% for the small, medium and large shade structures, respectively. From this research it can be concluded that the UV radiation levels in the shade in winter could cause erythema and other sun-related disorders.
