Scattered and Filtered Solar UV Measurements
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Any fabric used in a shade structure should have a UPF of 30 or higher. Look for our trusted Seal of Recommendation as you consider your options.
Density of the shade protection: A porch or lanai with a solid roof provides more consistent sun protection from above than the leafy branches of a tree, which shift in the breeze and allow some sun to penetrate. Proximity to buildings or other shade structures: A single tree is going to provide less UV protection than a tree surrounded by other trees in a dense forest. Similarly, a shady side of the street in a city provides more sun protection when the buildings are tall and tightly packed.
This is because surrounding structures can help to block more of the sky, lessening both direct and indirect exposure. Side Protection: As described above, UV rays can reach your skin by bouncing off reflective surfaces. Shade structures with side protection, like those nifty beach tents that have become popular in recent years, provide better protection than structures without any walls or side draping. In those cases, a good rule of thumb is to take note of the amount of open sky you can see from under the shade structure. Specifically, C14 measured an extreme UV-B — nm value of 8.
C14 suggest that their high value of 8.
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The standard deviation is Hence, a TOC of 75 would be more than 16 standard deviations below the average and therefore extremely unlikely. The high UV-B value could also be explained with an ozone column larger than 75 DU, but then the enhancement of UV-B by clouds would have to be much larger than any observation published in the past. According to results of radiative transfer model calculations presented by C14, an ozone column of 75 DU results in a clear-sky UVI of 66 for the conditions sza, altitude of the extreme event on January The measured UVI reported by C14 for this time is Our own model calculations result in a value of 62, which agrees with the results by C14 to within the uncertainties of the model input parameters.
Because the origin of this inconsistency is unclear, it is unknown whether it would be ameliorated by use of a more typical higher value of the TOC.
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We now turn our attention to the effect of clouds implicated in Figure 3 of C C14 made measurements of both UV-B and UV-A — nm radiation which is important because the former is quite sensitive to the TOC while the latter is much less so with respective radiation amplification factors, RAFs, of ca. Enhancements by clouds are a well-known effect and can arise for short times when both bright clouds and the direct solar beam are in the field of view Nack and Green, A coincidence between such rare cloud enhancements and record-low ozone is improbable.
Scattered and Filtered Solar UV Measurements
However, even more troubling are the variations in the few hours following the peak, with both UV-A and UV-B levels dropping sharply as summarized in Table 1. Reported UV-A radiation decreased by a factor of 3. If the UV-A decrease was due to clouds reducing rather than enhancing irradiance, ostensibly by now blocking the direct beam , and the same factor is applied to UV-B wavelengths, a remaining factor of 5. Thus, explanation of the UV-B observations requires not only an unprecedented low ozone column value for these latitudes 75 DU at the time of the maximum, but also a rapid increase to DU within a few hours.
We are not aware of such extreme spatio-temporal granularity of stratospheric ozone at any location around the globe. C14 suggest that an optically thick cloud at noon could have enhanced the absorption by tropospheric ozone, thereby contributing to the pronounced drop in UV-B and could thus explain the sharp noon-time UV-B reductions. Such a phenomenon is well known for heavy clouds over polluted regions because in-cloud scattering increases the total length traversed by photons before exiting the cloud base, increasing total absorption in accordance with the Beer-Lambert law.
For example, Mayer et al. Thus, neither a heavy cloud nor significant tropospheric ozone appears supported by the simultaneous UV-A and UV-B data. We also note that cloud enhancement effects under these relatively pristine conditions should be similar, and even somewhat stronger, at the longer UV-A and visible wavelengths compared to UV-B.
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Detailed radiative transfer models as well as observations confirm that the cloud effects generally deviate from unity more with increasing wavelength see for example Figures 2—4 of Crawford et al. Thus, based on previous theory and observations, we doubt that the larger UV-B variations are due to more profound cloud effects at the shorter wavelengths, be it as a result of absorption or scattering.
An alternative explanation of the results of C14 is that the large reported UV-B changes stem from instrument problems or data analysis issues. We recognize that accurate and reliable measurement of UV-B radiation can be extremely difficult, and much more so in the extreme environment of the high-altitude Andes. For example, any unquantified departures from the ideal cosine responses for the detectors would become more problematic because a larger fraction of the irradiance is in the direct solar beam. The particular type of instrument used in this study overcomes many of these difficulties, but has also been shown to be less stable than some other instruments, e.
The conversion factors from instrument response to the desired output e.
An example of this issue is clearly apparent in the inconsistencies between the measured UV-B and UVI values, and their ratios to model results discussed earlier. The overall uncertainty in C14's retrieved UV values relative to an irradiance scale established by a standards laboratory e. On this basis, we feel it would be premature to revise our understanding of shortwave radiative transfer or stratospheric ozone variability to match these results.
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The authors declare that the research was conducted in the absence of any commercial or financial relationships that could be construed as a potential conflict of interest. Badosa, J. Two methods for retrieving UV index for all cloud conditions from sky imager products or total SW radiation measurements. Bernhard, G. Ultraviolet and visible radiation at Barrow, Alaska: climatology and influencing factors on the basis of version 2 National Science Foundation network data.
Bodhaine, B. Spectral UV measurements at Mauna Loa: july july Google Scholar. Cabrol, N.
Record solar UV irradiance in the tropical Andes. Cordero, R.