Clinical climatology.

An appreciation of the influence of environmental factors regarding the quality and quantity of terrestrial ultraviolet radiation (UVR) is important when trying to optimize the conditions for climatic therapy of skin diseases or in order to minimize the harmful effects of the sun. The time of day, season of the year and geographical latitude all have strong influences on the amount of UVB energy reaching the surface of the Earth. For example, it is important to realize that there be may be more sunburning UVB around on a moderately cloudy day at midday than on a clear day in the late afternoon. Other factors, such as reflection from the ground, are also important. A common misconception is that water reflects much of the incident ultraviolet radiation. In fact water and a green grass lawn have about the same reflectance. Furthermore, the erythemal effectiveness of sunlight is actually greater in the autumn than in the spring due to a thinner ozone layer. Some of the factors described above are often neglected when we advise patients about their sunlight exposure.