SUNBURN 521 



the end of the summer. There is a tendency to think of the tropics as 

 receiving much more sunlight than the temperate zones and therefore to 

 anticipate greater danger of sunburn in the tropics. This is again a 

 (juestion of season. At the time of the simimer solstice the sun is over the 

 Tropic of Cancer, 23°27' north of the e(iuator. On that date there is 

 about the same sunlight at 47° north — roughly the latitude of Seattle, 

 Washington; St. John's, Newfoundland; and Paris, France — as at the 

 equator. On June 21, other things being ecjual (the ozone layer probably 

 would not be), there should be about as much danger of sunburning at the 

 one latitude as at the other. 



Another less obvious factor likely to mislead the unwary is the high 

 proportion of erythemal radiation that is contributed by reflection from 

 the sky. Sky radiation is sunlight that has been scattered, principally by 

 the gas molecules of the atmosphere. The shorter wave lengths are 

 scattered to a greater extent than the longer ones, and hence the sky radi- 

 ation is richer in the erythemal radiation than in visible radiation. ^^ At 

 noon on a very clear day in temperate latitudes, the sky radiation may 

 constitute only 10 to 15 per cent of the visible component falling on a hori- 

 zontal surface, whereas for wave lengths shorter than 0.32 /x, the direct 

 and sky radiation are about ecjual under the same conditions (Pettit, 

 1932; Luckiesh et at., 1944). Obviously one need not be directly exposed 

 to the sun to receive a sunburn if he is sufficiently exposed to the sky. 

 This explains why, for instance, one may sit under a beach umbrella pro- 

 tected from the direct rays of the sun and yet receive a severe sunbiuni 

 from sky radiation which the umbrella does not cut off. On a lightly 

 overcast day, particularly in a fog, the scattered erythemal radiation may 

 be many times the direct radiation and may cause a severe sunburn. At 

 high latitudes at midday the sunburn-producing component of the sky 

 radiation is also greater relative to that of the radiation coming directly 

 from the sun than it is at low latitudes^^ (Coblentz et al., 1942). 



Dust and smoke absorb the erythemal wave lengths very strongly, and 

 a slight haze that is barely perceptible to the eye may completely wipe out 

 this part of the spectrum. Hence sunburn is more likely in rural regions 

 than in the neighborhood of industrial cities. The seashore with an 

 onshore wind carrying away all traces of smoke or dust may be a particu- 

 larly favorable place for sunburning, as may be the high mountains. 

 Snow and ice reflect the erythemal spectrum to a high degree, explaining 

 in part at least the coining of the terms "snow burn" and "glacier burn." 

 Water reflects less than is commonly believed (Coblentz et al., 1933). 



Although sunburn is primarily produced by wave lengths at the short 



2" The sky is blue for the same reason, bhie and violet being scattered to a greater 

 extent than the longer wave lengths. 



2' DeLong (1884) reported in tlie log of the ill-fated Jeanetle the difficulties from 

 sunburn during the arctic summer. 



