The Editor,

The Journal of Glaciology

Sir,

It is not unusual to come across misconceptions in glaciological literature about the part played by direct solar radiation in the melting of snow and ice. For example, when discussing the processes causing ablation, O. Rogstad states on page 554, line 16, of the October 1951 Journal of Glaciology, that the amount of radiation is independent of the air temperature. Other examples of this type of misconception, although usually of a much less specific character, often occur in the literature.

In fact, the solar radiation which remains available for melting snow and ice is highly dependent upon the temperature of the free air above. This dependence is most easily appreciated from a few typical figures such as those given in the table below.

For these the solar heating was taken as averaging 60 calories per cm.²/hour on a horizontal surface, for a few hours round mid-day with clear sky in May to August, at about 60° N latitude. Owing to reflection only one-fifth of this would be available for heating the firn surface, leaving 12 calories to supply the outgoing radiation and cause melting. The values of atmospheric radiation used here are in accordance with recent observations.Footnote *

When the free air temperature is several degrees below freezing convection will tend to cool the snow and prevent the small surplus of solar heat from being effective for melting, but for air temperatures above freezing convection is mainly dependent on the wind.

Although low clouds may greatly reduce the radiation in both directions, they do not always diminish the surplus available for melting. Here again the latter is highly dependent on air temperature.

23 November 1951