E. Linacre and B. Geerts

11/'97

Measurements of the components of the energy balance at the surface of the Franz Josef glacier in New Zealand have been made at five different times of the year (1,2). At times of ice-melt, the relative magnitude of the various sources of energy (to melt the ice) varied a lot. These sources are: the net incoming radiation, the latent heat flux, the sensible heat flux of air, and the sensible heat flux of rain falling on the ice. The latter dominates during heavy summer rains, but it is rather insignificant on average.

Only just over half the heat used in melting the ice came from sensible heat in the air, so that air temperature alone is a poor index of the rate of melting, as pointed out elsewhere (3). Throughout the year there was similarity between the amount of net radiation and the latent heat flux, however the net radiation was a larger heat source.

The latent heat flux occurs mainly by the condensation of water vapour onto the ice; the higher the air’s dewpoint is above 0°C, the larger this flux. Sometimes, during a warm spell, when it is calm, a fog will form just over the glacier. This fog is due to the mixing of two airmasses, i.e. the air adjacent to the ice, and the ambient air. A similar fog is sometimes seen around icebergs. The ‘glacier fog’ will slowly slide down into the valley, past the glacier terminus; this rather shallow ‘glacier breeze’ is driven by a temperature difference.