Page:Scott's Last Expedition, Volume 1.djvu/484

308 both islands. It is clear that in the heaviest blizzard one could escape the wind altogether by camping to windward of the larger island. One sees more and more clearly what shelter is afforded on the weather side of steep-sided objects.

Passed three seals asleep on the ice. Two others were killed near the bergs.

Saturday, June 10.—The impending blizzard has come; the wind came with a burst at 9.30 this morning.

Simpson spent the night turning over a theory to account for the phenomenon, and delivered himself of it this morning. It seems a good basis for the reference of future observations. He imagines the atmosphere A C in potential equilibrium with large margin of stability, i.e. the difference of temperature between A and C being much less than the adiabatic gradient.

In this condition there is a tendency to cool by radiation until some critical layer, B, reaches its due point. A stratus cloud is thus formed at B; from this moment A B continues to cool, but B C is protected from radiating, whilst heated by radiation from snow and possibly by release of latent heat due to cloud formation.

The condition now rapidly approaches unstable equilibrium, B C tending to rise, A B to descend.

Owing to lack of sun heat the effect will be more rapid in south than north and therefore the upset will commence first in the south. After the first start the upset will rapidly spread north, bringing the blizzard. The facts