We had still to wait three days before the weather was as it ought to be. The high pressure region had spread itself long ago over the Arctic Sea, and the bad weather which passed Björnöya had moved to North Siberia, but right up to the morning of the 21st we had dull weather with snow now and again in King’s Bay. The reason was a slight local depression which had remained persistently over the warm current which the Gulf Stream sends along the west coast of Spitzbergen. On the 21st there was, for the first time, sufficient easterly wind to drive the snowy weather out to sea, so that from midday on we had radiant sunshine and a cloudless sky. At last the condition had arrived for which we had waited so long, the first useful condition since the planes had been ready to start. It _had_ to be used, especially as the season was getting on towards the end of May and the danger of fog was increasing each day. So far we had not seen any fog at Spitzbergen and if one had not had the knowledge about polar fogs which “Fram’s” observations, 1893–6, had given us, it would have been tempting enough to wait longer. It was still pretty cold, -9° c. in King’s Bay on the 21st of May and at the Pole one might risk calculating that the temperature would be down to -15° c. Both for the planes and the crews it would have been better and more comfortable to have had a more summery temperature. But of two evils choose the lesser. As soon as the summer arrives in North Europe, North Siberia, Alaska and North Canada, fog starts to reign over the polar sea. Each air current above the Arctic, no matter from which direction it comes, will bring with it warm air, which is exposed to a lowering of the temperature on contact with the polar ice. This cooling of the warm air which contains a great deal of dampness causes fog. This formation takes place quite independently regardless of whether there is high or low pressure. Even the best high pressure condition in the summer, might therefore be useless for flying. During the high pressure one will certainly be free from the clouds which produce snow and rain, and the flight can take place in radiant sunshine, but fog, even if it only reaches twenty meters up from the ground, will make a landing impossible. Fog of that kind was very unlikely on the 21st, in fact, one might say the possibility of its existence was quite excluded. The northeast wind on that day was so cold (-9° c.) that it must have come from the very central regions of the polar ice, and it is hardly probable that on its way to Spitzbergen it should have been exposed to the further lowering of temperature, which would have been necessary to produce fog.