First of all there must be no fog at the place where they have to land. Even if there is only a fog-belt extending a few meters above the ground, a landing is impossible and a “forced-landing” would almost certainly end in a catastrophe. Further, the flyers must avoid passing through thick snow. The two flying-machines might easily lose sight of each other, and if, in order to keep in contact with each other, they should fly close together, there is always the danger of a collision. An overcast sky without rainfall is also useless. At least it must clear now and again sufficiently to make it possible to navigate by the sun. It is of course known that steering by the magnetic compass is very uncertain so far north, as the extent of the deviations in the Arctic is not sufficiently known. Luckily so much is known about the weather in the polar ice region that it is possible to choose in advance the most suitable time of year for a polar flight. First of all, Nansen’s expedition by the “Fram” in 1893–1896 has given us this knowledge about the polar weather. During almost the entire time of their drift across the Arctic observations were made nearly every two hours in the course of the day, so that a singularly rich stock of information exists. The observations have been thoroughly gone through by the late Professor H. Mohn, so that we have got them set out now in a most perspicuous form. Both the observers’ and Professor Mohn’s calculations are published in the work, “The Norwegian North Polar Expedition XVII Meteorology.” [Illustration: THE TWO METEOROLOGISTS] [Illustration: N 25 ON THE WAY TO OSLO] I am going to cite some figures from this book which give a clear reply to the question, Which time of the year is the best for flying to the Pole? In the three years which the drift lasted the approximate number of clear days per month were: In January 14 „ February 12 „ March 9 „ April 8 „ May 7 „ June 0 „ July 0 „ August 0 „ September 0 „ October 4 „ November 11 „ December 15 Thus in midwinter (December and January) nearly half of each month is composed of clear days, but the number quickly declines towards the summer and there are none in the four months from June to September. It happens, of course, that in the summer time the sun breaks through the sky at some time of the day, but even _that_ is not very often. June had on an average twenty-six overcast days, July twenty-seven, August twenty-four, and September twenty-seven days. [Illustration: ROUTE OF THE AMUNDSEN-ELLSWORTH FLIGHT] As might be expected, downpours are much more frequent in the gray summer months than in the other part of the year. The number of days with rainfall were, on an average, as follows: