It is common knowledge that as you rise in the atmosphere so the pressure decreases:  in fact, it is usual to measure your height by reading the barometer.  Now the air on this last stretch to the Pole was rising, for the wind was from the south, and, as we have seen, the plateau here was sloping down towards the Pole.  The air, driven uphill by this southerly wind, was forced to rise.  As it rose it expanded, because the pressure was less.  Air which has expanded without any heat being given to it from outside, that is in a heat-proof vessel, is said to expand by adiabatic expansion.  Such air tends first to become saturated, and then to precipitate its moisture.  These conditions were approximately fulfilled on the plateau, where the air expanded as it rose, but could get little or no heat from outside.  The air therefore precipitated its moisture in the form of crystals.

Owing to the rapid changes in surfaces (on one occasion they depoted their ski because they were in a sea of sastrugi, and had to walk back for them because the snow became level and soft again) Scott guessed that the coastal mountains could not be far away, and we now know that the actual distance was only 130 miles.  About the same time Scott mentions that he had been afraid that they were weakening in their pulling, but he was reassured by getting a patch of good surface and finding the sledge coming as easily as of old.  On the night of January 12, eight days after leaving the Last Return Party, he writes:  “At camping to-night every one was chilled and we guessed a cold snap, but to our surprise the actual temperature was higher than last night, when we could dawdle in the sun.  It is most unaccountable why we should suddenly feel the cold in this manner:  partly the exhaustion of the march, but partly some damp quality in the air, I think.  Little Bowers is wonderful; in spite of my protest he would take sights after we had camped to-night, after marching in the soft snow all day when we have been comparatively restful on ski."[299] On January 14, Wilson wrote:  “A very cold grey thick day with a persistent breeze from the S.S.E. which we all felt considerably, but temperature was only -18 deg. at lunch and -15 deg. in the evening.  Now just over 40 miles from the Pole.”  Scott wrote the same day:  “Again we noticed the cold; at lunch to-day all our feet were cold but this was mainly due to the bald state of our finnesko.  I put some grease under the bare skin and found it make all the difference.  Oates seems to be feeling the cold and fatigue more than the rest of us, but we are all very fit.”  And on January 15, lunch:  “We were all pretty done at camping."[300] And Wilson:  “We made a depot [The Last Depot] of provisions at lunch time and went on for our last lap with nine days’ provision.  We went much more easily in the afternoon, and on till 7.30 P.M.  The surface was a funny mixture of smooth snow and sudden patches of sastrugi, and we occasionally appear to be on a very gradual down gradient and on a slope down from the west to east.”  In the light of what happened afterwards I believe that the party was not as fit at this time as might have been expected ten days before, and that this was partly the reason why they felt the cold and found the pulling so hard.  The immediate test was the bad surface, and this was the result of the crystals which covered the ground.