‘regardless of terrain’ (line 5)

These two factors the altitude and the weather, tend separately and together to defeat the climber. The height weakens, slows him down; it forces him to spend days and nights in the courses of his assault on the summit; the weather, besides adding to the demand of his energy and moral fortitude, conspires to deny him the time he needs to complete his mission. Whereas in lower mountains and on easy ground the weather may be no more than a handicap, in the high Himalayas it is decisive, regardless of terrain.
The deduction to be drawn from these two factors is was clear enough. We must either so fortify ourselves that we could continue, without detriment, to live and have our being above the limit of natural acclimatization, or, better still, we must solve the problem of speed. It was desirable; in fact, that we must meet both these requirements and thus give to those chosen to attempt the summit and to their supporting teams some measures of insurance against the vagaries of the weather, for safety in mountain climbing is as much a matter of swiftness as of sureness of foot. Either or both could be achieved only by the administration of oxygen in sufficient quantities to make up for the deficiency in the air, and for the duration of the upward journey above the limit of successful acclimatization