The air’s humidity governs the formation of clouds, fogs, dew and so forth. It also greatly affects the comfort of bushwalkers. Humidity measurements provided by the Bureau of Meteorology, state the amount of water vapour in the air as a percentage of the maximum amount that the air can absorb. Remember that water vapour is invisible, and only becomes visible as cloud, fog or precipitation when it condenses. The maximum amount of water vapour that a parcel of air can contain depends on its temperature and pressure (or height). If a parcel of air at 100% humidity cools, some of its water vapour will condense and become visible as cloud or fog.

Low humidity air has the capacity to absorb water from objects it moves over. High humidity air is unable to absorb extra moisture. When walking, our bodies generate heat which must be dissipated by perspiration. We cool as it evaporates. However, in periods of high humidity, the evaporation process is very inefficient and cooling is difficult. The air cannot absorb much additional moisture. Our bodies can overheat and become dehydrated as excess fluids are perspired. These are risks in tropical northerly regions, particularly when high temperature is often combined with high humidity and little wind.

At the other extreme, ski tourers in winter are faced often with very low humidity air as winds from the south move north, warm and increase their ability to absorb moisture. Low humidity air removes much more water from lungs than does humid air, increasing the need for frequent drinks during heavy exercise.

The difference in humidity between the outside air and air within a Gore-Tex jacket affects the ability of Gore-Tex fabric to pass water vapour to the outside. Water vapour passes from areas of high humidity to low humidity. Gore-Tex works exceptionally well in low humidity, particularly on skiing trips. In high humidity, when perspiring freely, you can feel as wet inside your Gore-Tex jacket as it is outside it.

Leaders need to be aware that human skin will be dried out during trips in low humidity in the snow. The cold weather can restrict blood flow, leading to frost bite and chilblains. Conversely, asthma attacks and hayfever can increase during high pressures as allergens such as pollen are carried aloft in the rising air currents.