The craft of climbing has been developed to avoid two main types of danger: the danger of things falling on the traveller and the danger of the climber falling himself. The things that may fall are rocks, ice and snow; the mountaineer may fall from rocks, ice or snow, or into crevasses in ice or snow. There are also dangers from weather. Thus in all there are eight chief dangers: falling rocks, falling ice, snow-avalanches, falls from difficult rocks, falls from ice slopes, falls down snow slopes, falls into crevasses, dangers from weather. To select and follow a route avoiding these dangers is to exercise the climber's craft.
Every rock mountain is slowly disintegrating due to erosion, the process being especially rapid above the snow-line. Rock faces are constantly swept by falling stones, which are generally possible to dodge. Falling rocks tend to form furrows in a mountain face, and these furrows (couloirs) have to be ascended with caution, their sides often being safe when the middle is stoneswept. Rocks fall more frequently on some days than on others, according to the recent weather. Ice formed during the night may temporarily bind rocks to the face but warmth of the day or direct sun exposure may easily dislodge these rocks. Local experience is a valuable help on determining typical rockfall on such routes.
The direction of the dip of rock strata often determines whether a particular face is safe or dangerous; the character of the rock must also be considered. Where stones fall frequently debris will be found below, whilst on snow slopes falling stones cut furrows visible from a great distance. In planning an ascent of a new peak mountaineers must look for such traces. When falling stones get mixed in considerable quantity with slushy snow or water a mud avalanche is formed (common in the Himalaya). It is vital to avoid camping in their possible line of fall.
Falls from rocks
The skill of a rock climber is shown by his choice of handhold and foothold, and his adhesion to those he has chosen. Much depends on a correct estimate of the firmness of the rock where weight is to be thrown upon it. Many loose rocks are quite firm enough to bear a person's weight, but experience is needed to know which can be trusted, and skill is required in transferring the weight to them without jerking. On rotten rocks the rope must be handled with special care, lest it should start loose stones on to the heads of those below. Similar care must be given to handholds and footholds, for the same reason. When a horizontal traverse has to be made across very difficult rocks, a dangerous situation may arise unless at both ends of the traverse there be firm positions. Mutual assistance on hard rocks takes all manner of forms: two, or even three, men climbing on one another's shoulders, or using for foothold an ice axe propped up by others.
The great principle is that of co-operation, all the members of the party climbing with reference to the others, and not as independent units; each when moving must know what the man in front and the man behind are doing. After bad weather steep rocks are often found covered with a veneer of ice (verglas), which may even render them inaccessible. Crampons are useful on such occasions.
Rapid ascents can lead to altitude sickness. The best (and very effective) cure is to descend immediately. The climber's motto in high altitude is "climb high, sleep low", referring to the regiment of climbing higher to acclimitize but returning to lower elevation to sleep. An alternative, traditionally used in South America is the chewing of coca leaves.
Lack of oxygen can start to be a problem for many people at heights below 5,000 m. Everyone needs to acclimatise, even those who can summit Everest without bottled oxygen. Generally speaking, experienced mountaineers do not normally start using bottled oxygen until they reach around 7,000 - 8,000 m.
In 2005, researcher and mountaineer John Semple established that above-average ozone concentrations on the Tibetan plateau may pose an additional risk to climbers.
The primary dangers caused by bad weather centre around the changes it causes in snow and rock conditions, making movement suddenly much more arduous and hazardous than under normal circumstances. Whiteouts make it difficult to retrace a route while rain may prevent taking the easiest line only determined as such under dry conditions. In a storm the mountaineer who uses a compass for guidance has a great advantage over a merely empirical observer. In large snow-fields it is, of course, easier to go wrong than on rocks, but intelligence and experience are the best guides in safely navigating objective hazards.
Summer thunderstorms may produce intense lightning which are attracted to the highest points on the ground. If a climber happens to be standing on or near the summit, they may now in fact be the highest point. There are many cases where people have been struck by lightning while climbing mountains. In most mountainous regions, local storms develop by late morning and early afternoon. Many climbers will often begin ascents "alpine style"; that is before or by first light so as to be on the way down when storms are intensifying in activity and lightning and other weather hazards are a distinct threat to safety.