is the sport, hobby or profession of walking, hiking and climbing up mountains. It is also sometimes known as alpinism. It may be said to consist of three aspects: rock-craft, snow-craft and skiing, depending on whether the route chosen is over rock, snow or ice. All require great athletic and technical ability, and experience is also a very important part of the latter.

Hazards
The craft of climbing has been developed to avoid three main types of danger: the danger of things falling on the climber (objective danger), the danger of the climber falling and inclement weather. The things that may fall include rocks, ice, snow, other climbers or their gear; the mountaineer may fall from rocks, ice or snow, or into a crevasse. In all, there are eight chief dangers: falling rocks, falling ice, snow-avalanches, falls, the climber falling, falls from ice slopes, falls down snow slopes, falls into crevasses and dangers from weather. To select and follow a route using one's skills and experience to mitigate these dangers is to exercise the climber's craft.

Falling rocks
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 stone swept. 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 rock fall on such routes.

The direction of the dip of rock strata often determines the degree of danger on a particular face; 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.

Falling ice
The places where ice may fall can always be determined beforehand. It falls in the broken parts of glaciers (seracs) and from overhanging cornices formed on the crests of narrow ridges. Large icicles are often formed on steep rock faces, and these fall frequently in fine weather following cold and stormy days. They have to be avoided like falling stones. Seracs are slow in formation, and slow in arriving (by glacier motion) at a condition of unstable equilibrium. They generally fall in or just after the hottest part of the day, and their debris seldom goes far. A skillful and experienced ice-man will usually devise a safe route through a most intricate ice-fall, but such places should be avoided in the afternoon of a hot day. Hanging glaciers (i.e. glaciers perched on steep slopes) often discharge themselves over steep rock-faces, the snout breaking off at intervals. They can always be detected by their debris below. Their track should be avoided.

Avalanche
The avalanche is the most underestimated danger in the mountains. People generally think that they will be able to recognize the hazards and survive being caught. The truth is a somewhat different story. Every year, 120 - 150 people die in small avalanches in the Alps alone. The vast majority are reasonably experienced male skiers aged 20-35 but also include ski instructors and guides. There is always a lot of pressure to risk a snow crossing. Turning back takes a lot of extra time and effort, supreme leadership, and most importantly there seldom is an avalanche to prove the right decision was made. Making the decision to turn around is especially hard if others are crossing the slope, but any next person could become the trigger.

There are two types of avalanche:
The slab avalanche: This type of avalanche occurs when a plate of snow breaks loose and starts sliding down; these are the largest and most dangerous.
Hard slab avalanche - formed by hard-packed snow in a cohesive slab. The slab will not break up easily as it slides down the hill, resulting in large blocks tumbling down the mountain.
Soft slab avalanche - formed again by a cohesive layer of snow bonded together, the slab tends to break up more easily.
The loose snow avalanche: This type of avalanche is triggered by a small amount of moving snow that accumulates into a big slide. Also known as a "wet slide or point release" avalanche. This type of avalanche is deceptively dangerous as it can still knock a climber or skier off their feet and bury them, or sweep them over a cliff into a terrain trap.
Dangerous slides are most likely to occur on the same slopes preferred by many skiers: long and wide open, few trees or large rocks, 30 to 45 degrees of angle, large load of fresh snow, soon after a big storm, on a slope 'lee to the storm'. Solar radiation can trigger slides as well. These will typically be a point release or wet slough type of avalanche. The added weight of the wet slide can trigger a slab avalanche. Ninety percent of reported victims are caught in avalanches triggered by themselves or others in their group.

When going off-piste or traveling in alpine terrain, parties have a moral responsibility to always carry:

avalanche beacons
probes
shovels (retrieving victims with a shovel instead of your hands is five times faster)
and to have had avalanche training! Paradoxically, expert skiers who have avalanche training make up a large percentage of avalanche fatalities; perhaps because they are the ones more likely to ski in areas prone to avalanches.

Even with proper rescue equipment and training, there is a one-in-five chance of dying if caught in a significant avalanche, and only a 50/50 chance of being found alive if buried more than a few minutes. The best solution is to learn how to avoid risky conditions.


 Falls from rocks
The skill of a rock climber is shown by one's choice of handhold and foothold, and his adhesion to those one 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, people 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 climber in front and the one behind are doing. After bad weather steep rocks are often found covered with a veneer of ice, which may even render them inaccessible. Crampons are useful on such occasions.


 Ice slopes

Mountaineers descending mixed rock, snow and ice slope in winter High Tatras. For travel on slopes consisting of ice or hard snow, crampons are a standard part of a mountaineer's equipment. While step-cutting can sometimes be used on snow slopes of moderate angle, this can be a slow and tiring process, which does not provide the higher security of crampons. However, in soft snow or powder, crampons are easily hampered by balling of snow which reduce their effectiveness. In either case, an ice axe not only assists with balance but provides the climber with the possibility of self-arrest in case of a slip or fall. On a true ice slope however, an ice axe is rarely able to effect a self-arrest. As an additional safety precaution on steep ice slopes, the climbing rope is attached to ice screws buried into the ice.
Newly-fallen snow quickly thaws on the surface and becomes sodden below, so that the next night's frost turns the whole mass into a sheet of semi-solid ice.
 Snow slopes

Snow slopes are very common, and usually easy to ascend. At the foot of a snow or ice slope is generally a big crevasse, called a bergschrund, where the final slope of the mountain rises from a snow-field or glacier. Such bergschrunds are generally too wide to be stepped across, and must be crossed by a snow bridge, which needs careful testing and a painstaking use of the rope. A steep snow slope in bad condition may be dangerous, as the whole body of snow may start as an avalanche. Such slopes are less dangerous if ascended directly than obliquely, for an oblique or horizontal track cuts them across and facilitates movement of the mass. New snow lying on ice is especially dangerous. Experience is needed for deciding on the advisability of advancing over snow in doubtful condition. Snow on rocks is usually rotten unless it is thick; snow on snow is likely to be sound. A day or two of fine weather will usually bring new snow into sound condition. Snow cannot lie at a very steep angle, though it often deceives the eye as to its slope. Snow slopes seldom exceed 40°. Ice slopes may be much steeper. Snow slopes in early morning are usually hard and safe, but the same in the afternoon are quite soft and possibly dangerous; hence the advantage of an early start.


 Crevasses
Crevasses are the slits or deep chasms formed in the substance of a glacier as it passes over an uneven bed. They may be open or hidden. In the lower part of a glacier the crevasses are open. Above the snow-line they are frequently hidden by arched-over accumulations of winter snow. The detection of hidden crevasses requires care and experience. After a fresh fall of snow they can only be detected by sounding with the pole of the ice axe, or by looking to right and left where the open extension of a partially hidden crevasse may be obvious. The safeguard against accident is the rope, and no one should ever cross a snow-covered glacier unless roped to one, or even better to two companions. Anyone venturing onto crevasses should be trained in crevasse rescue.


Weather
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.

Altitude
Rapid ascent can lead to altitude sickness. The best treatment is to descend immediately. The climber's motto at high altitude is "climb high, sleep low", referring to the regimen of climbing higher to acclimatize but returning to lower elevation to sleep. In the South American Andes, the chewing of coca leaves has been traditionally used to treat altitude sickness symptoms.

Common symptoms of altitude sickness include severe headache, sleep problems, nausea, lack of appetite, lethargy and body ache. Mountain sickness may progress to HACE (High Altitude Cerebral Edema) and HAPE (High Altitude Pulmonary Edema), both of which can be fatal within 24 hours.

In high mountains, atmospheric pressure is lower and this means that less oxygen is available to breathe. This is the underlying cause of altitude sickness. Everyone needs to acclimatize, even exceptional mountaineers that have been to high altitude before. Generally speaking, mountaineers start using bottled oxygen when they climb above 7,000 m. Exceptional mountaineers have climbed 8000-meter peaks (including Everest) without oxygen, almost always with a carefully planned program of acclimatization.

In 2005, researcher and mountaineer John Semple established that above-average ozone concentrations on the Tibetan plateau may pose an additional risk to climbers.