Weather

Lenticular Cloud before Bad Weather

The primary dangers caused by bad weather center on 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. If a climber happens to be standing on or near the summit, they risk being struck. 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 get an "alpine start"; 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. High winds can speed the onset of hypothermia, as well as damage equipment such as tents used for shelter. Under certain conditions, storms can also create waterfalls which can slow or stop climbing progress. A notable example is the Föhn wind acting upon the Eiger.