Mountain Weather

Mountain ranges are notorious for their rapid weather changes. The sides of mountains deflect winds, causing clouds and rain in their vicinity. During the day, as the air warms, it expands and rises up the slopes, referred to as a 'valley breeze'. As this air rises, it cools by 1°C (2°F) every 100m (330ft). The fast-rising air cools adiabatically (the term given to cooling by expansion resulting from a drop in pressure). This drop in temperature often leads to cloud formation as the pressure of the water vapour alters, leading to condensation.

Late in the day, the air cools further and moves downwards - the 'mountain breeze' - dispersing the clouds, often leading to the summer rainstorms characteristic of many mountainous regions.

Short-term instabilities can cause sudden and unexpected rainstorms, and meteorologist are understandably wary of predicting mountain weather on a local, short-term basis. Peaks like the Eiger in Switzerland, standing at the edge of a range, have their own weather patterns which vary with lightning rapidly. Various areas are also affected by long-term air movements, with high- and low pressure systems tending to 'migrate' North or South in different seasons. For instance, the Azores anticyclone which dominates the weather in central Europe in summer produces a semi-permanent high-pressure system. This generates weather cycles of 10 to 12 fine days, followed by a few days of stormy weather, as a result of interaction with the low-pressure system predominating over Western Europe and Asia. Generally, the weather stabilizes for a month or so in September to October. Similar 'cycles' can be identified in most mountain areas, and climbers are well advised to take cognizance of these when planning their trip.

An altimeter-barometer is a useful short-term predictor. Once at a fixed altitude, any rise or fall over a number of hours can indicate weather trends. A significant fall in pressure (shown as a 'rise' in altitude) predicts bad weather; the faster the drop, the more violent the storm is likely to be. Conversely a steady rise in pressure (a 'drop' in altitude) will herald an improvement in weather.

The high peaks of the world project into the jet stream, the name given to the fast-moving wind belts at the edge of the breathable atmosphere. This can lead to winds with speeds in excess of 160Kph (100Mph) with drastic consequences for climbers. Choosing the 'right' time of the year in terms of wind and weather patterns can dictate success or failure in the mountains. This is particularly true of the big ranges such as the Andes and Himalayas.

Lightning

Mountains frequently give rise to lightning storms - spectacular if you are far away, terrifying if you are uncomfortably close. Peaks and ridges produce the vertical updrafts and raincloud conditions that generate lightning, and climbers thus often find themselves in the strike zone. Electrical potential builds up in clouds in much the same way that we generate it by rubbing nylon against nylon on a dry day.

Electrons are transferred from one molecule to another as the air masses rush past one another or past the earth. Air is normally a poor conductor, whereas earth, trees, rock and the human body are better conductors.

Because air offers little by way of pathway, an imbalance of electrons builds up, creating a 'potential difference' or 'charge'. Any electrical charge follows physical laws which tend to neutralize it, or 'balance the charge'.

The accumulated electrons which constitute the charge will find the course of least resistance in order to achieve electrical balance. Because of the mutual repulsion of differing charges, the electrons tend to stream towards a 'point density' on the outside of a charged surface - for example a rock tower, a tree or a human head.

A phenomenon sometimes seen is 'St. Elmo's Fire' - a bluish-tinged corona accompanied by humming or crackling sounds. Often metal equipment starts to 'sing' or your hair stands on end. Although this does not indicate a definite strike where you are standing, it should be a sufficient warning to encourage evasive action! Even a strike a few meters away will generate sufficient free electrons to move a good few billion through your body. This shock can disrupt the action of the heart, knock out brain activity, or cause severe burns. When lightning threatens, climbers should:

1) Avoid moist areas, such as gullies and cracks. 2) Sit or crouch on insulated objects, such as a kit bag or rope. 3) Avoid ridges, stay off the top. 4) Keep out of overhangs or small caves - lightning may strike the ground via you from the cave roof.

5) On a ledge, crouch at the outer edge. Tie on via a short lead. 6) Try to find an area with something a bit above your head height, e.g. a small tree or rock. 7) Move away from metal objects if possible - they do not 'attract' a strike, but might intensify the effect via induced currents. If someone is truck by lightning you should begin cardiopulmonary resuscitation, and treat the burns.

Read more in Garth Hattingh's The Climber's Handbook. This book is a must have for the beginner and expert climber, offering expert information, with pictures and illustrations, on climbing and mountaineering. [Order online]

Source and extracts from: The Climber's Handbook, by Garth Hattingh.

About the Author: Garth Hattingh has over 30 years of experience and has written several climbing and mountaineering books including; Rock and Wall Climbing, Top Climbs of the World, Extreme Rock and Ice, and The Outdoor Survival Manual. These books are available from our online store.

Courtesy and Copyright by: Garth Hattingh. New Holland (Publishers) LTD, London, Cape Town, Sydney, Singapore. Struik (Publishers) LTD, Cape Town.

Distributed by: Struik (Publishers) LTD, Cape Town. ISBN 1 85368 7170.

Updated Wed, 10 October, 2001 http://www.rocksport.co.za