A party venturing into mountainous areas has to be prepared for weather conditions vastly different to those in low-lying country. The following aspects of weather are affected by mountains.
Air pressure at the tops of mountains is less than at the bottom. Air flowing over them will therefore be forced into an area of lower pressure and will expand and cool. In cooling, it sheds moisture and can produce cloud, rain and snow. Have you caught glimpses of sun filled valleys through mist swirling around a summit? On the lee side of the mountain, the descending air increases in temperature and clear periods are more frequent, as shown in Figure 18.5. If precipitation occurs, the clouds will dissipate at a higher level on the lee side than they formed on the windward side. If you have a choice of routes to leave the high country in bad weather, head for the lee side, with the wind on your back.
Temperature and sunshine
If the air is well mixed (as in windy conditions), the air temperature decreases with increasing altitude at a rate of 0.6–1.0°C per 100 m for humid air, and about 1.0°C per 100 m for dry air. For example, if the temperature at sea level is 10°C, at 1500 m it will be below freezing point—so take warm clothing for a summer weekend on mountains above 1800 m or so.
As atmospheric dust and moisture content decreases with increasing altitude, on a clear day the direct sunlight at 1200 m contains twice the amount of ultraviolet light as at sea level. So in high country, without clouds, ‘slip on a shirt, slap on a hat and slop on sunscreen’.
Wind is the movement of air to equalise atmospheric pressure. Because of friction at sea level, winds are gusty but have reduced velocity. At high altitudes, they are stronger and steadier. Knowledge of wind speed is important, and vitally so in winter conditions, where even a light breeze can reduce the environmental temperature (the temperature as felt by the human body) to an uncomfortable level. The wind speed on open land above 1000 m is about two to two and a half times stronger than on more sheltered low-lying land. Environmental temperature is also affected by humidity: the raw conditions of winter and the mugginess or oppressiveness on a sultry summer’s day are well-known effects.
To cool down, you need to lose heat to the environment, mainly through conduction to the air layer surrounding your body. If this does not provide sufficient cooling, your body perspires, and the evaporation helps the cooling process. In both cases, the rate of evaporation, and hence effectiveness of cooling, is influenced by wind speed and humidity in the air. Wind chill is a measure of the effectiveness of wind speed as a coolant. Air with an actual temperature of 4°C and a wind speed of 24 km/h, has a wind chill temperature of –5°C; and if moving at 48 km/h, of –11°C. Wind chill is discussed further in Chapter 21.
The efficiency of cooling depends on the amount of surface skin area exposed to the air flow. In extreme conditions, leaders should monitor:
- thin, tall walkers who, while having very efficient cooling in summer, may be prone to hypothermia in winter (particularly children)
- shorter, squat walkers who may have trouble shedding heat in summer, but who are efficient storers of body heat in winter.
Large mountains may strongly affect rainfall patterns. Moisture-laden air approaching a mountain range is forced to rise. It expands as it rises (because of the decreased weight of air above) and therefore cools. This cooling can force cloud formation and precipitation on the windward side and tops of the range. As the air descends the lee slope, it is compressed and warms. It can contain more moisture and so precipitation can cease and clouds disappear, as shown in Figure 18.5. A rain shadow exists on the lee side of the mountains and can give rise to a different vegetation regime.
In southeast Australia, rain-bearing winds normally come out of an arc from west to south. The Great Dividing Range provides a barrier for many of the less intense, low-pressure systems, with rain precipitating on the southern and western faces. The southeast coast of New South Wales and Victoria often experiences relatively warm weather during cold and wet westerly or northwesterly winds elsewhere. These winds rise up the windward side of the mountains, cooling and shedding moisture. The relatively dry winds warm as they descend the lee side and move over the coastal regions. The coastal hills of southwestern areas of Western Australia are wetter than areas inland from the highest points of the ranges, and the Adelaide Hills in South Australia cause a rain shadow on the plains to their east towards the Murray River.