Part 3 How mountains influence the weather  

The earlier articles in this series showed how local and global weather is created by the sun, as it unevenly heats the world, which in turn heats the air around it. This final article in the ‘weather series’ will examine how mountains influence the weather, locally, on a regional level and at the level of the whole global climate. I will also look at the signs of approaching bad weather when you are in the mountains and what the weather is like in the Alps and the Himalaya.

Depressions and storms form as the ‘Polar Front’ buckles, this forces warm air to rise over the cold ‘front’, forming clouds. Mountains work in much the same way by forcing air to rise and this explains why mountains tend to have much wetter climates than the low lying areas on the opposite side to where the weather is coming from - the rain shadow. For example the west coast mountains of Scotland receive 250 cm of precipitation every year, whilst the east coast doesn't often reach 75 cm. The rainfall created by mountains and hills is called ‘orographic’ or ‘relief’ rainfall.

A good example of mountains affecting the weather regionally is the Andes range of mountains, which forms a barrier between the Atacama Desert and the Amazon Rainforest with the result that the Atacama is dry while rainfall in the Amazon is enhanced. The influence of large mountains however, extends far beyond their immediate vicinity, sometimes a thousand miles away. Most of the earth’s airflow is orientated from the west to the east. Consequently, mountains that run north to south influence the general circulation of air around the earth. Unfortunately for the UK the Canadian Rockies are known to create the ‘Rossby’ waves that create the buckling of the polar front and form the depressions that flow across the UK.

It gets cooler as you go higher

When air meets a hill it will either find a way through the valleys or be forced over the top. When air is forced to rise it cools because the air holds less water and therefore absorbs less heat. The higher you go, the thinner the air is and thus the effect of cooling becomes more pronounced. The fall in temperature with height is called the ‘Lapse rate’ and it depends on the humidity of the air. Unsaturated air loses about 1°C per 100 m (dry adiabatic lapse rate), whereas saturated air loses an average 0.5°C per 100 m (saturated adiabatic lapse rate) - the average in the UK is a drop in of 2°C for every 300 m.  In the alps or Greater Ranges the air is drier and the lapse rate can be as much as 3°C for every 300m. This means that at the summit, the temperature will be much cooler than in the valley.

Higher ground is also windier, because the air that is forced to rise must  combine with all the other air molecules as it rushes over the top. It can be twice as windy at 1000m than at sea level and it can therefore feel much colder due to the ‘wind-chill’ effect. However it should be understood that the wind chill doesn't change the actual temperature of the air it simply feels colder because the air takes moisture away from the skin cooling it down. Unfortunately we sweat when we walk up mountains so the effect can be great even when the air is above freezing.  A wind of 65 kph and an air temperature of 3 °C (not uncommon in summer), can make it feel more like -10 °C. To avoid this wrap up to avoid exposeing damp skin and reduce sweating by changing damp layers.

But it can get warmer as you get higher?

Air temperature usually decreases with a gain in height, but the opposite can  occur with warm air above cold air, this is called a temperature inversion. The most common way they form is on still cold evenings with clear skies. The ground and the air near the ground then cools quickly and because it does not mix easily with the warmer air above the inversion is not easily dispersed. The inversion will only disappear the following morning as the sun heats the ground. Temperature inversions can also occur in the winter when the pressure rises because above high pressure the air is descending and warming up again resulting in a band of warm air above cold air. Another occasion when hills are warmer than valleys is during clear nights with little wind, particularly in winter. As air cools it flows downhill (katabatic wind) and gathers on the valley floor or in pockets where there are dips in the ground. This can sometimes lead to fog and/or frost forming lower down.

Mountains affect the wind

The heating of the earths surface locally and globally creates differences in air pressure and it is these these differences that create the winds with air tending to flow from higher to lower pressure areas. Locally, winds along lake and ocean shores are the result of the temperature differences between land and water, which cause a pressure difference and hence the wind. The shape of the hills also affects the wind strength. Winds will rush rapidly through mountain passes and, depending on the shape of the hill the airflow can remain turbulent and erratic for some distance from the mountain. The highest winds in the UK are often found on Scotlands Cairngorm plateau during periods of high pressure. Strong wind are then forced to rise but are forced through a narrow gap formed by the high pressure with result that the winds accelerate dramatically. 

When winds are driven over the mountains or becomes heated by the sun the air rises, expands, forms clouds and rain. As this air descends on the other (lee) side, it becomes compressed again, dries and warms up. This wind can be a quite strong, gusty, and dry wind. This dry and warm wind is called the Föhn in Europe, the 'Chinook' in the USA, Halny Waitr in Poland, the Zonda in Argentina and in New Zealand the Nor’west arch. They are infamous for their ability to make snow melt rapidly due to their warmth and dryness.

However as we have seen earlier not all downslope winds are warm. Even when air warms as it descends, it can still be cooler than the air it is displacing. They can be biting cold and in France are called the Mistral. 

Alpine weather

The location, shape and variation in shape of the European Alps gives rise to extreme changes in weather patterns across the alps and within regions of the alps. Unlike the UK which is an island the alps are centrally located and are affected by four air flows

 

  • The mild moist air from the Atlantic
  • Cool or cold polar air from northern Europe
  • Continental air masses, cold and dry in winter and hot in summer, dominate the east of the alps
  • The southern alps are affected by warm Mediterranean air that flows northward.

  The warm and dry Fohn winds can also have a dramatic affect on the alpine weather and afternoon storms are frequently generated by valleys releasing their moisture laden and very warm air that create huge and turbulent cumulous clouds.

Himalayan weather

The Himalaya is so big that the weather varies greatly from one region to another. The Himalaya obstructs the passage of cold continental air from the north into India in winter and also forces the southwesterly monsoon winds to give up most of their moisture before crossing the range northward to Tibet.

There are two periods of rainfall/snow:

 

  •        the small amounts brought by winter storms and
  •      the heavier precipitation of the southwesterly   monsoon winds.

 

During winter, depressions advance from the west and cause heavy snowfall, which is greatest over the high mountains and is greater in the west than the east. In January, for example, Mussoorie in the west receives almost 7.5cm, whereas Darjiling to the east receives less than 2.5cm. By the end of May the situation is reversed with southwesterly moisy ‘monsoon’ air moves toward the eastern Himalayas, where the moisture rising over the steep terrain cools and condenses to fall as rain or snow. The rain and snow cease in September, after which the best weather in the Himalayas prevails until the beginning of winter in December.

How to predict the weather

With modern computing devices it is easy to pick up a weather forecast but what happens when the is no signal or it runs out of battery?

Read the sky -   Changes in the shapes and movements of clouds indicate changes in the weather. The approach of a depression and its associated fronts are indicated by the appearance of high altitude wispy cirrus clouds formed by ice crystals. Isolated cirrus indicates the arrival of warmer moist air, but dense, broader cirrus usually means a depression is on the way. When they form into longer straight rows with a halo affect, there is commonly prolonged rain soon afterwards. As the weather system moves toward your location watch for a thickening and lowering cloud cover. If over the course of the day the clouds change from high cirrus to lower layered stratus clouds rain is only three to six hours away. When the stratus are low-lying they usually bring drizzle or light rain and when they’re higher and greyer, they bring periods of heavier rain.

Monitor the air pressure and temperature - Cold fronts can develop rapidly and move swiftly, causing temperatures to drop, wind directions to shift and barometric pressure to fall. The barometer and thermometer on an altimeter watch can be used but the air pressure needs to be monitored at the same location in order for any changes to be meaningful.  This is easy enough overnight, but if you are leaving camp and returning later it may be worth noting the pressure in the morning and evening to keep an eye on any significant changes (a barometer memory function is very useful here). In general, rising air pressure is a good sign - as high-pressure systems usually bring stable, settled weather.  Conversely, a rapid or significant drop in air pressure (e.g. decreasing at 2mbar an hour, or a 10mbar drop overnight for instance) is invariably a warning that bad weather may be on the way. If your watch doesn't have a barometer function, you can still use it to monitor changes in air pressure by keeping an eye on the altitude reading when you are at a fixed location. A 30m increase in altitude roughly equates to a 2 mbar drop in air pressure - so, if the altitude reading has gone up by 120m overnight for example, then the air pressure has dropped by around 8 mbar and you would be well advised to keep an eye out for further signs of deterioration in the weather. 

A warning about using long range weather forecasts

The atmosphere is a chaotic system, so small changes to one part of the system can grow to have large effects on the system as a whole. Forecasting weather is an uncertain science, particularly beyond two or three days, It is a best guess based upon observations from ships, aircraft, oil rigs, buoys and balloons, satellites, radars and manned land stations around the world. Computer models use this information to create a weather map and predict the weather. They are rarely wrong but mistakes are frequently made about the timing and severity so watch the weather when you are out even if good weather is predicted.

 

 

 

 
 

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