A QMD publication
Bill Fear 2020
These views and opinions are my own and are not representative of, or linked to, any organization or group
This is a summary of the section on mountain weather in the Military Mountaineering, July 2012, handbook by the Department of the Army, Washington DC. It is limited to considerations in the UK and Ireland mountains. Note that farenheit have been converted to Celsius and feet to metres so there are some odd measurements.
Mountain weather is more erratic than on lower ground and is highly changeable. Understanding the weather and how to plan accordingly is important. Safety or danger can depend on a few degrees of temperature and terrain can quickly become dangerous and even impassable. Severe weather can impact on morale and increase, or exacerbate, problems.
High pressure generally means better, more stable, weather.
Low pressure usually means the weather will worsen.
With high pressure the air flows clockwise and out (anticyclone). It is usually associated with clear skies and mild wind at most. With low pressure the air flows counterclockwise (cyclone). It is usually associated with bad weather. Low pressure builds vertically and pulls air inwards.
Pressure is shown by isobars on a weather map. Isobars are like contour lines. Areas of high pressure are called “ridges” and lows are called troughs.
Wind. High ridges and passes are seldom calm but protected valleys rarely have strong wind. Winds accelerate through mountain passes and canyons and can increase in force on exposed mountainsides or summits.
The force exerted by wind quad quadruples each time the wind speed doubles. Gusts can be 50% higher than average wind speed.
In the Northern Hemisphere, there are three prevailing winds:
- Polar Easterlies
- Prevailing Westerlies
- Northeast trade winds (although these winds generally blow in from the west; they originate in the north east and pick up parcels of air.)
In the mountains we also have localised mountain specific winds that do not usually affect the weather:
- Anabatic wind; blows up mountain valleys
- Katabatic winds; blows down the slopes. Occasionally strong.
All air holds water vapor. The warmer the air, the more moisture it can hold. Fully saturated air is at 100% humidity. When air cools beyond its saturation point it releases moisture (clouds, fog, dew, rain, snow, etc.). The temperature at which this happens is called the condensation point. Air that is holding a lot of water can reach condensation point at 200C. In drier air it can be a as low as 00C or even below freezing. Air cools as it rises and warms as it descends. This is the adiabatic lapse rate. This rate varies depending on the moisture content of the air. Moist air warms and cools at about 1.790C per 305m of elevation gained or lost. Drier air warms and cool about 3.080C per 305m feet of elevation gained or lost.
Clouds are indicators of weather conditions and it is possible to forecast weather conditions b reading the clouds. Clouds can be formed by:
- Convective lifting; air rises from the heated ground
- Frontal lifting; air masses collide and warm air is forced over the colder air mass. This usually results in precipitation
- Cyclonic lifting; Low pressure pulls air into its center. Once there it goes up.
- Orographic lifting. Air is pushed up over a mass of higher ground.
- Clouds are classified into five categories: low-, mid-, and high-level clouds; vertically developed clouds; and less common clouds.
Low-level clouds are either cumulus or stratus. They are mostly water droplets. When temperatures are cold enough they may also contain ice particles and snow. There are two types of precipitating low-level clouds: nimbostratus and stratocumulus.
Nimbostratus clouds are dark, low-level clouds accompanied by light to moderately falling precipitation. The sun or moon is not visible through them. Their bases are difficult to diffuse and difficult to accurately determine.
Stratocumulus clouds usually appear as low and lumpy or as rounded masses with clear breaks of sky. They may be accompanied by weak precipitation. If you extend your arm to the sky altocumulus clouds are about the size of a thumbnail. Stratocumulus are about the size of a fist. Low-level clouds account for most of the precipitation. If they are dark at their base they are thick and usually indicate impending precipitation.
Mid-level clouds have the prefix alto-. They are less distinct than low level clouds. Warm clouds are less distinct than cold clouds. Middle clouds usually indicate fair weather, especially if they are rising. Lowering mid level clouds usually indicate a coming storm.
Altocumulus clouds appear as parallel bands or rounded masses. Usually a portion is shaded. They form in advance of a cold front. On humid summer days they usually indicate later thunderstorms. Scattered altocumulus usually indicates high pressure and fair weather.
Altostratus tend to obscure the sun or moon and there is no halo around the sun or moon.
High-level clouds are usually frozen cloudswith a fibrous structure and blurred outlines. They often obscure the sun or form a ring around the moon. They indicate moisture and the approach of a storm around 24-36 hours away.
Cirrus cloud thickens and lowers as the storm approaches. Temperatures are usually warm with rising humidity. Cirrus is the most common of the high-level clouds. They usually occur in fair weather and are associated with an approaching warm front.
Cirrostratus are sheet-like, relatively transparent, high-level clouds. The sun or moon can be seen through them. They tend to thicken as a warm front approaches.
Clouds with vertical development can grow to great heights with tremendous energy. The two types of vertical development clouds are fair weather cumulus and cumulonimbus.
Fair Weather Cumulus looks like floating cotton balls and last for 5 to 40 minutes. Under the right conditions they can develop into towering cumulonimbus clouds associated with powerful thunderstorms. They are fueled by thermals rising from the earth’s surface.
Cumulonimbus are larger and more vertically developed than fair weather cumulus Under the right conditions they can develop into large cumulonimbus associated with powerful thunderstorms known as super cells. These storms tend to develop during the afternoon and early evening when the effects of heating from the sun are the strongest.
Other cloud types include orographic clouds, lenticulars, and contrails.
Orographic clouds develop in response to the forced lifting of air by the earth’s topography, over mountain tops and high passes for example. The air is lifted by the mountain, cools, sinks down and becomes warmer then accelerates back up. As it cools the water vapour condenses into cloud. This is also called lenticular cloud.
Lenticular clouds are cloud caps above pinnacles and peaks. When they are flying saucer shaped they indicate extremely high winds. If they grow and descend they tend to bring bad weather.
Contrails are water vapour trails made by the exhaust of jet engines. If they take longer than two hours to evaporate that indicates impending bad weather.
Serious errors can occur in interpreting the extent of cloud cover. Cloud cover always appears greater on or near the horizon, especially if the sky is covered with cumulus clouds, Cloud cover estimates should be restricted to sky areas more than 40 degrees above the horizon. You can assess cloud cover by diving the full circumference of the sky into eighths and noting the coverage and type in each eighth.
Fronts occur when two air masses of different moisture and temperature contents meet. One indicator of an approaching front is the progression of the clouds. The four types of fronts are warm, cold, occluded, and stationary.
A warm front occurs when warm air moves into and over a slower or stationary cold air mass. Because warm air is less dense it rises up and over the cooler air. The cloud types indicating a warm front are cirrus, cirrostratus, nimbostratus (producing rain), and fog. Cumulonimbus clouds will sometimes be seen during the summer months.
A cold front occurs when a cold air mass overtakes a slower or stationary warm air mass. Cold air forces the warm air up. Clouds types are cirrus, cumulus, and then cumulonimbus.
Occluded fronts. Cold fronts usually move faster than warm fronts and overtake warm fronts lifting the warm air from the surface. The zone of division between the cold air ahead and the cold air behind is called a cold occlusion. If the air behind the front is warmer than the air ahead, it is a warm occlusion. Most land areas experience occlusions more than other types of fronts. Cloud types include cirrus, cirrostratus, altostratus, and nimbostratus. Precipitation ranges from light to heavy.
A stationary front is a zone with no significant air movement. Warm and cold fronts can become stationary. When it begins moving again it becomes a warm or cold front. There is usually a noticeable temperature change and shift in wind direction when crossing from one side of a stationary front to the other. Weather is usually clear to partly cloudy along a stationary front.
Temperature drops 0.6 to 0.9 degree Centigrade for every 100 metres of gain in altitude in motionless air. When air is moving up the mountain with no cloud forming the temperature drops 1 degree Centigrade 100 metres of gain in altitude.
Temperature inversions occur, often in the morning and evening, when the air is warmer higher up than in the valley. Temperature inversion is caused by cool air sinking into the valley and staying there until it is warmed by the sun.
Air cools on the windward side of the mountain as it gains altitude. This happens more slowly if clouds are forming. On the leeward side of the mountain the air is heated and is warmer than on the windward side.
Weather forecasting. Weather reports should always be used in conjunction with the locally observed current weather situation to forecast weather patterns. The weather can vary at different elevations and across different topography.
There are five ways to forecast the weather:
The persistence method. This method assumes the best predictor of the weather is the current weather conditions. That is, the conditions will not change.
- Trends method. This involves determining the speed and direction of fronts, high and low pressure centres, and clouds and precipitation.
- Climatology method. This uses average weather statistics over many years.
- Analog method. This uses historical data to compare to current weather conditions.
- Numerical method. Computer analysis of data.
Recording weather data. Recording conditions, or just keeping an eye on them, can help forecast the weather to some extent. Pay attention to:
- Wind direction – the magnetic direction it is blowing from.
- Wind speed – you can use the pint rule.
- Visibility – asses the distance to the furthest visible object.
- Current conditions – precipitation: rain, rain showers, snow, fog, haze, mist, mizzle, thunderstorms, heat.
- Cloud cover – divide the sky into eights and note cloud cover in each eighth.
- Ceiling height – estimate the height at which the cloud intersects with the terrain.
- Temperature – above or below freezing, how hot.
- Pressure trends – if you have a barometer or if you can learn to feel changes in air pressure.
- Changes in weather – note changes or trends in observed weather conditions:
- Wind direction shifts; high pressure system wind flows clockwise, low pressure system wind flows counterclockwise.
- Wind speed increases.
- Changes in visibility.
- Increasing cloud coverage.
- Increase in precipitation.
- Lowering cloud ceilings.
- Cooling temperature, which could indicate a cold front.
- Increase in humidity.
- Decreasing barometric pressure
- Wind is steady from one direction.
- Decreasing wind speeds.
- Increasing visibility.
- Decreasing or ending precipitation.
- Decreasing cloud coverage.
- Increasing height of cloud ceilings.
- Getting warmer.
- Humidity decreases.
- Increasing barometric pressure.