Download Mountain Weather - Rondebosch Journey

MOUNTAINEERING KNOWLEDGE & SKILLS Sheet 2 Version 1:1 Revised: 29062006 Written & Compiled by Elmarie Meyer © Venture Forth 2006 Mountain Weather Mountain weather is very unpredictable and sudden dramatic changes could occur at any given moment. The experienced hiker is always aware of these possible changes and always anticipates the worst possible conditions. Experience is however not the only requirement. The following knowledge is very valuable to make the correct predictions and decisions.
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A basic understanding of meteorology.
Knowledge of the weather conditions in the area you plan to hike in, including the effects of altitude.
Knowledge of how to obtain and interpret weather information. Terminology: It is important to distinguish between the terms climate and weather. Climate refers to the weather conditions experienced in a certain region over a certain period of time. Weather is defined as the atmospheric fluctuations on a day to day basis. WINDWARD AND LEEWARD SLOPES Mountains create their own weather and for people living in Cape Town Table Mountain is a vivid example. Cloudy and rainy weather may be experienced in the Newlands area, whilst warm sunny conditions may be experienced in the city at the very same time. The weather Camps Bay is also quite different to that in Newlands, since it is situated on two different sides of the mountain, namely the windward and leeward slopes. These slopes are sometimes also referred to as the north face or south face. In the Northern Hemisphere the north face is known to be colder and more dangerous than the southern slope. In the Southern Hemisphere, it would however be the south face that will be the daunting one. The explanation for these different weather conditions, lie in the spherical shape of the earth. The mountain slopes facing the equator receives more direct sunlight that those facing the poles. This results in them being warmer that the slopes facing the poles. The slopes facing the poles are normally your leeward whilst your windward slopes face the equator. OROGRAPHIC LIFTING The windward slope receives more direct sunlight since it faces the equator and, due to this, the air on the surface of the mountain warms throughout the day. When air warms, it starts to expand causing the density and weight to decrease and the air thus starts rising.
With an increase in height, there is a decrease in temperature. Although not only the altitude influences the temperature, it is important to note that rising air cools down approximately 10ºC for 1km gained. When this decrease in temperature happens, the water particles in the air start to move slower and tend to stick together. These water particles condense around dust particles in the air and a cloud develops. As the cloud droplets grow in size, they become too heavy to stay in the air and falls down as rain, snow or hail – known as precipitation. When rain develops in this manner, it is called orographic rain.
It is also possible that precipitation could not occur at all as the amount of moisture in the air will determine the severity of the precipitation. Air movement due to a difference in temperature above water (the ocean) and the land mass, may result in moisture carrying winds being forced to lift when a mountain is in their way. This air mass will produce strong rains. Dry air may only create cloud cover or sometimes nothing at all. Mountain ranges close to the sea will subsequently have a higher rain fall than those located more inland. When the warm air on the windward slope starts rising, the air vacuum it leaves gets filled. This movement of air is called wind. Air always flows from a high pressure to a low pressure. As explained earlier the rising warm air on the windward side cools down as it increases in height, thus, when it reaches the top of the mountain, this cold, moisture depleted are mass starts to sink down the leeward side of the mountain. As it moves down, it starts to warm. We thus experience a cold rising wind on the windward slope and warm sinking wind on the leeward side. It should now be clear that the windward slope of a mountain receives more rain than the leeward side. This is why the leeward side is also called the rain shadow slope. Just as ‘n normal shadow is a place blocked off from sunlight, this slope of the mountain is blocked from precipitation. Sometimes the cold air mass sinking down on the leeward slope has to cover such a big distance that it becomes especially warm. This is often found in high altitude regions. The air particles cool down with the decrease in height and rub against the ruff surface of the mountain, creating heat. This is known as adiabatic heating. The exceptionally warm wind that is produced is called a Chinook wind. ANABATIC AND KATABATIC BREEZE The movement of air, as described above, also happens on a small scale on hillsides. During the day the air in the valleys are warmed and starts rising. As we know by now, a vacuum created by moving air is always filled immediately. A circular effect arises as in the sketch below, with are rising, cooling and then sinking. This rising of air against the slopes is felt as a gentle breeze and is called an anabatic (upward moving) wind. In the evening the air on the hills cools down quicker than the air in the valley. It starts sinking, collecting at the bottom of the valley. The cold air forces the air in the valley to rise, since it is more dense and colder than the air in the valley. As this air rises, it cools down and starts sinking – creating a similar circular effect as with the anabatic wind. The downward breeze is known as a katabatic wind.
When deciding where to pitch your tent at night, it would be advisable not to do so in the bottom of the valley, since it would be the coldest place at night. On the hillside again, it might be open to all kinds of serious weather changes. Try to find a good level spot against one of the slopes. Choosing between the windward and leeward slope may depend on the area. Whilst the windward slope might be warmer, it would also be the slope receiving more direct rain, whilst the colder leeward slope will be sheltered from precipitation. FRONTAL LIFTING Except for elevation and heating during the day and cooling at night there are also other factors causing air masses to move. Warm and cold ocean currents cause big air masses to become hot or cold and to subsequently move. This creates all kinds of interesting circular air movements around the earth. For the mountaineer, the most important information is to know the effect of a cold and warm air mass meeting. When this happens a front is formed. Fronts are normally experienced near the coast. On contact, the warm air mass – due to its light, less dense nature – rises over the cold, heavier and more dense air mass. This is sometimes only experienced as a cold or warm front. Most of the times however, a middle latitude cyclone is formed causing cold and wet weather conditions as we experience every winter in the Western Cape. When a cold front develops it is due to a cold air mass moving into an area occupied with warm air. A warm front on the other hand, develops when a warm air mass moves into an area occupied with cold air. A middle­latitude cyclone occurs when a warm and cold air mass meets, and due to the movement of the earth, starts spiralling inward. In the southern hemisphere this movement is clockwise, whilst it is anti­clockwise in the northern hemisphere. It consists of both a warm front and a cold front. The downward spiralling air creates a low pressure system. At sea level, the standard air pressure is 1013hpa. An air pressure reading of less than a 1000hpa is associated with a low pressure system and subsequently bad weather. These spiralling air masses are also not statutory, but are migrating. The low pressure systems we experience in South Africa, moves on the Westerlies Sub­polar wind, moving the low pressure system from east to west. The following figure is explanatory of the conditions experienced during the passing of a cyclone.
The warm front is followed by the cold front. Before the warm front pleasant, sometimes abnormally warm, weather or warm winds could be experienced. There will be a drop in atmospheric pressure and clouds will appear lower and become darker in colour. Light rain would start falling, followed by a drop in temperature. The rainy conditions will get heavier and more severe, with possible thunder storms. Strong winds are prevalent. The coldest condition will be experienced behind the cold front and snow is possible. These cold conditions will be without rain or thick cloud cover. But what are high pressure systems then? It is also the circulation of air mass, but it is an upward anti­clockwise motion in the southern hemisphere. In the northern hemisphere the rotation is clockwise. A high pressure system is also called an anti­cyclone. It is associated with warm pleasant weather, as Capetonians experience in summer. In South Africa three high pressures systems are present throughout the year. The Kalahari High Pressure System is present over the Kalahari area in the north of the country and also over our neighbouring countries. The South Indian High Pressure System is present along the south east coast while The South Atlantic High Pressure System is found along our west coast and is by far the biggest of our three high pressure systems. Due to her very prominent presence in summer over big parts of the south west of South Africa, middle latitude cyclones are unable to reach the country. CLOUDS It should be clear by now that clouds are formed by rising air masses. It is however important to know exactly how certain cloud types develop and what weather conditions are associated with them. Keeping an eye on the clouds is an excellent way of predicting the weather. Cloud formation can happen in four ways.
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Air is forced to rise due to the relief, e.g. a mountain range (oriographic).
Heavy cold air can wedge in under a warm air mass, forcing it to rise (frontal).
There can be a convergence of air in the lower regions of the atmosphere (frontal – inward spiralling low pressure).
Convection currents can carry moist air upwards into the higher atmosphere. As described earlier, clouds are formed when upward moving air cools down to such an extent that the air becomes over saturated and microscopic small water particles condense around dust particles. These dust particles are called condensation nuclei. These small particles must be present for the water particles to condense on. Clouds are classified on the basis of their shape and altitude. There are four basic cloud types: a) High clouds: Cirrus (fleecy, white and above 6000m) b) Middle clouds: Altostratus & altocumulus (grayish, fairly dense, uniform shape and between 2000m and 6000m) c) Low clouds: stratus, stratocumulus & nimbostratus (could be white or grey, uniform shape and below 2000m d) Vertically developed clouds: Cumulus (big and billowy and stretches over low and medium altitude)
Cloud names with a “Nimbus” in front, means that it is rain clouds. “Stratus” means that they are thick low clouds which will produce soft continuous rain as we experience in the Western Cape in winter. Cumulonimbus clouds are vertically developed and are associated with heavy showers, lightning and high winds that characterise the Gauteng thunderstorms. In the picture depicting the middle­latitude cyclone, we can clearly see how certain cloud types predict the weather to follow. Cirrus clouds are always associated with warm weather. If the clouds however increase in number, a cyclone may be on its way. Altostratus clouds that thicken and move lower, is a sign of impending widespread rain. Altocumulus clouds that combine in a flatter cloud layer are a sure sign of rain that same day. Cumulus clouds are good weather clouds. If these clouds grow in a vertical manner and become dark in colour, thunderstorms are sure to be experienced very soon. Stratus clouds are typical of coastal regions and are formed by a difference in temperature in land and sea air masses. Stratus clouds are the cause of thick mist along coastal mountains. Stratocumulus clouds bring cool conditions and are slightly higher and thicker than stratus clouds. Nimbostratus clouds bring soft continuous rain associated with a middle­latitude cyclone. SYNOPTIC WEATHER MAPS It is not always easy to read the clouds to predict the weather. A much easier solution is to phone the Weather Bureau in the area you plan to hike in. Radio, television and the press are also sources of weather information, but do remember that their information is limited to a few days in advance and not always as accurate. Although the weather bureau is by far the better option, it is important to note that very few reporting stations are in mountainous areas and REMEMBER: mountains create their own weather. Should you ever be given a synoptic weather map of a region, it could prove very handy to know how to interpret it. See if you can identify any of the high and low pressure systems on the following map. The lines drawn around them are called isobars, and connect places with the same atmospheric pressure. You will note the difference in pressure between the high and low pressure systems. Since we only see the cold front leg of the middle­latitude cyclone, it is common practise to only refer to this cyclone as a cold front. Although it is not meteorologically correct, it is acceptable. The accompanying key will help to interpret the different signs at the weather station. At each weather station you will see two numbers. The top number is the maximum temperature for that specific station for that day. The bottom number is the dew point temperature. It is at this temperature where the air will be saturated and clouds will develop. The closer the two temperatures are, the more cloud cover will be experienced. You will also note that the coastal weather stations have a higher dew point temperature and cloud cover is more likely to occur.