Table of Contents
Peculiarities of the Mountain Climate Affecting Local Winds
The features of the mountain climate are connected with the local winds that are formed due to differences in pressure at different heights. The term mountain climate generally implies the climatic conditions in the high altitude mountainous areas.
The common features of mountain climate include the low atmospheric pressure, the high intensity of solar radiation, the air qualities such as visibility and haze, the low temperature and absolute humidity that is rising to the level of rainfall, different types of winds such as mountain-valley winds, etc. (Whiteman, 2000).
There are several peculiarities about the climate conditions of the mountains as compared to other types of climate. First, it is the impact of the height. With the increase in height, the temperature is lowered. For every 100 meters in altitude, the temperature drops by 0.5-0.6 degrees Celsius. To a large degree, this is connected with a high cloud cover as well as the presence of the cold winds and hurricanes. Secondly, there is a low atmospheric pressure. The level of atmospheric pressure reduces considerably with the increase in height. The decrease in atmospheric pressure leads to an interesting peculiarity of the mountains: the boiling point of the water is lowering with the increase in height. While at the sea level it is 100 degrees Celsius, at a height of 4300 meters, 86 degrees are enough to boil the water (Whiteman, 2014). Third, there is a decrease in barometric pressure. With every 11 meters, it decreases by 1 millimeter. In the mountain climate, the intensity of solar radiation and the share of the ultraviolet rays in sunlight are increased. The rate of evaporation of the liquid increases also. The same situation is with the increase in precipitation (observed up to a certain height) (Spellman, 2013).
The mountain climate influences the areas surrounding the mountains. The main impacts are the precipitation, the amount of solar radiation, and the airflow. Mountain ridges that are perpendicular to the direction of the air masses movement delay rainfall. The so-called “rain shadow” is common for the leeward side of the mountains (Durran, 2015). The opposite slopes can differ dramatically because of climate peculiarities. On the windward side, the area is characterized by excessive humidity, while on the opposite leeward side, there are dry slopes and the surrounding valleys. For example, the Alps come as a a sharp boundary between the subtropical and the Mediterranean temperate climate of the rest of Europe. In the mountains, different slopes have different sunlight. Consequently, on the sunny side of the mountains, the climate is much warmer and generally more favorable. Therefore, in the mountain valleys, people prefer to settle on the sunny side (Spellman, 2013).
The specific airflow forms the mountain wind systems. The mountains are an obstacle on the path of movement of the air masses. The mountains therefore provide a special condition for the air, which flows through the shimmer ridge under its own weight and rolls down the slopes. In this way, specific mountain winds are formed, which have a significant impact on the climate of the mountains and the surrounding areas.
The Formation of Local Wind Systems
The mountains influence the wind formation in two ways: they either cause the wind formation or act as a barrier to the air flows. Above the hills, the air is heated more than the air at the same height above the lowlands, creating a zone of low pressure over the mountains, which leads to wind formation. This effect often leads to the formation of mountain-valley winds that are prevailing in the areas with rough terrain. The increased friction of surface valleys lead to the deviation of the wind that blows parallel to valley height of the surface on the surrounding mountains, which leads to the formation of high-altitude jet stream. A tall jet flow may exceed the ambient wind speed by up to 45%. Bypassing the mountains can also change the direction of the wind (Durran, 2015).
The difference in the height of the mountains significantly affects the movement of the wind. Thus, if the ridge that the wind flows by has a mountain pass, the wind passes it by with an increased speed because of the Bernoulli effect. Even small height differences cause fluctuations in wind speed. As a result of the significant gradient in the velocity of the air, it becomes turbulent and remains so even at a certain distance on the plain away from the mountains. These effects are important for aircrafts that are passing through the mountainous regions. Quick cold winds blowing through the mountain passes received a variety of local names. In Central America, there is Papagayo near Lake Nicaragua; the Panama wind on the Isthmus of Panama, and teuano wind in the Isthmus of Tehuantepec. Similar winds in Europe are known as boron, Tramontana, and mistral (Whiteman, 2000).
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Another effects associated with the passage of wind over the mountains are the lee waves and the standing waves of air flow that occur behind the high mountains, which often lead to the formation of lenticular clouds (Whiteman, 2000). Because of this, as well as due to other effects of the wind that is passing through the obstacles, there are numerous vertical currents and eddies over the rugged terrain. In addition, on the windward slopes of the mountains, there are heavy rainfalls caused by adiabatic rises of the cooling air and the condensation of moisture in it. On the contrary, on the leeward side, the air becomes dry, which causes the formation of a rainy dusk. Therefore, in the areas where the prevailing winds overcome the mountains, the windward side is dominated by the humid climate, and the leeward side often has droughts. The winds blowing from the mountains to the lower areas are called the descending winds. These winds are warm and dry (Coulter, 1967). They also have many local names. Thus, the downward winds coming down from the Alps in Europe are known as the foehn; the term is sometimes used in other areas as well. In Poland and Slovakia, suh wind is known as thalny; Argentina has zonda winds; there are koembang winds on the island of Java; New Zealand has winds called Nor’west arch (Brinkmann, 1971). The Great Plains in the United States have winds that are known as Chinook, and in California, there are Santa Ana and sundowner (Durran, 2015). The speed of the downward wind can exceed 45 m/s.
The most widespread mountain local winds are types of slope winds and mountain-valley winds. Slope winds are characterized by the dependence on the time of the day. They rise during day or night on the mountain slopes; slope winds are one of the reasons of mountain-valley winds.
During the day, the slopes are more heated than the air, so the air in proximity to the slope is heated faster than the air farther from the slope. Thus, there is a horizontal temperature gradient in the atmosphere that is directed from the slope into the free atmosphere. More hot air from the slope begins to climb up the hill, as it happens in convection in the free atmosphere. This amount of air on the slopes leads to an intensive formation of clouds on them. At night, when the slopes cool down, the conditions are reversed, and the air flows down the slopes. These slope winds join the transfer of air on a larger scale between the valley as a whole and the surrounding plain. During the day, the air temperature in the valley is generally higher than the corresponding levels above the plain, since it is affected by warmed slopes. Therefore, similar to the way the sea breeze forms over the bank, the pressure in the valley is lower than the rest of the ridge and is higher at higher altitudes (Coulter, 1967).
Mountain-valley winds are the winds with a daily frequency similar to breezes that are observed in the mountain systems. During the day, valley winds blow from the down of the valley upwards and up the mountain slopes. At night, the mountain breeze is blowing down the valley towards the plains. Mountain-valley winds are well expressed in many valleys and basins of the Alps, the Caucasus, and Pamir, mainly in the warmer months of the year (Brinkmann, 1971). These winds are a typical characteristic of the mountain climate. The vertical power of these winds is significant and is measured in kilometers: the winds fill all the cross-section of the valley, up to the sides of its ridges. As a rule, they are not strong, but sometimes reach 10 m/s or more (Brinkmann, 1971).
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It is possible to distinguish at least two independently acting causes of mountain-valley winds. One of the reasons is the daily rise of the air on the mountain slopes (that is, the slope winds). The other reason is the general transfer of air up the valley during the day and down at night, and these are mountain-valley winds in the narrow meaning of the word. Sometimes, this process is called mountain-valley circulation, that is, the mountain-valley winds observed between the ridge and the valley (Parish, 2015). During the day, the slope warms up, forms a layer of warm air, which then rises. This is a valley wind. The ascending wind is also called anabatic wind. At night, the slopes are cooled more than the surrounding air. Cold air flows down from the high places in the valley. This is katabatic wind (mountain wind, or falling wind). Going up the hill, the air cools adiabatically and, if the temperature drops to the dew point, the condensation occurs. From the sea, it would look like a cloud, and for the residents of the slopes, it would be a fog. The orographic fog is associated with mountain-valley winds. In the mountainous shores, the mountain-valley circulation merges with the breeze circulation, reinforcing each other.
Specific Local Wind Systems
Foehn is a strong, gusty, warm and dry local wind blowing from the mountains to the valleys. Cold air from the high mountains quickly falls down the relatively narrow mountain valleys, causing adiabatic heating. With lowering for every 100 m, the air is heated by about 1°C; thus, the air coming down from the height of 2,500 meters is heated to 25 degrees and becomes warm, even hot (Durran, 2015). Foehn usually lasts less than a day, but sometimes it may last up to 5 days, with changes in temperature and humidity that can be quick and dramatic. Foehns are particularly frequent in spring, when the intensity of the general circulation of air masses sharply increases. Unlike foehn, the invasion of dense masses of cold air forms a local wind called bora (Whiteman, 2014).
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Bora is a strong cold gusty local wind that occurs when cold air meets a hill on the way up. Overcoming the obstacle, the bora wind hits the coast with great force. The vertical dimension of bora occupies several hundred meters. There the local winds usually affect smaller areas with low mountains directly bordering the sea (Whiteman, 2000).
In Europe, the most famous boras are those of the Adriatic Sea (near the cities of Trieste, Rijeka, Zadar, Sen etc.). In Croatia, the wind is called bura. Similar to the Bora wind and the “Nord” in the districts of Baku, which directly border the sea, there are kinds of bora like mistral on the Mediterranean coast of France from Montpellier to Toulon, as well as “nortser” in the Gulf of Mexico (Durran, 2015). Bora winds usually last from several days to several weeks. The daily difference in temperature during these winds may reach 40° C.
Bora occurs in some regions of the Northeastern Russia as well as the Adriatic coast. It occurs when a cold front approaches the coastal ridge. Thus, a cold front crosses a mountain range that is not high. Under the influence of gravity, the cold air rushes down the ridge, acquiring thereby a greater speed (Whiteman, 2000).
Before the advent of bora, the winds at the top of the mountains can be seen as thick clouds. Initially, the wind is very unstable, changing its direction and strength, but it gradually acquires a certain direction and a great speed that can reach up to 60 meters per second (Durran, 2015). The speed of the bora wind in the winter can reach 20 meters per second. Having fallen on the water surface, this downdraft causes a gale, thus causing heavy seas. This dramatically lowers the air temperature, which had been warm above the sea before the bora wind occurred (Whiteman, 2014).
Sometimes bora winds cause considerable damage in the coastal zone. The sea breeze causes big waves on the sea; the amplified waves flood the coast and bring destruction. At strong frosts, bora winds make waves freeze, thus forming ice crust (Durran, 2015).
There are two kinds of bora winds, black and white. In Croatia, the black bora wind is called skoura. It is a kind of bora that occurs on the coast during the passage of the cyclone by the sea. In the rear, there are cyclonic northeasterly vortex winds, which amplify after the collapse of the flow on the leeward slopes of the mountains on the coast. The wind is accompanied by precipitation and powerful clouds of the lower tier. The white bora occurs when there is an anticyclone, with a strong northeasterly wind. White bora wind does not bring precipitation; there are usually clear skies, but the force of the white bora wind is usually greater than that of the black bora (Durran, 2015).
The other types of bora winds are Tramontana and Sarma.
Tramontana (ital. Tramontana – «from the mountains”) is the cold north and northeastern wind in Italy, Spain, France, and Croatia. It arises from the difference between the high pressure in the mainland Europe and the lower pressure in the Mediterranean Sea. Tramontana can reach the speed of up to 130 km/h (Spellman, 2013). The wind name is different in different languages. In English, it is taken from Italian (tramontana), which, in turn, is a Latin word altered from trānsmontānus (trāns- + montānus). In Catalonia and Croatia, the wind is called Tramuntana (Spellman, 2013).
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Sarma is a strong squally wind blowing from the valley of the river Sarma, which runs to the Small Sea Strait (Baikal). Is a kind of a bora wind similar to Tramontana (Spellman, 2013). This wind arises when cold arctic air goes to the plateau, passing through the seaside ridge, and enters the orographic narrowing to the shore of the Baikal lake called the Sarma valley, which is a kind of a natural wind tunnel. At the exit from this natural tunnel, the wind speed reaches the speed of a hurricane. In general, the speed of the sarma wind is 40 m/s, but can sometimes reach 60 m/s. The wind can blow continuously for several days, causing damages so heavy that there are uprooted trees, turned courts, teared off house roofs and other consequences. This wind is the most frequent and fierce in autumn and winter. On average, sarma blows 10 days in November and 13 days in December (Spellman, 2013). Usually, the wind covers the waters of the Gulf of the Small Sea, but its echoes can sometimes be as high as the eastern shore of Lake Baikal. The wind speed increases rapidly and quickly reaches the hurricane strength.
Mistral is a dense, cold northwest wind blowing from the Cevennes on the Mediterranean coast of France. Mistral is a kind of a katabatic wind. Katabatic winds are tight and cold flows of air blowing from the slopes and peaks of the mountains. Katabatic winds have high speeds that may reach the speed of a hurricane. For example, in the dry valleys of McMurdo, they accelerate up to 320 km/h (Parish, 2015). Mistral is a real scourge for the Provencal farmers. The wind is blowing so frequently and with such a force, that under its influence, the trees acquire an inclined form (Parish, 2015).
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The mistral comes from the pressure difference between the ridge of high pressure over the near Atlantic Northern Europe and the minimum low pressure over the Mediterranean Sea (Gulf of Genoa). A cold front associated with a low-pressure system over the Northern Europe moves eastwards. The flow is moving northwest after the passage of the front, bringing the fresh ocean air. The flow generated by this configuration is channeled and accelerated in the Rhone Valley (corridor wind) up to the altitude of about 3000 m (bypassing the Alps) (Spellman, 2013). The mistral can blow all year round, but it is most common in winter and spring.
Northeast winds occur in the mountains of the northern hemisphere and blow towards the sea. At sea, they cause severe storms. They also usually cause a drop of temperature. These winds are typical of the northeastern coasts of the USA, Yakutia (Russia), and some former Soviet Union countries (Uzbekistan, Tajikistan, Kyrgyzstan, Ukraine) (Spellman, 2013).
Chinook is a southern foehn that occurs on the Eastern slopes of the Rocky Mountains in Canada and the United States as well as the adjacent areas of the prairies. It is accompanied by a very fast and sharp increase in temperature, which may sometimes reach 20-30°C, and which contributes to the melting of snow, accelerates ripening and so on (Coulter, 1967). This local wind is observed during all seasons, but most often it blows in winter. Chinook is also a moist southwestern wind blowing from the Pacific Ocean to the West coast of the US.