Local winds that are hot are caused by the advection of hot air from a warm source region. They may also be produced by dynamic heating of air as it descends from an elevated area to lowland.
Foehn is a warm and dry wind on the lee side of a mountain range. Its high temperature is largely due to adiabatic heating during its descent down the mountain slopes. These warm, dry and gusty winds are common on the northern side of the Alps in Switzerland.
With the arrival of these winds there is a great and rapid rise in temperature. The foehn-type winds occur when air originating elsewhere must pass over a mountain range. These wind> are invariably initiated by the advance of an upper-level low-pressure centre.
When a relatively moist west wind crosses a mountain range, a low-level pressure trough is formed on the lee side of mountain. As the upper system crosses the mountain, the air descends on its leeward slopes and is heated by compression. Thus, high temperature and low relative humidity of the foehn winds result from the adiabatic heating of the down-slope winds.
Foehn winds intensify on the windward slope of the mountain barrier and have ascending motion with resultant condensation and precipitation. The ascending air currents cool at the moist adiabatic rate (about 6″ Celsius per kilometer), but the same air, while descending on the lee side of the mountain chain, is heated at dry adiabatic rate (10° Celsius per kilometer).
Thus, these down-slope winds are considerably warmer and drier than the air of that area. In winter and early spring, rapid melting of snow is caused by such warm and dry winds.
In this way, foehn winds are helpful in spring farming. Being associated with the passage of extra-tropical cyclones, these local winds occur mainly during the cold season. It summer, they are quite rare.
The beginning of a foehn is marked by a sharp rise in temperature and a sudden drop in the relative humidity of air. Sometimes, the sudden rise in temperature accompanying such winds proves damaging to the standing crops. These warm and dry winds bring a drastic change in local weather.
Like foehn these winds are also very warm and dry. On the eastern slopes of the Rocky Mountains in Wyoming and Montana, these local winds are called Chinooks. The mode of origin of these winds is similar to that of the foehn-type winds.
They are more common in winter and early spring. Under favourable conditions such winds may occur over any mountain range.
Although the actual temperature of these winds is only about 4.4° Celsius during the cold season, the affected area may be having below freezing temperatures. Thus, in comparison with the prevailing low temperatures all around, these winds bring about a drastic change.
With the onset of this warm and dry wind, the snow present on the ground rapidly melts away. At times, these winds are so dry that in spite of their below freezing temperatures the entire snow cover on the ground disappears by the process of sublimation. That is why these winds are called Chinook, which literally means “snow eater’.
An increase of about 22° Celsius in the temperature of the air of the locality affected by chinook within 24 hours is nothing unusual. In Montana the air temperature at a place named Kip registered a record increase of 19° Celsius in a very short period of seven minutes.
At Denver, Colorado, a temperature rise of-14° Celsius occurred in two hours with the onset of a chinook on 27 January, 1940 shortly after midnight.
The effect of chinook-type winds is not only limited to the foot of mountains, but it may also extend up to hundreds of kilometers away from them. In the western part of the Great Plains in North America, the chinook proves beneficial to ranchers, because the grasslands are kept free from snow during much of winter and early spring.
It is due to the occasional onslaughts of these warm and dry winds that the severity of winter in the western part of the Great Plains is much reduced. However, these benefits are offset by the loss of soil moisture that the snow would have provided by its melt water, had it not been evaporated earlier.
Santa Ana :
Santa Ana is a warm and dry local wind that occurs in the United States of America. It resembles Chinook. This hot and desiccating wind is experienced in Southern California where it blows out from the Santa Ana Canyon towards coastal low lands.
This wind occurs when cold air accumulates in the Great Basin and starts overflowing through the mountain valleys and spreads out on to lowlands. Recent investigations show that the air reaching down the low lands, in fact, starts several hundred meters above the mountain peaks in free air.
Santa Ana- type winds are so warm and dry that they pose a great danger to Californian orchards. Under their impact the trees get dried up and there are large- scale forest fires. Sometimes these dry winds are laden with so much of fine dust particles that breathing becomes troublesome.
Similar warm and dry winds that blow down the steep valleys in Japan are called Yamo. Such winds blowing down the Andean valleys to the plains of Argentina in South America are named as zonda. In the valleys of Central Europe, they are called tramontane.
Sirocco is the local name given to hot, dry and dusty winds blowing from Sahara Desert over central Mediterranean and southern Italy in front of an advancing depression. In general, the term sirocco is applied to any hot wind associated with the warm sector of an advancing extra-tropical cyclone and heated by passing over hot and dry land area.
Moving depressions over the Mediterranean Sea draw hot and dry air from over Sahara. On the southern coast of the Mediterranean, hot, dry and dust laden winds persist for several days together. These winds have a relative humidity of 10 to 20 per cent.
The presence of dust particles makes the atmosphere so turbid that even the sun is hardly visible. The sirocco which extends to southern France and Italy is warm and humid, because it picks up moisture from over the Mediterranean. These winds are called levech in Spain and leste in Madeira and Morocco. The warm air-mass type hot northerly wind in Australia is called a brickfielder.
It is a hot, dry, southerly wind that bows in Egypt in spring time. Since these local winds have their origin in hot deserts, they are not only hot but also carry a lot of fine sand particles. Whenever temperate cyclones pass over the Mediterranean, such dust storms originating in the hot deserts occur on their forward side.
Simoom is an intensely hot and dry wind of the Asiatic and African deserts. They take their origin from the warm tropical continental air masses. These winds are very dusty and suffocating. They produce very oppressive weather.
Harmattan is a dry, dusty wind of the west coast of Africa blowing from the deserts. These winds also originate in Sahara Desert in winter. These dry and dusty winds blowing towards Guinea, where the air is hot and humid, provide some relief to the inhabitants of the affected area. In hot and humid regions, harmattan has a cooling effect because of the process of evaporation taking place in them.
Many other warm and dry winds are known to occur in different regions of the world. Black-roller is the local name given to a very strong and dust-laden wind blowing from the southwest or north-west in the Great Plains of North America.
There are so much of sand particles in these winds that at times buildings falling in their path are buried under the heaps of sand.
They cause tremendous economic loss to the affected regions. Shamal is a similar warm and dry northeast wind blowing in Mesopotamia and the Persian Gulf. In New Zealand, the warm, dry and gusty winds descending from the high mountains are called norwester.