Mountain meteorology

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The effects of mountains on the atmosphere, ranging over all scales of motion, including very small (such as turbulence), local (for instance, cloud formations over individual peaks or ridges), and global (such as the monsoons of Asia and North America).

The most readily perceived effects of a mountain, or even of a hill, are related to the blocking of air flow. When there is sufficient wind, the air either goes around the obstacle or over it, causing waves in the flow similar to those in a river washing over a boulder. Since ascending air cools by adiabatic expansion, the saturation point of water vapor may be reached in such waves as they form over an obstacle, and a cloud then forms in the ascending branch of the wave motion. Such a cloud dissipates in the descending branch where adiabatic warming takes place. The shapes and amplitudes of these lee waves (they form over and to the lee of mountains) depend not only on the thermal stability and on the vertical wind shear in the overlying atmosphere but also on the shape of the underlying terrain. See also Cloud; Cloud physics; Wave (physics).

On a grander scale, mountain ranges, such as the Sierras of North and South America, place an obstacle in the path of the westerly winds (that is, winds from the west), which generally prevail in middle latitudes. Such a blockage tends to generate a high-pressure region upwind from the mountains (this may be viewed as air piling up as it prepares to jump the hurdle), and a low-pressure area downwind. Thus, there is a stronger push against the mountains on the high-pressure western side than on the low-pressure eastern side. The net effect is the slowing down of the atmospheric flow (mountain torque). See also Torque.

Less subtle than mountain torque effects are the large-scale meanders that develop in the global flow patterns once they have been perturbed, mainly by the North and South American Andes and by the Plateau of Tibet and its Himalayan mountain ranges. These meanders in the large-scale flow are known as planetary waves. They appear prominently in the pressure patterns of hemispheric or global weather maps. See also Weather map.

The major monsoon circulations interact with the global circulation, shaped in part by sea-surface temperature anomalies in the equatorial Pacific. The various aspects of mountain meteorology, therefore, have to be viewed within the larger picture. There is a continuous interaction between the weather effects on all space and time scales generated by the mountains and the weather patterns that prevail elsewhere on the Earth. See also Meteorology.

Mountains give rise to distinctive climates. Upland areas are cooler than lowlands of the same latitude, since temperatures fall by 1 °C per 150 m. Heating of the valley floor to a higher temperature than the valley sides may lead to anabatic mountain winds. Equally, colder and heavier air at the peaks may spill down into the valley floor as a katabatic wind.

Warmer, moist air will be forced to rise over mountain barriers. As the air is chilled with height, orographic rain falls. When such air has passed over the barrier, it descends and is adiabatically warmed. This may bring warm winds, like the chinook. Since the warmed air is now capable of holding the remaining moisture, a ‘rain shadow’ of drier air develops in the lee of mountains.

On a larger scale, mountain barriers affect low pressure systems as they pass over and lee depressions may also develop. The location of major mountain barriers such as the Rockies and Himalayas affects the formations of air in the upper atmosphere.