Alpine climate

For the climate of the mountains named the "Alps", see Climate of the Alps.

White Mountain, an alpine environment at 4,300 metres (14,000 ft) in California

Alpine climate is the average weather (climate) for the regions above the tree line. This climate is also referred to as a mountain climate or highland climate.

In the Köppen climate classification, the alpine climate is part of "Group E", along with the polar climate, where no month has a mean temperature higher than 10 °C (50 °F). Certain highland climates can also fit under the hemiboreal climate or semi-arid climate groups of climate classification.

Description

Diagram showing heat transfer in the Earth's atmosphere

The climate becomes colder at high elevations, due to the way that the sun heats the surface of the Earth. Practically all the heat at the surface of the Earth comes from the sun, in the form of solar energy. The sun's radiation is absorbed by land and sea, which is warmed. The warm land loses heat by convection within the atmosphere, and long-wave radiation back to space. This radiation can move freely through gases composed of diatomic molecules, such as the atmosphere's oxygen and nitrogen, but is readily absorbed and re-radiated by triatomic molecules, such as carbon dioxide and water vapor. When the heat is re-radiated, some of the heat that would be lost to space is instead reflected back towards the Earth. Thus, the troposphere, as a whole, acts as a blanket for the Earth. This blanket effect is known as the "greenhouse effect". The higher the altitude, the less of this blanket there is to keep in the heat. Thus, higher elevations, such as mountains, are colder than surrounding lowlands.

The rate at which the temperature drops with elevation, called the environmental lapse rate, is not constant (it can fluctuate throughout the day or seasonally and also regionally), but a normal lapse rate is 5.5 °C per 1,000 m (3.57 °F per 1,000 ft). Therefore, moving up 100 metres (330 ft) on a mountain is roughly equivalent to moving 80 kilometres (45 miles or 0.75° of latitude) towards the pole. This relationship is only approximate, however, since local factors, such as proximity to oceans, can drastically modify the climate. As the altitude increases, the main form of precipitation becomes snow and the winds increase. The temperature continues to drop until the tropopause, at 11,000 metres (36,000 ft), where it does not decrease further. However, this is higher than the highest summit.

Distribution

Although this climate classification only covers a small portion of the Earth's surface, alpine climates are widely distributed. The altitude where alpine climate (and the tree line) occurs varies dramatically by latitude. For example, at 68°N in Sweden, alpine climate occurs as low as 650 metres (2,130 ft), while on Mount Kilimanjaro in Africa, alpine and the tree line occurs at 3,950 metres (12,960 ft).

The Sierra Nevada, the Cascade Mountains, the Rocky Mountains, the Appalachian Mountains, the Alps, the Snowy Mountains in Australia, the Spanish Pyrenees, Cantabrian Mountains and Sierra Nevada, the Andes, the Himalayas, the Tibetan Plateau, Gansu China, Qinghai, the Eastern Highlands of Africa, high elevations in the Atlas Mountains and the central parts of Borneo and New Guinea are examples of regions that have alpine climates. The mountain climate in the Northern Andes is particularly known for the notion of four zones of elevation:

• Tierra caliente or hot land
• Tierra templada or temperate land
• Tierra fría or cold land
• Tierra helada or frozen land

In mountainous areas with an alpine climate, the dominant biome is alpine tundra.

See also

• Alpine plant
• Climate of the Alps