Mountain climate

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.

There are multiple definitions of alpine climate.

One simple definition is the climate which causes trees to fail to grow due to cold. According to the Holdridge life zone system, alpine climate occurs when the mean biotemperature of a location is between 1.5 and 3 °C (34.7 and 37.4 °F), which prevents tree growth. Biotemperature is defined as the temperature, except all temperatures below 0 °C (32 °F) are treated as 0 °C (32 °F), because plants are dormant below freezing.

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).

The temperature profile of the atmosphere is a result of an interaction between radiation and convection. Sunlight in the visible spectrum hits the ground and heats it. The ground then heats the air at the surface. If radiation were the only way to transfer heat from the ground to space, the greenhouse effect of gases in the atmosphere would keep the ground at roughly 333 K (60 °C; 140 °F), and the temperature would decay exponentially with height.

However, when air is hot, it tends to expand, which lowers its density. Thus, hot air tends to rise and transfer heat upward. This is the process of convection. Convection comes to equilibrium when a parcel at air at a given altitude has the same density as its surroundings. Air is a poor conductor of heat, so a parcel of air will rise and fall without exchanging heat. This is known as an adiabatic process, which has a characteristic pressure-temperature curve. As the pressure gets lower, the temperature decreases. The rate of decrease of temperature with elevation is known as the adiabatic lapse rate, which is approximately 9.8 °C per kilometre (or 5.4 °F per 1000 feet) of altitude.

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