#foehn

Foehn Winds

Foehn winds are warm, dry winds that sometimes flow down the lee side of mountains ranges. Two well known examples of Foehn winds are the Chinook in the Canadian Rockies and the Santa Ana in California. Chinook roughly translates to ‘snow eater’, named for their ability to melt accumulated snow. Often occurring in the late fall and winter months, these winds can dramatically affect weather hundreds of kilometers away from the mountain range due to the strong winds and large, rapid temperature increases. In 1972 a Chinook wind caused an incredible temperature increase of 57°C (from -48 to 9°C) in Loma, Montana - the largest temperature change ever recorded over a 24-hour period.

Foehn winds occur because of the adiabatic heating and cooling of the air as it travels up and over the a mountain range. The adiabatic lapse rate is the rate at which the temperature of a parcel of air changes as it ascends or descends, varying depending on its moisture content. The average adiabatic lapse rate of unsaturated air (dry adiabatic lapse rate) is 3°C /1,000ft, and the average adiabatic lapse rate of saturated air (moist adiabatic lapse rate) is 1.5°C /1000 ft. The difference between the dry and moist adiabatic lapse rates is due to the latent heat of condensation. When air becomes saturated and excess water vapour begins to condense to form clouds, heat is released, decreasing the adiabatic lapse rate.

When unsaturated air is forced upwards by a mountain range it expands and beings cooling at the dry adiabatic lapse rate. If rising continues the air will eventually reach its dew point and become saturated, where the air can not hold any more water vapour and the excess will condense and form clouds. This elevation is called the lifting condensation level. Most of the condensed moisture will fall as rain. As the saturated air continues to ascend the range it will cool more slowly, at the moist adiabatic lapse rate. When the air reaches the peak of the mountain range and begins descending on the lee side it warms by adiabatic compression. The air is now unsaturated because some of the moisture has been removed, so it warms at the dry adiabatic lapse rate. This creates areas, where at the same elevation, the air on the lee side of the mountain is much warmer than the air on the windward side.

Here’s a simplified example of how a Foehn wind warms as it passes over a 5000ft mountain range. Let’s say that the air temperature is 20°C at sea level on the windward side of the mountain range and that it will reach its dew point at 14°C or 2000ft. For the first 2000ft that the air must ascend the mountain range, it cools at the dry adiabatic lapse rate (3°C/1000ft) to 14°C. For the last 3000ft (from 2000ft to 5000ft), the air is saturated and excess moisture from the rising air condenses into a cloud and it rains. This saturated air cools at the moist adiabatic lapse rate (1.5°C/1000ft) as it rises, further decreasing its temperature by 4.5°C. This means that the air temperature at the top of the mountain range is 9.5°C. This air now descends the 5000ft lee slope of the mountain where it is compressed and warms at the dry adiabatic lapse rate, increasing the sea level temperature by 15°C to 24.5°C. By the process of adiabatic cooling and heating over the mountain range, the temperature of the air has increased by 4.5°C.

CD

Sources https://www.skybrary.aero/index.php/Lapse_Rate https://bit.ly/2MduL4Q https://www.metoffice.gov.uk/learning/wind/foehn-effect https://bit.ly/2Mvtd2J https://bit.ly/2KETuu3