Well, Blow Me Down!

Katabatic and Chinook winds are two types of downslope winds that can be extremely powerful. Katabatic winds are bitterly cold and dry and chinook winds are warm to hot and dry. Katabatic winds are mainly winter winds but chinooks are not limited to any one season.

Katabatic winds are gravity winds that begin in a shallow dome of cold, dense air. The air mass develops over a snow-covered plateau or other highland area. Siberia and Antarctica are both classic examples of places that get immensely cold and produce domes of frigid air. Thermal high-pressure systems, like the Asiatic or Antarctic highs, push this cold, dry air outward in all directions for great distances.

This air turns into a katabatic wind when it is pushed over a mountain range or over terrain that drops steeply. This cold, dense air rolls downslope under the pull of gravity because it is heavier than the surrounding air it replaces. When air moves downslope, it warms by compression (adiabatically) but it is so cold, it never gets warm.

Katabatic winds have many names world-wide but two of the most famous are the mistral winds of the Rhone River valley in France and the bora winds of the eastern Adriatic Sea coast. These winds can be very strong and can damage buildings and equipment as well as stop everyone from doing anything outdoors. It can render driving or even walking impossible and raise fierce seas in coastal areas. In some places, mountains funnel katabatic winds through narrow mountain valleys, which can double the speed. Steep mountain slopes can also exacerbate the winds. On Greenland and in the Antarctic, for instance, katabatic winds often exceed 100 mph and the Antarctic has recorded speeds above 200 mph in a few places where funneling occurs.

Chinook winds are also downslope winds but the mechanisms at work are different. Chinooks develop when relatively mild air is compressed and warmed as it descends the lee slopes of a mountain or mountain range. Because the air is relatively warm at the start, it is not as influenced by gravity as katabatic winds are. Instead, chinooks are pulled down slope by outside phenomena. In the case of the chinooks of the US Rockies, strong west winds associated with large-scale low-pressure systems east of the mountains pull the air downslope. Foehns are chinook winds and occur all over the world from the tropics to both poles.

Chinooks bring significant, often dramatic warming and drying when they occur. Air temperatures often abruptly climb tens of degrees in just a few minutes. There have been instances when the temperature rose nearly 40 Fahrenheit degrees in less than 5 minutes! The humidity can drop so much crops are desiccated and fires break out in suddenly tinder-dry forests. Chinooks are not just warm and dry; they can also violent. They are typically gusty and can be locally destructive, especially along the Front Range of the American Rockies. Boulder, Colorado once had a chinook event that reached 160 mph and more. It did considerable damage to buildings, toppled telephone poles, and caused widespread power failures.

Chinooks and katabatic winds can be very dangerous and any forecaster who works in an area these occur need to know when and where they happen. After all, these big bad wolves can really do a number on anyone or anything unlucky enough to be in their paths!

Written by 14 WS/DOPA (Melody Higdon)