Unusually cold Santa Ana winds

[Lettucing Gutted]

Southwestern California's Santa Ana northeasterly wind is often hot - even in the winter.

However according to this website there were unusually COOL Santa Ana winds in November in Southwestern California. B)

http://www.topix.net/content/trb/387303151...327800901494139

Does anyone have any theories as to why the Santa Ana wind was so cold?

Edited December 17, 2006 by Craig Evans

[Nick F]

I'd imagine the interior deserts of Nevada and Arizona between the Sierra Nevada and the Rockies to the East would be cold with sub-zero temps at this time of year, so not sure how winds could be 'hot' from the NE across SW California in the winter B) .

There's probably been some unusually cold air sweeping South over the Western deserts of the US this month so far, so I guess the Santa Ana winds are feeling colder than normal over SW California

[Lettucing Gutted]

so not sure how winds could be 'hot' from the NE across SW California in the winter .

The Santa Ana wind

I know it sounds a bit of a paradox having "hot Northeasterlies" Nick F. However the Santa Ana's source regions - The Great Basin and Colorado Plateau are very high up and are positioned between The Rockies in the east and The Sierra Nevada Mountains in the west.

This means that any cold wind that develops up there has to travel downslope to Los Angeles in Southwestern California. All downslope winds experience adiabatic heating.

Adiabatic heating is caused by downslope winds moving from a low thickness tropospheric layer (500mb in the middle troposphere and on a high plateau) to a high thickness tropospheric layer (1000mb at sea level).

The movement of air from a low to a high thickness layer compresses the air that is descending from the high plateau to the sea level.

Compression generates Adiabatic heating - a non-solar heating effect on the downslope wind. To understand the heating effect of compression you have to think of the warmth that is generated by inflating a car tyre with air. The valve on a car tyre feels very warm after it has been inflated. Another example of generating heat by compression is by increasing the air pressure in a pressure cooker.

In a downslope wind - compression causes the air molecules to move closer together in descending air as it descends from the middle troposphere to the surface. As the air molecules move closer together - friction caused by the colliding air molecules generates heat.

The Western Sierra Nevada Mountain slopes - which the Santa Ana downslope wind MUST descend are very high and steep. This means that the adiabatic heating effect usually warms the Santa Ana's maximum air temperature from a max of about -5c on the High Sierra Nevada Mountain slopes to a max of at least +30c in all seasons by the time the Santa Ana wind reaches Los Angeles, California.

The above adiabatic heating process modifies the Santa Ana downslope wind in all seasons UNLESS the High Sierra Nevada Mountains are unusually very cold.

When the air temperature on top of the High Sierra Nevada Mountains is unusually cold for example -20c - adiabatic heating would only rise the maximum temperature in the Santa Ana wind to maybe a max of 17c in the winter in Los Angeles and a max of 24c in the summer in Los Angeles.

For more information on the Santa Ana wind follow the website link listed below:-

http://www.islandnet.com/~see/weather/alma...06/alm06sep.htm

The Shamal (Seistan) wind

Another example of "hot Northerlies" is the hot, humid and dusty Shamal (Seistan) Northwesterly winds that effect Afghanistan and the Middle East in the summer.

In the winter the Shamal (Seistan) Northwesterly winds produce cool, dry and dusty weather in the Middle East and Afghanistan because the Shamal is an extension of the Maritime Arctic Northwesterly winds from Greenland.

However in the summer the Shamal (Seistan) Northwesterly wind picks up heat and moisture as it crosses the hot Persian Gulf and hot Red Sea. The maritime modified Shamal Northwesterly wind brings hot, humid and dusty weather to the Middle East and Afghanistan in the summer.

For more information on the Shamal and Seistan winds follow the website link listed below:-

http://www.netweather.tv/forum/index.php?s...9&hl=Shamal

Edited December 18, 2006 by Craig Evans

[WhiteFox]

The Western Sierra Nevada Mountain slopes - which the Santa Ana downslope wind MUST descend are very high and steep. This means that the adiabatic heating effect usually warms the Santa Ana's maximum air temperature from a max of about -5c on the High Sierra Nevada Mountain slopes to a max of at least +30c in all seasons by the time the Santa Ana wind reaches Los Angeles, California.

In fact, LA recorded temperatures in the 90s Fahrenheit in early November with quite strong Santa Ana winds. I do recall hearing of coolish Santa Ana winds later in November, but I think this was a relative term and was around the same time that Seattle was experiencing a very cold blast so I guess there was some cold air higher up.