Mountain Effects on Climate

 

Rain Shadow Effect - Revisited

 

As recounted here and in other recent posts, the Rain Shadow Effect is the primary Regional "cause" of the four North American Deserts with the Mid-Latitude High-Pressure Zone being a secondary cause.

While the Rain Shadow Effect is a Regional Climate Effect in the case of the Western United States, on a smaller scale, it can be a temporary, local Weather Effect (paired with Orographic Lifting) in a direction "contrary" to the regional (Prevailing) trend.

In regard to the Organ Mountains in Southern New Mexico and the Franklin Mountains in extreme West Texas (part of the same Basin and Range Fault Block), the prevailing winds are generally from West to East.  But on rare occasions, Weather Fronts from the Northeast and East do happen in the region. 

Downtown El Paso is about 3,700 feet above Mean Sea Level and it often gets below freezing, but it is usually dry, so to get a rare "good snow", the moisture has to come from one direction, while the frigid air comes from another.

As with early 1985 (I think), a Cold Front arrived into the El Paso area from the Northeast and due to Orographic Lifting, the east side of the mountains received a coating of ice on the streets and 2 - 3 inches of snow.  Normally, that small amount of snow would not be much of a local travel impediment, but when it overlays a coating of ice, that's a different story.

While the eastern slopes were getting the ice and snow, downtown El Paso and the western slopes were just getting cold rain.  

Orographic Lifting and the Rain Shadow Effect can happen in wetter parts of the country where there is sufficient Topographic Relief.  The mountains of Rabun County, in the northeastern corner of the state, is the wettest part of Georgia, averaging about 70 inches of rain per year.  In contrast, the Augusta area, on the edge of the Coastal Plain gets about 40 inches of rain per year.  

Chihuahuan Desert Rain Shadow Effect

 

Not to over-dwell upon this subject (with this and the two following videos), but the Rain Shadow Effect is the primary "cause" of the four North American Temperate Deserts, the Mojave, the Great Basin, the Sonoran, and the largest, the Chihuahuan Desert.  

The Rain Shadow Effect is the "2nd half" of the Orographic Effect.  The first "half" occurs when prevailing air currents bring moist air masses into contact with linear mountain ranges.  Over the long-term climate effects, due to our Jet Stream position, usually westerly wind currents bring moist air masses in from the Pacific Ocean.  As the moist air is uplifted, it chills, condenses, and brings precipitation to the Windward side of the mountain ranges.

Once the air currents "top" the mountain range(s), as it starts to fall in altitude (down the Leeward side of the mountain ranges), it expands, warms, and dries.  Due to the mountainous areas west of the Rockies Front Range and west of the eastern-most parts of the Basin and Range Province, the Rain Shadow Effect is the primary reason for the prevailing dryness of the "American West".

A secondary "cause" of the four North American Temperate Deserts is the "Mid-Latitude High-Pressure Zone", related to vertical atmospheric cells, called the "Ferrel Cell", in which persistent Atmospheric High-Pressure cells (and associated falling winds) tend to suppress cloud growth.

What are Foehn Winds?

Foehn Winds (or Föhn winds) are locally-strong examples of cross-mountain winds (in mountainous areas) that cause warming and drying of descending air on the lee (leeward)  side of the mountain.  In some areas, there are local names, such as "Chinook Winds", for these winds that are a component of the Rain Shadow Effect.

In this particular post, there is a video on the types of rainfall and within the video, the Rain Shadow Effect is explained at approximately 5:19 in the video.   

In the linear mountain ranges of the North American West Coast (including Canada and Alaska), when the prevailing winds bring Pacific moist air inland, the Orographic Effect occurs as the air moves upslope and chills along the western mountain slopes, triggering precipitation.

Once the winds cross the ridgelines and start to descend the eastern, leeward slopes, the winds become warmer, drier, and faster (due to gravity), producing the Rain Shadow Effect and when particularly strong, the Foehn (Chinook) Winds.

There are times when temporary conditions "reverse" these prevailing directions.  When the sub-tropical Northern Jet Stream is in a "non-wavy" shape, inland High-Pressure Systems "stalled" in the warm Western Interior produce winds that blow westward towards the Pacific Ocean.

As these winds blow to the west and southwest, when they descend the western slopes of the Sierra Nevada and the Pacific Coast Ranges, they "reverse" the "normal" West-to-East Rain Shadow Effect.  When these winds are funneled through mountain passes, their speeds further increase due to the "Venturi Effect".  These are the yearly "Santa Ana winds" or "El Diablo winds" that dry the seasonal vegetation and help spread wildfires.

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For several reasons, what happens in the Pacific Ocean has influences on North and South America and the Atlantic Ocean that we probably don't yet fully understand.

Among these:

1). Oceans store a great deal of heat (and release it gradually).

2). The Pacific Ocean is the largest of the Earth's oceans.

3).  Do we fully know the effects of the submarine volcanoes and rift zones in the Pacific Ocean, vis-à-vis El Niño/La Niña?  Probably not.

4). All of the Jet Streams (Northern and Southern) "blow" from West to East.

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