Rocky Mountain High Means Hot California Wind

Really. Glen Campbell sang, at least metaphorically, about Rocky Mountain Highs; but real high pressure weather systems in Colorado can produce hot conditions in Southern California known as Santa Ann winds.

While the prevailing winds in California come from the west off the cool Pacific ocean, the Santa Ana blows from the east and comes from the high, dry desert plateaus of the mountain ranges east of Los Angeles and San Diego. More than just a dry mountain breeze; a Santa Ana is a hot, powerful wind that can quickly turn small brush fires into unstoppable infernos.

The Santa Ana is a special type of wind, known as a katabatic wind, well known in other parts of the world. In parts of the American West, this wind is called the Chinook; in southern France it is called the Mistral; in Japan it's the Oroshi; and in Austria and Germany it is called the Foehn.

Here's how the wind is created (it's not from Glen Campbell's singing).

First, air tends to pool at high elevations, where it cools. As winds pass over the top of the mountains, water-like eddies form and some air breaks away from the main wind stream, where it pools and cools.

Second, a weather condition that will cause this cooled air to spill down from its high-elevation position is required—typically a high pressure in the northern Rocky Mountains. The clockwise air circulation of this high pressure system forces the winds downward from the high plateau. Once the air is dislodged from its high elevation, it sinks rapidly since it is cooler—and more dense—than the surrounding air below.

As it sinks, the third factor cones into effect: air pressure increases, which compresses the cool, dry air. As it's compressed (it's called adiabatic compression), the air temperature increases at a rate of 29°F per 5000 feet of elevation).

Now the fourth factor comes into play. As the air warms it tries to expand. often, though, the downslope winds are foreced between mountains that form natural venturis and force the winds down through valleys, increasing the wind speed.

What began as cool, dry air pulled down by gravity becomes hot dry wind propelled by the increasing pressure of the air as it heats up, the fifth and final factor.

These winds can commonly reach speeds of 25-35 miles per hour, often exceed 60 miles per hour, and has been known to reach 200. Hot, dry winds traveling at such speeds are enough to whip common brush fires into intense, region-wide fires that can scorch thosuands of acres of Southern California.


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