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Mountain Breezes
Wed Jun 23, 2004
Listen in RealAudio 
Mountain Breezes, flowing downslope on The Weather Notebook.
Hi, I’m Bryan Yeaton for The Weather Notebook. If you hang around mountains long enough, you will experience refreshingly cool nocturnal breezes sliding downslope after a hot day, or frosty winds rushing down at night. Such breezes are produced by vertical temperature gradients over the local terrain.
Mountains react with overflowing wind in many ways as air is forced to go over ridges and through gaps. But, mountainous terrain also generates its own wind flows when higher elevations heat or cool more rapidly than regions below.
The most common mountain wind arises after the sun’s heat has waned. Without that heat, air around the summits cools rapidly. Compared to the air below, this colder air is denser, and therefore, heavier. It rushes down the slopes pushed by the newly established pressure gradient and pulled by gravity.
Such winds are called “katabatic” winds. These cool flows bring refreshing coolness to lower elevations during summer, but in other seasons, the chilly air flooding downslope may pool in valleys or low spots, often producing fog or frost.
When day breaks, the highest reaches of mountain terrain receive sunlight well before the lowest slopes and valleys. If the heating is strong and regional winds are low, the heated air rises rapidly from the upper slopes forming localized areas of low pressure at the higher elevations.
The resulting pressure gradient along the slope draws cooler air up from lower elevations producing an upslope wind. Upslope winds are also known as valley breezes, or more generally, “anabatic” winds. When the air drawn upslope reaches its condensation point, clouds appear around or over the summit.
Thanks to our contributing writer, meteorologist Keith Heidorn. You can find our past Weather Notebook shows online at www.weathernotebook.org. Our show is made possible through the support of The National Science Foundation, and Subaru of America.
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