Chinook Wind

A mountain man choosing a wintering spot wanted to pick someplace that had mild winters, or at least periodic warm spells throughout the winter.  He’d want to make a good choice for two reasons: first, a warm winter would allow him to get out of his lodge occasionally, wander around outside, and maybe kill some fresh meat to replace the dried meat he’d been eating for weeks.  Second, if he picked a bad spot, his buddies might make fun of him for years.  Captain Benjamin Bonneville, for instance, picked a bad spot to build what he envisioned as a year-round fort.  He sited it just west of modern Pinedale, Wyoming, where the winters are bitterly cold, and the fort – which he immodestly named Fort Bonneville – was abandoned the first year.  For the duration of Captain Bonneville’s stay in the west, the trappers called his construction “Fort Nonsense.”

I think Big Timber, Montana, the town just south of the cabin where I’m spending the winter, would have made a good spot.  It’s along the Yellowstone River, so there was plenty of water.  Game was likely plentiful, and there were plenty of cottonwood trees to supply bark on which horses could feed after snow covered the grass.  Best of all, throughout the winter, periodic warm Chinook winds sweep in from the northwest to warm up the valley.  A few weeks ago in this blog, I wrote about the cold.  For three or four days I don’t believe the thermometer topped 0°F.  At one point it hit -15°.  But two days ago, amid a hard west wind, the temperature rose to 40°.  Yesterday and today, it topped 50°.  John Cosgriff, whose family has ranched around Big Timber for generations, says that’s what he likes about the winters west of the Crazy Mountains – it gets cold, but every once in awhile nature cuts you a break.

Chinook winds come about this way.  When a west-moving mass of air hits the western edge of the Rockies, the mountains force the air up.  As the air moves up, it cools because the decreased atmospheric pressure at higher altitudes allows the air molecules to spread out.  This cooling of air as it rises is called “adiabatic cooling,” and it happens all the time.  But all airmasses have a temperature below which the moisture that’s locked into the air will precipitate, or condense into droplets and fall as precipitation.  The point below which moisture will precipitate in a given airmass is the airmass’s “dewpoint.”  So if west-moving air cools enough as it rises along the Rockies’ western slopes – i.e., if the adiabatic cooling is sufficient – to drop the air temperature below the dewpoint, then rain, sleet, or snow will fall.  The airmass continues to move west, but now, having dumped its moisture, the air is drier.  After the air crosses the Rockies, it descends again.  And as it descends, the atmospheric pressure on the airmasses increases, shoving the molecules closer together.  The air heats up.  This is called adiabatic heating.  And here’s what causes the Chinook wind: the rate of adiabatic temperature change is different for moist and dry air.  Moist air being forced upward cools at a rate of approximately 3.5°F every 1000 feet.  But dry air being forced downward warms at a faster rate, about 5.5°F per 1000 feet.  So when the dried-out airmass descends to the base of the Rockies on the eastern side, it becomes warmer than it was when, moisture-laden, it first climbed the mountains’ western slopes.  The dried-out, warmed-up airmass continues moving east and when it arrives in a town east of the Rockies, such as Big Timber, the townspeople call it a Chinook wind.

Two days ago, when the Chinook wind blew through.  The wind picks up around the microphone when I step from the lee side of the cabin.

I am a big fan of Chinook winds.  The warmer weather feels great; it makes me want to lace up my hiking boots and take to the hills.  Now I can go outside in just one jacket and can wear gloves instead of mittens.  If I were a cold-inured Montanan, I’d go outside in only a tee shirt.  But hey – I’m no mountain man.  I’m just writing about them.