When the atmosphere is upside down
During summer we climb mountains to cool off.
But other than serving as effective natural air-conditioners, mountains have a rather more sinister reputation when it comes to temperature.
Mountains are where you get frostbite and hypothermia.
Most generally this reputation is deserved.
Under what meterologists call a “normal temperature gradient” — and everyone else just calls “normal” — the air temperature drops as you gain elevation.
Typically the change ranges from about 3 to 5 degrees for every 1,000 feet of elevation.
Anyone who’s ever fled the Baker or Grande Ronde or Wallowa valley on a sweltering August afternoon for the comparatively comfortable air at Anthony Lakes or atop Mount Howard can attest to this phenomenon.
The difference in temperature between the valley and the peak often is 15 degrees or more.
But occasionally this situation gets flipped, as it were, upside down. The high country is balmy while we shiver in the valleys.
Which is just another way of saying “inverted.”
This is much more common in winter.
We’ve been subjected to week-long temperature inversions a few times over the past couple months
These have produced some interesting statistics.
One morning in late November, the temperature at the Baker City Airport, elevation 3,373, was 10 degrees.
At the same time, at the top of the chairlift at Anthony Lakes Ski Area — 4,600 feet higher — the temperature was 41.
The inversion in place this week isn’t so extreme, nor as widespread.
Neither the Grande Ronde nor the Wallowa valley has been affected as much as the Baker and North Powder valleys.
At 3 p.m. on Wednesday North Powder was still below freezing, at 27 degrees, and Baker City was barely above, at 34.
In the nearby Elkhorn Mountains, though, the temperature at a snow-measuring station near Bourne, 2,400 feet higher than Baker City, the temperature was 49 degrees.
The La Grande Airport, at 2,717 feet, was 46 degrees.
Yet Meacham, a thousand feet higher and usually colder than La Grande, was milder still, at 53 degrees.
A couple of factors probably contributed to La Grande avoiding the frigid air that has been entrenched in Baker and North Powder valleys, said Mike Vescio, a meteorologist at the National Weather Service office in Pendleton.
One is wind.
The Grande Ronde Valley, as anyone knows who has lived there long, can get pretty gusty.
But even a gentle breeze can thwart an inversion, Vescio said.
Wind, in effect, acts like a blender, keeping the atmosphere mixed and allowing the warmer air at higher altitudes to mix with the cooler air lower down, he said.
The second factor is snow — there isn’t much in the Grande Ronde Valley, but both North Powder and Baker valleys still have snow cover.
Snow contributes to, and exacerbates, inversions because the white surface reflects the sun’s heat, keeping the air close to the ground cooler than it would be without snow.
The Wallowa Valley’s story is different, Vescio said.
The key there is elevation. At around 4,000 feet, the valley is in effect above the inversion, he said.
The temperature at the Joseph Airport warmed to 46 degrees on Wednesday afternoon.
The ingredients for an inversion come together most often in the winter, when the region’s extreme topography serves as the perfect kitchen, as it were.
High pressure is the main culprit, Vescio said.
What that means, basically, is that the air high in the atmosphere is sinking. And when air sinks it warms.
But high pressure, at least soon after its arrival, also usually means a scarcity, or altogether absence, of clouds.
Clouds act rather like blankets, holding warm air near the ground. Without clouds this warmth, as warm air is wont to do, rises, leaving a relatively shallow layer of cold air (shallow, in this case, can mean a few hundred to several hundred feet) next to the ground.
Snow, as was mentioned, accentuates this effect. On clear, calm nights, even an inch of snow can cause temperatures to drop 15 or 20 degrees farther than in an area without snow cover.
Once the high pressure is in place — what meteorologists often call a “ridge” — the relatively warm air is an effective ceiling, preventing the cold air below from rising.
Cold air trapped in valleys isn’t the only hallmark of inversions.
Fog is common in valleys during inversions because the cold air can’t hold as much moisture as warmer air, and some of that moisture precipitates into fog.
The ceiling of warm air also prevents smoke and other pollutants from escaping. The air quality in valleys stuck in inversions tends to degrade over time.
Baker City, which usually has about 20 to 25 days per year with air quality in the “moderate” category, has had more than 50 such days since Nov. 1, 2013.
La Grande has had five days in the “good” category in the past month. The rest were either “moderate” (20 days) or “unhealthy for sensitive groups” (five days).
Enterprise has had 11 days in the “moderate” category, the rest in the “good” range.