High Pressure Low Pressure

You’ve been researching an area’s weather and climate too long . . . when you can reconstruct the weather systems (highs, lows, frontal passage) by reading the complaints about wind and lack there of in ranch records from the late 1800s early 1900s.

High pressure, low presure, or fixin' to blow up a storm. Photo by Michael Lapoint.

High pressure, low pressure, or fixin’ to blow up a storm. Photo by Michael Lapoint.

No one in the Panhandle likes calm days, except perhaps 1) guys repairing power lines while standing in lift-buckets and 2) aerial applicators. Usually calm wind means that high pressure has settled over the area and it is going to be hot and dry and miserable, or bitter cold and dry and miserable. Unless we are under the center of a low-pressure system, and we are in the eye of the storm. Or it means you’d better be tying down garbage can lids, bringing in laundry, and checking the leashes on small dogs and yappy children (or vice versa) because the wind is about to change directions with a vengeance.

I suspect a few of my readers are not quite as familiar with weather drama as those of us who either deal with outdoor living (professional, avocational, or general curiosity) or who live in variable climes. If you are from Seattle, Portland, San Francisco, Naples Italy, or other places with more standardized weather, the “clash of the air masses” and “polar vortex” are probably things you see on the SyFy channel listing, roll your eyes and change the station. Or you see the blue and red lines on the map, note the local forecast, and go on with normal life. Lucky. 😛

Low and high pressure systems are what cause the larger weather patterns. They are pockets of rising or sinking air, sometimes warm, sometimes cool, sometimes very hot or cold, and are shoved around by the upper level (jet stream-type) winds. High pressure means stable air, air that is sinking from higher in the atmosphere toward the surface and flowing away from the center of pressure. In summer in the High Plains, the persistent high pressure ridge that settled over the area was called the Mexican Plume or just “the lid.” You can guess why the first name is no longer used, in part because it’s not entirely accurate. Yes, we do get southwesterly wind, sometimes, depending on where the center of highest pressure is located, but not always. High pressure is stagnant. I’ve flown into Amarillo from Dallas or Denver when high pressure had settled in. Amarillo was 110 degrees and you could see the top of the air mass as a thick brown line in the sky. Blue above, brown below, full of trapped dust and smoke and smog and pollen and yeah.

The air does have that brown-red tinge to it when seen from 35,000 feet.

The air does have that brown-red tinge to it when seen from 35,000 feet.

It acts like a lid, keeping things in and down. We get little summer puffy clouds but no thunderstorms unless . . . unless it gets so hot that the atmosphere under that lid is unstable enough to punch through into the much colder air above. Then you grab the kids, the dog, and dive for cover, because the storm is going to be a doozie. But there is little to no wind, much of the time. The air lies heavy, calm, and stagnant. Windmills don’t even creak. The cicadas and grasshoppers sound tired. The horses and cattle stand, heads down, half-ways making a token effort at grazing, and humans loaf along.

Which way does it rotate? Depends on where you are relative to the equator.

Which way does it rotate? Depends on where you are relative to the equator. This illustration is from an Australian website.

Low pressure is a gigantic vacuum cleaner, pulling air into it. Hurricanes are the most famous low-pressure systems, although the storms that form in the Gulf of Alaska are pretty impressive. Larger lows do the same thing, but not quite as dramatically or visibly unless you are looking at isobars and wind arrows on some weather maps, or know what to look for in the sky. Low pressure is unstable and tends to be stormy, windy, and more “interesting.” High pressure is “good weather” while lows usually bring “bad weather.” When you get a low, a cold front, and a sharp dry-line, it means “very interesting.”

The center of a low is much like the eye of a hurricane. The wind stops or tapers off into gentle whispers. The air may clear, or it might not. When it picks back up, it will be from a different direction, a bit like frontal passage but not quite as dramatic.

Frontal passage is when the wind stops abruptly and you start looking for shelter, at least if it is a cold front running after a warm front. I was down in Midland some years ago in early October, helping guide parking in a grass lot. The wind was out of the south and chilly, and persistent. Around eleven that morning or so, it went dead calm. I mean dead, nothing moving other than people. Even the little skittery lizards disappeared. I thought to myself “Ut oh.” Turned my head very slowly to look over my shoulder and saw a brown wall to the north. I had about enough time to think, “Oh sheet” before the dust and north wind slammed into me. Two hours later we had a nice northerly breeze, clearing skies and lovely brisk weather. And I’d gotten more than enough minerals for the day, thank you.

And people wonder why calm air makes me jumpy.

8 thoughts on “High Pressure Low Pressure

  1. A variation on the theme, is the Inversion. It’s a wintertime phenomenon in mountainous regions, when the jet stream wanders off after a cold snap, warm air seals the bitterly cold air in the valleys, high pressure sets in, and there you sit, until the next front comes through and clears it out.
    I’ve seen the accumulated smoke and exhaust get thick enough to resemble fog.

    • Wildland fires in mountain terrain are also affected by inversions. You can have a fire in an inversion; not much activity, though it’s usually too smoky for air support. Where it gets interesting is when the fire has built up enough heat to punch through the inversion. The air starts moving and the fire goes bonkers. Best to be way far away from the burning section at that time.

        • The first day (July 6th) of the Bootleg fire was the first time I saw a pyrocumulus cloud. The fire was reported as 100 acres at 1:40 that afternoon, but it looked considerably larger from the smoke column when I saw it coming home at 2:00. At that time it was about 10-12 miles from us with no indication which way it was going, beyond “outward”. We saw PC clouds every day for quite a while, along with a few of its evil-twin, the pyrocumulonimbus. Now the fire moved considerably east of us, so other communities get the brunt of the fire. Not a good place to be.

          It’s not growing much, but at 413,500 acres, it doesn’t have to grow to cause trouble. There were several days when the fire doubled in size. Burning in (among other things) beetle killed Ponderosa Pine and seriously drought-stressed forest gave it plenty to work on. I think it was laughing at the DC-10 VLAT. Most of the big guys are LATs, 737/DC-9 sized. First time I saw jet tankers working fire around here. The usual fires warrant a C-130 or a repurposed P-3 Orion for the big stuff, plus a fleet of helicopters with Bambi buckets.

  2. Ah yes, living on the dry line IS just such fun… As a retired aviator, it’s just automatic to check the weather so I know what is going to hit me, as I no longer have to worry about flying into/through it…

  3. Well, we have plenty of weather and wind up here in Ohio, and we keep an eye on the weather. But most of the time, the patterns are pretty consistent. When they’re not — that’s when people worry. And the valleys aren’t really deep enough to do the inversion thing. What’s unusual is for a day to have no wind — usually only for a short while before a thunderstorm. You get wind picking up, it goes still, more wind, and then the storm comes.

    Still a little bit cool of a summer, although we’ve had some pretty hot days. We’ve had some good rain and thunderboomers, but really not all that much. Shrug.

  4. Then there’s New England, where “if you don’t like the weather, wait five minutes” is close to being literally true. We can get weather systems coming in from basically any point on the compass. I used to live in southwestern Ohio before I moved here, and the difference in forecasting precision was staggering. In Ohio the weekly forecast has better than even chance of being accurate. In New England, I never trust the forecast more than 24 hours in advance.

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