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 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.

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. 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.

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