When High Pressure Isn’t Always Good

Courtesy: blog.minitab.com

All pilots are amateur meteorologists out of necessity. And one weather fact we think we know is that high pressure systems make for good flying weather. And that’s generally true – but not always at this time of the year.

As you remember from private pilot ground school, high pressure systems typically clear out the atmosphere, bringing good visibility and generally clear skies. The soggy stable air of low pressure systems can collect lots of moisture that leads to widespread clouds and reduced visibility. Lows usually spawn fronts that add their own mix of flying weather challenges. 

But in the winter and early spring high pressure systems can be so strong that they can spontaneously generate flying weather problems that are very difficult to forecast.

The hallmarks of high pressure systems are generally low moisture levels and unstable air. Unstable air means the air cools with altitude at a pretty steady rate, while under low pressure systems the air may actually be warmer aloft than at the surface with that temperature inversion holding clouds and low visibility close to the ground.

However, winter highs can be so powerful that the rapidly rising and cooling air they contain can squeeze what little moisture exists into clouds. Such small buildups are sometimes called “instability showers.” These clouds can form just about anywhere without the need of a front to get them growing. And that makes them particularly difficult to forecast.

When it’s cold at the surface the scattered to broken instability clouds typically create snow showers that can be very threatening to pilots flying VFR. The high pressure air gets the vertical development of a cloud really moving almost like a mini thunderstorm, except the moisture is frozen in the form of snow. The cloud sucks in all of the moisture around so the snow rates can be very heavy right under the cloud. And significant snow knocks flight visibility down to almost zero. You can see the ground below you in the snow, but you can’t see ahead to find a horizon.

The best forecasters can do is note that there is a “chance” of broken clouds or snow showers but they can’t be certain where, or when during the forecast period, the instability clouds and snow showers may form.

In this recent weather snapshot, Jason Warren, a weather enthusiast and Trained Severe Weather Observer for the National Weather Service, predicted that cold air will sweep into northwestern Pennsylvania and use instability spurred on by Lake Erie to produce snow showers over the area east of the lake. Courtesy: Jason Warren

Instability snow showers can be fed by a significant body of water such as a Great Lake or coastline supplying moisture. But the clouds may form some distance from the water body source. And the clouds also tend to cluster in areas or along lines, making it potentially very difficult for a VFR pilot to find a way around them.

Some of the biggest weather scares I have ever had were in those pop-up instability snow showers when I was flying VFR only in my Cessna 140 many years ago. The airplane had only an ancient turn and bank instrument to help me know up from down when I blundered into a snow shower. And often the snow under the cloud doesn’t look from a few miles away like any big deal – until you fly into it. But once in the snow – clear of the cloud but in the snow – the visibility can vanish in an instant.

I don’t know how to tell you to avoid all instability showers, except to say that if it is very cold and the altimeter setting is high, beware of all puffy little clouds and don’t fly under them or you will learn a new meaning to the term “white out.”

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8 Responses to When High Pressure Isn’t Always Good

  1. CHARLES LLOYD says:

    This quote from the article confuses me.
    “However, winter highs can be so powerful that the rapidly rising and cooling air they contain can squeeze what little moisture exists into clouds. Such small buildups are sometimes called “instability showers.” These clouds can form just about anywhere without the need of a front to get them growing. And that makes them particularly difficult to forecast.”

    My understanding about highs is that the core air is falling. Does the cool air out of the high core then cool the moist air below and create the snow shower?

    I enjoy your blog and “Sport Pilot” articles. Keep’em coming.


    • Mac says:

      High pressure air is typically unstable which means the air cools at a steady rate with increase in altitude. That means warm air will try to rise into the cooler air above and if there is any moisture present the heat in the moisture will increase the velocity of the rising air. That’s how instability clouds form.

      In a more general way, high pressure air is more dense than low pressure so when a high fills in behind a low the overall air mass does sink so maybe that’s what you are thinking of.

      Mac Mc

      • Charles Lloyd - Goddard, KS says:

        Thanks Mac, that is exactly what had me going down the wrong track.

        Fly Safe,

      • To add a bit of confusion: <:-))
        Meteorology: In a high the air descends and in a low it rises. The Coriolis effect turns the out flowing winds clockwise around the high. The opposite is true for a low. Air rises and the winds are turned counterclockwise.

        Now that does not mean all of the air in a low goes up nor down in a high. Both can have severe turbulence. In meteorology this does not translate to "unstable" air.

        Hot, calm, most air that is capped might provide a stable flight, but it is classed as unstable air because of the energy trapped and what it will do if it breaks through that cap (lifting coefficient) . Cold windy air associated with a strong high may provide a reason for Dramamine and a few extra "lunch bags" with a very unstable ride, but the air system is generally considered stable to the weather man, but not most pilots.

        I've lived in Michigan all my life so those little and sometimes intense snow storms are a fact of life. Many pilots don't pay much attention to them if the visibility is good enough to be considered marginal VFR whether you can see the horizon or not, at least if it's cold enough for the snow to be what we call "dry snow". IE it doesn't stick to anything.

        Wet snow, when it's just below freezing is something to stay away from. Oh so many years ago my instructors took me through a lot of that as a primary student.

        Often even major snow storms follow the same pattern. After one goes through, look at the patterns on the ground. They look like some one took a paint brush and just slopped white paint on the ground. You may see a white strip a few miles wide that may have a foot or two of snow with bare ground on either side. Those are true IMC inside when they go through.
        They are also typically, "Lake Effect" storms and may cross the entire lower peninsula with those white stripes. Winds in these can top 50 to 60 mph at times. These come from moisture picked up from the great lakes where the water is warmer than the air and can provide substantial snow and even blizzard conditions.

        OTOH lots of isolated showers are things to go around due to the down drafts you are likely to find under them. If you see a few clouds with rain shafts under them, it's probably wise not to go get the plane "washed" as some of those can provide a surprising ride with more excitement than you might want with downdrafts you can not out climb. You might still see a rate of descent past 2000 fpm at full power and the nose up.

        The wise thing would be to avoid small storms, be they rain or snow. Severe down drafts, icing, having a boat load of snow stick to the plane and then freeze. These all can ruin your day or add far more excitement than you bargained for. After all they are just a little storm you can see through. Don't bet your life on them being benign.

  2. Dennis Koehl says:

    Hi Mac,

    Out here in the west, high pressure causes problems for a different reason in the winter. In the mountain west, high pressure will cause inversions, the temperature in Truckee (6300 feet) will be significantly warmer than in Reno (4700 feet). The inversion serves as a “cap” on the atmosphere, and what should be a CAVU winter day can be really muddy with limited visibility.

    Once a front approaches and the breeze kicks up to stir the atmosphere, the inversion eases and visibility improves.

  3. Ron says:

    I fly in New England and have seen this alot. They can often be flown over if not over the mountains. A better way to avoid them is to have in cockpit xm radar. Also if you do get into the snow showers get on the guages and keep flying , the snow does not stick so you wont accumulate ice.

  4. Pat says:

    Have to agree with rodger, been flying in Michigan a long time. Ron is truckee the same as Donner pass? I just lost my apetite. Great blog thanks Mac!

  5. Joe Smith says:

    One more reason to have, and be current with, your instrument rating. The good news about such snow showers is that they most often contain snow on the “dry” side — that is, that plays heck with vis, but doesn’t really want to stick to anything moving. Back when I lived in Eastern Oregon I flew better than once around the world between home and Portland, and encountered dry snow many times. It did indeed erase the horizon — but didn’t add a pound to the airplane.
    One caveat: if you fly a Mooney, make sure you close the ram air.

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