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.
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.”