Wx Watch: A Fright-Free Fall

Most of the time, high pressure prevails in autumn

September 1, 2006

Weather educators are forever scaring us with doomsday scenarios, and at times I've been one of them. But let's keep things in perspective. The fact is, most times the weather is relatively benign, and conducive to VFR flying. Very often, cloud layers stay above 3,000 feet agl, visibilities will extend beyond the magic six-mile limit that aviation forecasters use to define "unlimited" visibilities, turbulence doesn't reach severe levels, and surface winds stay below 15 mph.

How do I know? I once wrote a book on aviation weather — Flying America's Weather — that used a climatological approach to America's regional weather patterns, and the accidents that happen within them. I scoured climatological atlases and other data as part of the research, and I learned that many regions experienced instrument meteorological conditions (ceilings below 1,000 feet, visibilities below three statute miles) as little as 10 percent of the time (say, three days a month) in large areas of the western United States, and no more than 20 percent of the time (six days per month) in the rest of the nation. And that's in the winter months, when weather conditions are the worst, statistically speaking.

Of course, "benign" is in the eye of the beholder. A neophyte pilot without an instrument rating may feel comfortable only when skies are crystal clear and surface winds are dead calm. Those with 500 or so hours, an instrument rating, and time in the clouds will probably feel confident enough to take on conditions where ceilings and visibilities are at the 1,000-and-three limit, and won't be scared off by gusty surface winds or forecasts of moderate turbulence.

At the other end of the spectrum is the high-time, seasoned instrument pilot who regularly flies on instruments — in the soup, that is — and who feels competent enough to fly in and out of airports in instrument conditions — as long as there are no unnavigable thunderstorms or icing conditions along the route.

But back to the premise. Instrument conditions are, in point of fact, rare. Feel uncomfortable about the weather?

Then simply wait a day or two, and good VFR will be yours.

You know what to watch for: large areas of high pressure. These are heavily advertised days in advance of their arrival, and their forecasts are very accurate, too. The Weather Channel will sound their approach, and so will low-level prognosis charts. The good thing about big highs is their duration. They can linger for days, giving you plenty of chances for long cross-country flights unimpaired by anything but perhaps a scattered layer of shallow cumulus clouds at approximately the 4,000-foot level.

But all is not roses with high-pressure systems. Their worst weather typically comes at the arrival and departure stages. As a high moves in, it pushes colder air ahead of it, on westerly or northwesterly winds. This air runs into the warmer, more humid air masses ahead. Presto! A cold front occurs, along with the chance of thunderstorms and/or low clouds that they create.

As highs move away, the clockwise circulation around their centers draws warm, moisture-laden air northward on winds out of the south. This is what meteorologists call the "back side" of a high-pressure system. And with the back side of a high, the stage is once again set for showers and thunderstorms.

The best weather is located in the center of the high, the spot where meteorologists place a big H on a surface weather chart. Here is where you'll find the calmest winds — aloft and at the surface — the clearest skies, and the best visibilities.

Sure, morning fog can occur on cool mornings when a high is parked overhead. And if surface temperatures go high enough during the day, and conditions aloft are cold enough, air-mass thunderstorms can form. But for the most part, highs are our steadfast friends. This is especially true in the fall months, when, on average, two or three big highs move through per month.

Let's give our weather paranoia some time off. Wait for those ideal fall flying days, when cool temperatures make our aircraft perform better, we can have unrestricted views, and our passengers can have stress-free flights that boost the spirit.

E-mail the author at tom.horne@aopa.org.

Links to additional information about high-pressure systems may be found on AOPA Online.