High pressure’s tricks

You look at the surface analysis chart and see that indeed, there’s a big “H” parked over a huge section of the country. There’s no mention of airmets or convective sigmets, so you make the “go” decision to fly.

Yes, big highs usually mean fair weather, but there are some qualifiers. They mostly have to do with the nature and direction of the wind circulation around areas dominated by high pressure. We all learned that wind blows in a clockwise direction around highs (in the northern hemisphere, that is), and this is the key to being savvy about the weather they can make. The location and movement of a high-pressure system is also important. Sometimes you hear people say that there’s “no weather” when high pressure’s around. Not so!

Weather systems mainly move from west to east, so think about a big high moving east. At its leading edge—or “front side”—winds blow from the north or northwest. Wind and weather are reflections of their origins, so this means they are affected by conditions to the north. In this case, colder temperatures, lower dew points and humidity, and strong winds are the first signs that strong high pressure is moving in. Expect strong winds out of the northerly points of the compass, and a fair amount of turbulence. This is especially true if the high is following a cold front ahead of it. There have been warmer temperatures ahead of cold front, and the warmed ground it created will generate rising air with the arrival of a high’s cold winds. That, and the shear from a change in wind direction after frontal passage often means a bumpy flight—and perhaps a workout during takeoffs and landings as you correct for crosswinds and airspeed fluctuations.

When the center of high pressure moves overhead, expect little or no wind at the surface. Remember, high pressure consists of sinking air, so there is usually less chance of turbulence or cloud buildups. Clear skies are often the rule—especially at night when surface heating drops. Temperatures may drop low enough to match the dew point and create visibility-restricting fog in the early morning hours. During the day, the high’s descending air can create a temperature inversion that can trap air near the surface, causing it to stagnate, and trap haze, smog, and smoke.

After two days or so, the high will have moved east. Now we’re affected by the “back side” of the high, and the clockwise flow brings in warm, moist air out of southerly directions. Many times it comes in the form of a warm front, as the warmer air rides up and over the colder, higher-pressure air it replaces.

The back side of the high can set the stage for the entry of low-pressure systems from the west. Their attendant warm fronts bring widespread cloud cover as soggy air from the Gulf of Mexico is drawn northward. As the ground warms under this influence, cloud buildups form to create thunderstorms. Some systems can cover territory across several states, making diversions difficult or impossible for VFR operations. The north-south alignment of most of these cloud and storm systems means that flights east or west can be impacted. Better to make a 180-degree turn before going there.

Once the warm front and its clouds move east, colder air follows as the typically inevitable cold front passes, winds switch around from the north, and cold air arrives. The next high-pressure system makes its entry, and the cycle starts all over again.

The exception is the Bermuda high, which is a quasi-stationary high-pressure system that parks itself over the Atlantic, off the United States’ east coast. Get ready to see its back side clouds and storminess regularly affect the weather east of the Mississippi for most of the summer. Still think high pressure is no big deal?

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