As pilots, we don't need any scientific proof that the atmosphere is undergoing big changes. Every time we fly near an urban or industrial complex, evidence of environmental stress is all around. It's the dense, brownish or grayish haze that seems to be a semipermanent feature of summertime flying life at lower altitudes. As a restriction to visibility, haze is the most prevalent problem we face in the summer months. It can persist for days, producing instrument meteorological conditions over a vast area. Even when haze drops visibilities to marginal VFR (3 to 5 statute miles) status, it can be bad enough to prevent you from seeing that rainshower or thunderstorm that's just a few minutes ahead.
To put it simply, haze is caused by stagnant air — air with no place to go. This stagnation is most often caused by one or more of the following: an inversion, a warm front, or a blockage of an air mass. All of these mechanisms are slow-movement phenomena, which helps explain why haze can last for so many days.
Inversions do their work by providing warm air aloft — warm, that is, relative to the temperature of the air in lower altitudes. This is not the "usual" state of things. Ordinarily, temperatures are supposed to drop with altitude, a fact that was drummed into our heads in ground school, in terms of lapse rates.
In the summer months, this usual state of affairs sets up conditions where surface heating causes air near the surface to rise. As it rises, it encounters progressively colder air. This, in turn, makes the warm, rising parcel of air ever warmer — compared to the surrounding air mass — as it goes higher. And higher and higher, growing into a thunderstorm, perhaps, because this is a good example of how an unstable atmosphere feeds the convective potential energy of a small wad of hot air. In a matter of minutes, that hot air can condense its moisture and turn into a cumulus cloud, grow to become a giant towering cumulus, and then transmogrify into a full-blown thunderstorm. The point here is to show that this is a condition that allows for the rapid rise of air.
Inversions discourage convection by preventing any vertical air motions. The typical temperature lapse rate is switched around — inverted — so any heated, rising air beneath the inversion layer is quickly stopped the moment it encounters the warm air aloft. That's because the surrounding air is close to the temperature of any air rising from below. True, hot air rises, but it'll only rise if it meets colder air on the way up. With an inversion, that doesn't happen.
Because inversions are basically stable situations, there's no way for the air trapped below them to escape. As the days pass, particulate matter, pollutants, and water vapor build up, and visibility goes down.
Large, slow-moving high pressure systems are great for producing inversions. These are masses of comparatively heavy, descending air. This subsiding air mass does two things: It becomes warmer as it descends (if rising air cools, descending air increases in temperature), and it meets the air at lower altitudes, air that's been lingering near the surface.
This meeting point is easily visible as the top of a haze layer. Climb high enough and there, flat as a board, is the murky line that's proof positive of an inversion at work.
Additional proof comes with checking the outside air temperature gauge. Note the temperature just before takeoff, then look for a small jump as the airplane passes through the top of the inversion/haze layer. This jump is most pronounced during the early morning hours, after the earth has had a chance to cool overnight. As the day progresses, surface heating can push the inversion/haze layer higher. After a few days, the top of the layer can reach as high as 10,000 feet or more.
Warm fronts supply warm air aloft because a wide band of warm air slides over the colder air ahead of the frontal boundary. Often, however, haze isn't a persistent problem with this setup. Instead, rainshowers and precipitation fog tend to form as the two different air masses mix in the unstable boundary layer.
Pilots in the eastern half of the nation should know that a strong Bermuda high can also make for extended periods of hazy, low-altitude weather. The Bermuda high is a large, semipermanent high pressure area centered over the southwestern North Atlantic ocean. The air circulating off the back (western) side of the Bermuda high shunts hot, moist air from the Gulf of Mexico to America's interior. When the dog days of August beset the eastern United States, you'll probably have the Bermuda high to thank for those muggy, hazy week-long stretches.
The duration and intensity of the dog-days syndrome is proportional to the strength of the Bermuda high. Strength, in this case, is defined in terms of the heaviness of the air (as reflected in high barometer/altimeter readings) and the size of the high.
By definition, the Bermuda high is large. But its size waxes and wanes according to arcane laws of atmospheric physics. Sometimes it stays offshore and permits air-cleansing cold fronts to pass through the eastern United States. At other times, it swells in size, becoming large enough to extend as far west as the plains states. When it's that big and has a strong pressure value, it blocks any frontal progressions. Until it weakens, the weather under the Bermuda high doesn't change. Lows and fronts may sluggishly move around the periphery of the high, but that's about the extent of any nearby active weather systems. Meanwhile, haze and rotten visibility persist beneath the high.
Eventually, however, things change. Given a weakening Bermuda high and/or a strong cold front moving through the central United States, the zone of high pressure can be moved eastward. Frontal passages are likely to be violent affairs, as the colder, drier air pushes the haze and mugginess away. Expect thunderstorms to reach severe levels, and expect them to form in lines and clusters ahead of — and within — the frontal boundary.
This all goes to prove that high pressure isn't always a pilot's best friend. When it's producing an inversion and sits around for days, it'll ruin an otherwise perfectly good flying day. And when it leaves, it leaves with a bang. All the more reason to stay in touch with daily weather trends and to keep an eye on the sky after an extended period of hazy, humid conditions.
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