Wx Watch: Air Blocks

Why some highs are bad news

August 1, 2003

Low pressure bad. High pressure good. Right? Well, not always. Now that we're deep into summer's core heat, it's time to understand that high pressure systems aren't always benign. This is particularly true of a few common types of high pressure that assert themselves this time of year. These are the blocking highs, so called because their size and dynamics prevent the usual west-to-east movement of lows and fronts. The result? Atmospheric stagnation and day after day of reduced visibilities in haze and smog.

Think of high pressure as a large dome of cold, sinking air. As this air descends, it warms as it's compressed by the increased atmospheric pressure in the lower levels of the atmosphere. This warming is responsible for drying out the air and creating the sunny, comparatively cloud-free conditions we all associate with highs. Meanwhile, another sort of warming is generated by the Earth's surface, as those sunny skies let the terrain bake all day and radiate that heat skyward.

Now we have warm air rising from the ground meeting the sinking air from the high. This meeting place can be seen quite distinctly as the top of a well-defined layer of haze. Often, pilot reports will mention the top of the haze layer, and it's nice to know this information — because visibilities in fog, haze, smoke, smog, or smaze (a smoke and haze combination) can drop to IFR levels in that brownish murk down low.

Below that haze and murk layer, there's typically a temperature inversion, meaning that temperatures rise with altitude. This is a stable condition because any heated air parcels attempting to rise don't get very far. Hot air rises, true, but only if the air surrounding it is colder. The inversion traps the hot air below it, caps all the murk below this layer, and explains why highs can cause such lousy flight- and slant-range visibilities. The view straight down may be so-so, but the view ahead — through all those trapped particles — is often like that in a cloud.

When a blocking high sets in, the temperatures, haze, and inversion layers rise — and the visibilities go down — with each passing day. And a truly huge blocking high can last for two weeks.

There are three main types of blocking highs:

The Bermuda High. Everybody's heard of this. This happens when a center of high pressure anchors itself near Bermuda, then sits there, spinning hot, humid air up the eastern half of the United States via the high's counterclockwise wind flow. This gives us south winds from the "back side" of the high — the westernmost half of the system.

The Omega Block. This type of high gets its name from its distinctive shape, as seen on 500-millibar constant pressure charts; this roughly corresponds to the 18,000-foot level, which is a reliable depicter of upper-air dynamics. The shape looks like the Greek letter omega ( ), and the wind flow around this system enters from the west, then rides the high's margins up and around the northern extension of the "omega."

The Rex Block. This high resembles a large ridge, or a wedge, between two 500-millibar lows. It's named after meteorologist Daniel F. Rex, who identified the phenomenon in 1950. Rex Block airflows run north-south (except for the sharp clockwise bends at the crest of the ridge), and it's this mechanism that prevents the westernmost low from overpowering the ridge and replacing the eastern low.

When any of these blocks take hold, thunderstorms can form at their edges. What usually happens is that the moist, unstable, southerly flows around the back sides of the highs set the stage for the storms, which then make clockwise-moving tracks around their peripheries (again, as depicted on the 500-millibar constant pressure chart). One day storm cells may pop up at the eight o'clock position, the next day they move to the 11 o'clock location, and so on as the storms make their way completely around the block. In some cases the unstable air at the fringes of a blocking high can create a simultaneous arc of thunderstorms in the afternoon hours. Meteorologists call this phenomenon the "ring of fire," because of the storms' circular tracks and patterns.

This is one more reason to sit up and take notice when forecasters call for extended periods of high pressure. Haze and high density altitudes are one thing. Stumbling out of a massive high into a line of thunderstorms is quite another.


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