What are high pressure systems and how do they contribute to our weather?

When the weather is dry, tranquil and nice, you can typically thank high pressure systems for keeping stormy and rainy weather at bay.

A high pressure system is essentially a clockwise flow of dry, sinking air that typically builds into a region behind a departing storm system. High pressure systems can be linked to the jet stream by finding areas where the jet bulges northward.

The jet stream is essentially an atmospheric river of air located at the level where jets cruise. Winds in the jet stream often reach 250 mph.

On the eastern side of a high pressure, winds coming out of the north generally drag chillier air southward from northern latitudes.

Winds blow out of the south on the western side of a high pressure and bring warmer air northward from more tropical climates.

The calmest conditions and sunniest skies are found near the center of a high pressure system, where air sinks most efficiently and warms as it does so.

Most frequently, high pressures move around the globe in a west-to-east manner. However, these systems can sometimes reverse course or “buckle” and stop over a region for as long as a couple of weeks.

It is when these systems become semi-permanent over southeastern Canada and/or Greenland during the winter months that the jet stream is forced southward into the central and eastern United States, as well as parts of Europe. This type of weather pattern can breed dangerous blizzards and lead to bitter cold snaps in both Europe and the eastern U.S.

“High pressure systems can help strengthen nor'easters by funneling moisture from the Atlantic Ocean into these storms," AccuWeather Senior Meteorologist Alex Sosnowski said.

"At other times, moisture can develop on the southern side of high pressure areas," he added. "During the winter, this moisture can help produce bands of heavy snow downwind of the Great Lakes."

Another example of when high pressures can spoil the weather occurs when fog forms and becomes trapped in valley locations for days at a time.

"Light winds and cold, moist air trapped near the ground lead to this persistent fog, which is common in the broad, extensive valleys of the western United States in the winter," according to Sosnowski.

Sosnowski mentioned that fog created in the same manner may take much of the day to burn off in the deep valleys of the Appalachians during autumn, especially in the months of September and October.

In the summer months, high pressure systems become better established and semi-permanent over the central Atlantic and Pacific oceans.

The high over the Atlantic, deemed the “Bermuda High” by meteorologists, can lead to dangerous, long-duration heat waves in the Interstate 95 corridor, from Boston to Washington, D.C., when it becomes abnormally strong or shifts closer to the Eastern Seaboard.

Two heat waves, one in August of 2001 and the other in July of 2011, that shattered records in the eastern U.S. were a direct result of an abnormally strong Bermuda High. Newark, New Jersey, set an all-time record high temperature of 108 degrees F on July 22, 2011, at the peak of that heat wave.

"These enormous and nearly stationary areas of high pressure can keep calm winds, clear skies and oppressively hot air in place for well over a week at a time, so the relatively high concentrations of vehicle and industry emissions from the Midwest to the Northeast have nowhere to go," AccuWeather Meteorologist and Air Quality Blogger Faith Eherts said. "As a result, an increasingly poor air quality situation can develop."

The Bermuda High also plays a key role in the movement of tropical storms and hurricanes in the Atlantic Basin.

Easterly winds to the south of the Bermuda High force many tropical systems on a westward course from the eastern Atlantic to the Caribbean Sea and/or Gulf of Mexico. The Bermuda High can ultimately put dangerous tropical cyclones on a collision course with the mainland of the U.S.

When the Bermuda High shifts to the east of its typical position near Bermuda, the Islands of Bermuda may instead be most prone to a direct hit from a tropical system. Southerly winds to the west of the high force tropical systems to veer northward more quickly, toward Bermuda, than if the high were directly over the islands.

The summer monsoon season in the southwestern U.S. is often triggered and/or enhanced by a high pressure system over the Plains.

"Trapped moisture that gets recycled on a daily basis can lead to thunderstorms over the Rockies and Southwestern states," Sosnowski said.

So, despite the fact that high pressure systems traditionally yield pleasant, sunny weather, there are many cases in which they can enhance or create an undesirable to potentially life-threatening situation.

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