Low pressure means bad weather

In the past couple of weeks, weather has managed to make headlines from the central U.S. to the East Coast. The weekend of April 13, springtime turned wintry for folks in Kansas with some locations receiving a foot of snow (with Kansas being as flat as a pancake I’m not talking about mountain snows, either.) Tornadoes moved through the Dallas/Fort Worth area, and as the storm continued eastward it developed into a “Nor’easter.” This late season Nor’easter brought nearly 8 inches of rain to Central Park in New York City April 15 and 16 while blasting other areas of the Northeast with rain, wind and snow.

The blame for all this awful weather, and indeed all the storminess that occurs across the globe throughout the year, can actually be pinned on one thing … and I’m not talking about global warming. The perpetrator of stormy weather wears a scarlet letter on the weather map. Actually, it is a scarlet letter L, for low pressure. You’ll find low pressure at the center of tornadoes and hurricanes, and as the focal point for larger storm systems that trek across the country, leaving, at the very least, unsettled weather in their passing. Just what is an area of low pressure on the weather map, and why does it result in such stormy conditions?

Air always flows from higher to lower pressure. Low pressure results when moving air molecules exit a particular location. Nature does not like a vacuum, so other air molecules rush in to fill the void. When air molecules converge on an area of low pressure in the lower levels of the atmosphere, the air is forced to rise. With sufficient moisture, this rising air can eventually produce clouds and precipitation. The type of precipitation depends on the temperature profile of the atmosphere at the time. Deeper low pressure areas (deeper meaning lower) can create a tight gradient, or difference in air pressure from one point to another. This results in stronger winds – strong enough to cause damage in some situations.

Over subtropical waters, warm, moist air leads to vertical motions that cause strong areas of low pressure to form. Convergence over this area of low pressure can lead to the formation of hurricanes. Likewise in a severe thunderstorm, intense rising motion along with other factors can lead to the formation of a tornado. The strong winds or “sucking” force of a tornado occur as air rushes toward the center of low pressure like a vacuum. Contrary to popular belief, however, the pressure difference between a tornado and the inside of surrounding buildings, does not cause them to “explode.” The strong force of the tornadic winds, however, is sufficient to cause damage that might resemble an explosion.

Another popularly held belief that is only sometimes true, is that winds always rotate counterclockwise around an area of low pressure in the Northern Hemisphere (clockwise in the Southern Hemisphere.) While this phenomenon, caused by the rotation of the Earth (the Coriolis force), is true for large-scale storm systems and hurricanes, it does not play a role in the circulation of say, winds in a tornado or dust devil. Likewise, water in sinks and/or toilets does not “swirl” in a counterclockwise fashion because of the Coriolis force, either. The Coriolis force only acts on events of a larger time scale, i.e. storm systems or hurricanes which persist for days, rather than the hours, minutes, or seconds during which smaller-scale features like tornadoes might exist.

Short-term outlook

“Upper-level” low pressure across the Inland Northwest can be blamed for the continuation of slightly below normal temperatures which will persist over the next week to 10 days.