Under Pressure: How Atmospheric Pressure Impacts the Weather (Photo Credit: University of Wisconsin SSEC)
Perhaps you have heard a local meteorologist talk about a low pressure system or an area of high pressure. These are both common terms used to describe meteorological features and weather patterns. The big question: what is atmospheric pressure and why is it such an important factor in determining weather conditions? Let’s take a closer look.
In a general scientific sense, pressure is simply a way to quantify how much force (or in many cases weight) is being applied to a particular area. Think about what happens after diving into a swimming pool. As you move downward from the water’s surface toward the pool floor, the amount of water above you increases and the pressure exerted on you grows larger. This change in pressure is why oftentimes our ears will “pop” underwater. When you return upward to the surface and the mass of water above you decreases to zero, the pressure applied by the pool is much less than it was near the pool floor. As we will see, a similar principle applies in our atmosphere.
First we will consider a hypothetical situation in which a vertical column of air extends from the surface upward. If air within the column is cold, it is denser and sinks from upper-levels toward the ground. The air in this scenario is more compact near the surface, it occupies a larger volume close to the ground. Therefore the weight of this air is larger, corresponding with a high value of surface pressure. The opposite is true within a column containing relatively warm, less dense air. In this case, air is less compact just above the surface which means less weight is applied downward and pressure is fairly low. These opposite situations play a major role in both the overall weather pattern and specific meteorological conditions on any given day.
When we have an area of high pressure in lower-levels of the atmosphere, air sinks from aloft and spreads out horizontally, blowing away from the center of circulation. This situation is usually associated with sunshine, clear skies, and calm winds. During the day in the summertime, this lack of cloud cover leads to high values of solar radiation and thus warmer temperatures. At night, this radiation escapes into the atmosphere and temperatures at the surface can cool dramatically. When pressure at the surface is instead relatively low, air rises, cools, and condenses. This process results in stronger winds, less variable temperatures and the formation of clouds and precipitation.
These pressure patterns continuously change over time and in fact, a shift between high and low pressure over any geographic area can occur on the order of only a few days. However, climatological evidence has shown some common trends in system location and path. For example, you may have heard of the “Alberta clipper”, a low pressure system that tends to form just east of the Rocky Mountains in Canada’s Alberta province. While not always a certainty, historical data can help meteorologists understand where these extreme pressure areas are likely to develop and impact weather conditions!
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©2019 Weather Forecaster Dennis Weaver