We often hear meteorologists refer to high- and low-pressure systems in relation to the forecast. But what exactly does that mean? Simply put, the weather is primarily dictated by the flow of air in the atmosphere. These changes in flow are caused by a host of variables, of which the barometric pressure and jet stream are the primary forces. If it weren’t for fluctuations in barometric pressure, we wouldn’t get any of the powder days we live for.
What is the difference between high- and low-pressure?
Barometric pressure refers to the amount of pressure exerted on the surface of the earth by the atmosphere at any given time. When discussed in terms of weather patterns, high and low pressure is thought of as relative to surrounding geographic areas--air always moves from areas of higher pressure to areas of lower pressure. Given this, a significant decrease in the barometric pressure indicates the approach of a low-pressure system, where an increase indicates the approach of a high-pressure system. Of the two phases, low-pressure systems are thought of as the major storm makers.
How do low-pressure systems create storms?
When understanding how these pressures affect the weather it is important to think of the flow of air on a horizontal plane, and the pressure of air on a vertical plane. On the horizontal plane, air currents circle counterclockwise around the center of a system. In a low-pressure system, the air is naturally seeking equilibrium, so the flow of surface air is towards the center of the low-pressure system. This flow towards the center of the low-pressure point forces the air to converge and rise in the atmosphere. On the vertical plane, as air temperature cools it flows upward in the atmosphere and condenses, thus creating water molecules that can form clouds and precipitation. Think low-pressure = rising air = increased moisture.
Typically, large storms have three major components: a low-pressure system, a warm front, and a cold front. Over the winter months in North America, the jet stream dips lower and drives cold and dry polar air south across the United States. Meanwhile, warm moist air is rising up from the Gulf of Mexico. Because the flow of air is dictated by pressure systems, when a low-pressure system forms, the cold air flows south on the west side of the system, while the warm air flows north on the east side. Storms and other weather events occur when warm and cold fronts meet and both are driven into the center of the low-pressure system.
In the winter months, we receive large snowfalls when a massive cold front reaches and overtakes a warm front at the center of a low-pressure system. When the warmer, lighter, and moisture-laden air from the Gulf meets the cold, dense air mass from the north, the warmer air is forced up into the atmosphere. Thus, creating clouds and the subsequent precipitation.
Long story short, when you hear your meteorologist say that a low-pressure system is approaching, it means warm and cold fronts will be forced to converge, pushing moisture into the air to create clouds and precipitation. Otherwise known as snow. Grab your board, and head to the hills.