Yesterday, we covered the mechanics and significance of high-pressure systems. Today, we will examine the more impactful of the two pressure systems: Low Pressure.

Low-pressure systems offer almost all of the impactful weather on Earth. From something as mild as a brisk breeze to something as destructive as a tornado, low-pressure systems can be a root cause for all.

 

 

Low-pressure systems feature converging air at the surface, or air coming to a point in all directions. Due to the Coriolis effect, air spins counterclockwise around the system. This means if someone were due east of the center of a low, then they would likely experience winds from the southeast, both to the center and in a counterclockwise direction. With so much air coming to a single point, it must be going somewhere to escape more air coming. This is how air diverges, or spreads apart, aloft above the center of circulation. As a result, the column of air rises above the center of surface low pressure.

 

 

Looking at a cross-section, you can equate this to a geyser. As air, or in this case, water, comes together at the base of the geyser, it must exit the area as more water rushes in. As a result, the water spreads upwards before going in all directions at its highest point.

 

 

How does weather come from a low-pressure system, though? It all comes in the way air moves between elevations. In this case, air rises from below, and as it rises, the pocket of air expands, which cools it down. Cooling air can moisten up the parcel, because you are bringing the bit of air down to its dew point temperature (See more about dew point here). With moist air, you get the fuel for rain, snow, and other forms of precipitation. This is the basis for why weather can be attributed to low-pressure systems.

Low-pressure systems, much like highs, can be seen easily by meteorologists. They can come in several different shapes and sizes, as well. A hurricane is a type of low-pressure system that forms in the tropics. Heavy winter storms, which only form in the mid-latitudes, are another type of low. These are examples of mid-latitude low-pressure systems, which impact our area all the time. The best time to recognize one is when it is in its mature stage, where the alignment of its clouds looks like a comma. This is because the cold front associated with the low expands, with a large streak of cold air overrunning, allowing for widespread cloud development.

 

 

Lows, unlike highs, also contain fronts, or boundaries of differing air masses. Cold and warm fronts, the two most common in our area, separate cool and warm air masses. Stationary fronts, from the name, separate air masses but struggle to move. Occluded fronts, common in dying low-pressure systems, occur when cold air “overruns” another bit of cold air, essentially choking the system. Several other boundaries exist, which will be covered more fully in a future Weather 101 deep dive.

 

 

From rain to storm surge and even hail, low-pressure pressure form the dynamic duo of pressure systems around our planet. While just designated as a red “L” on maps, these have an extraordinary influence on climate, weather, and human life.