The “low-down” on Northern Hemisphere low pressure systems
A lot of people have a generalized idea about the weather conditions they can anticipate when a big red “L” is in the vicinity of their location on a weather map. This piece will explain how and why low pressure systems form, and how they are also an integral part of the balancing of Earth’s heat and moisture budgets.
The Golden Rule of balance
In short, we need only remember one simple rule of Mother Nature, and that is that She likes it best when her atmospheric checkbooks of temperature and moisture balance. After all, who wouldn’t like their checkbook to balance every time they went to reconcile it? What would cause Nature’s checkbooks to be out of balance? Let’s look at two of the key players: Heat and moisture.
Areas closer to the Equator receive more heat from the sun. The angle of the sun in the sky is higher as we head toward the Equator from the Poles. Thus, more concentrated solar energy reaches the Earth’s surface the closer we get to the Equator. To understand why this matters, try an experiment with a flashlight. Shine the flashlight down directly down on top of a table or a wall, notice how intensely the beam of light shines on that surface. The heat from the flashlight beam may also be noticeable if you put your hand on the table or wall directly underneath where the beam is shining (see figure 1 below in the slide show).
Now, compare this to shining the flashlight at an angle to that same surface. Notice how the beam is more spread out and the heat not as intense as it was when the beam was directly overhead. The Equator receives the most direct sunlight (flashlight shining directly overhead), while as the further north we go toward the poles, the sun’s rays are not as direct (flashlight at an angle to the surface as opposed to directly overhead). Note that the closer we are to be being “overhead” the more intense the beams, and the more energy they impart to the surfaces they come in contact with.
Consider also that the Earth is pretty close to 75% oceans, which water is constantly evaporating into the atmosphere from. Upon throwing moisture into the mix, things can get quite complicated. Suffice it to say, Nature has quite a bit to balance out if she wishes to keep things in check.
In summation, warm and moist air tends to “pile up” as we move closer to the Equator, while as cooler, less moist air, tends to “pile up” as we head toward the Poles. In Her attempt to balance her atmospheric checkbook, Nature has to develop a way to mix these two “pools of air” and balance them out. This is done by creating a circulation, our buddy and the main topic of this piece, the low pressure area.
Rising air & counter-clockwise circulation
Low pressure areas are associated with rising air. The circulation of air in the vicinity of a low pressure center is counter-clockwise, with air flow into the center of the low pressure area, as illustrated in the slideshow figure 2. This circulation causes a southerly flow of air on the eastern side of the low pressure, driving the warmer equator-ward air northward, while as on the western side a northerly flow of air is present, driving the cooler pole-ward air southward. The rising motion of the air may also cause precipitation and cloud cover to occur.
As this circulation evolves, the leading edge of both the warmer air and the colder air, knows as “fronts” tend to become a bit more pronounced and able to be spotted easily on a weather map. These areas usually mark themselves by changes in the direction of the wind. We will now investigate these areas of temperature contrast associated with this circulation of air caused by a low pressure area.
Don’t be “’front’in’”
Slideshow figure 3 shows the weather map symbol for a warm front, or warm frontal system. In general, low pressure systems tend to have warm fronts on their eastern flank, in advance of the low pressure center itself. The warm air tends to spread out in front of the low pressure area, and thus, the arc extending out and in advance of the system. Warm fronts are generally associated with weaker rising motions with respect to their partner, the cold front. However, they can still be hefty precipitation producers, especially if they happen to be circulating moisture-rich air into a region.
On a weather map, a warm front can be spotted by sudden rises in temperature over a given area, and a southerly or southwesterly wind. Surface atmospheric pressure tends to drop with the passage of a warm front as well.
Figure 4 in the slideshow below shows the weather map symbol for a cold front, or cold frontal system. In general, low pressure systems tend to have cold fronts on their western or southwestern flank, trailing behind the low pressure area that caused it to form.
All other things being equal, cold air is denser than warm air, and thus will sink, and in the case of a warm and cold front pair, will tend to undercut the warmer air, thus displacing it upward, as shown in the slide 5 of the slideshow. Thus cold fronts have a bit more “bang for the buck” when it comes to forcing air upward. They are usually associated with moderate to heavy precipitation and thunderstorms given adequate moisture being present.
On a weather map, cold fronts can be spotted by sudden drops in temperature over a given area, and westerly or northwesterly wind shifts. Surface atmospheric pressure tends to rise upon a cold front’s passage.
It’s a matter of speed
As a general rule cold fronts tend to move faster than warm fronts. It would then stand to reason that at some point in time, the cold front would eventually catch up to the warm front and lift the warm front off the surface of the Earth, thus completing the task Nature created the low pressure area to complete. That task being to take the warm air north and the cold air south. When this “catching up” occurs, the low pressure is said to be “occluding”, and thus an “occluded front” (slideshow figure 6) forms, signaling where the catch up process has already occurred. This also signals the beginning of the final stages of the life cycle of the low pressure system. Having completed its task, the initial area of low pressure will “spin down” and the circulation will cease. This is not always the end of the story though!
Often times, new low pressure circulations can form at the point where the occluded front, warm front, and cold front all meet up, as shown in slideshow figure 7. This area where all three fronts meet is appropriately named the triple point. If a new low pressure area forms at the triple point, the process begins all over again.
There may also be certain scenario’s where warm air and cold air do battle in the atmospheric arena, and the contest is a draw. In other words, neither air mass is able to gain control from the other. This causes a stationary front to form (slideshow figure 8). These frontal systems often times cause a lot of trouble in the weather forecasting community because they tend to oscillate back and forth over the same region for what may be prolonged periods of time. The greatest concern from stationary fronts is their ability to produce copious amounts of precipitation over the areas which they hover. In addition, temperature contrast is abundant in the vicinity of a stationary front, and thus, forecasting high and low temperatures can be quite a challenge.
Caught in the middle
Slideshow figure 9 shows the area between a low pressure system’s warm front and cold front, termed in the meteorological community as the “warm sector” of the low pressure system. This area is often an area of great atmospheric instability. If the sun is allowed to break through the clouds in a low pressure system’s warm sector, the atmosphere may become quite unstable, and when a cold front approaches and forces that unstable air upward, heavy precipitation and strong to severe thunderstorms may, but do not always, occur. It also tends to be breezy to windy in this area as well.
In conclusion, Nature is always attempting to keep things in balance, and the circulation known as a low pressure system is one of the main methods by which this task is accomplished.