Fronts and pressure areas mix it up
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| Figure 1 shows an extra-tropical cyclone as it might be depicted on a surface weather map. |
| Figure 2 depicts a cross-section of the same system shown in Figure 1, along the line from A to B. |
One of the big problems in understanding weather is making sense of three-dimensional phenomena, such as winds, clouds, rain, and snow, that are always moving and changing and never seem to be quite the same as the last time you encountered them.
The best way to cope is to find a simplified mental model that helps you by making the details part of a bigger picture. Once you have grasped the big picture, you can start understanding details that could force you to cancel today's planned flight.
Norwegian scientists working during and soon after World War I developed a weather model that's stood up well for understanding the weather that dominates most of the United States most of the year. In fact, you see this model every time you look at a weather map that depicts fronts and centers of high and low air pressure. It works pretty well as a very generalized picture of day-to-day weather changes across the middle latitudes, which includes all of the United States except Hawaii.
Even if you paid little attention to weather until you started flying, you almost surely heard about one of the model's key features: weather fronts. Fronts dominate weather across most of the United States from late fall through spring.
The Norwegian meteorologists came up with the term front in 1919, the year after World War I ended. From soon after the war began in 1914 through its end on November 11, 1918, "The Western Front" had constantly been in headlines. To the Norwegians, the word front was appropriate because they identified weather fronts as battle lines between warm and cold air.
The storms the Norwegians studied, and the ones we see sliding generally from west to east across weather maps, are extratropical cyclones--systems that form outside the tropics. Hurricanes are tropical cyclones, and the model doesn't work for them.
Figure 1 shows an extratropical cyclone as you might see it depicted on today's surface weather map, and as the Norwegian scientists envisioned such storms back in 1919.
In Figure 1, the "L" is the center of low air pressure, and the thin black lines with numbers under them are isobars, which are lines of equal air pressure. For instance, every point on the black line around the "L" nearest to it has a pressure of 1,000 millibars; the next black line out is 1,004 millibars, and the third one 1,008 millibars.
The red line with half-circles pointing to the upper right shows a warm front where warm air is moving toward the upper right, replacing cooler air at the surface. The blue line with triangles shows a cold front, with cooler air moving toward the lower right, replacing warmer air at the surface. The gray areas show where precipitation--rain or snow--could be falling.
Figure 2 is a cross-section of the same system along the A to B line in Figure 1. In it, you see that as the dense, cold air arrives, it pushes up the lighter, warm air that it's replacing. As the warm air rises, it cools, and the air's humidity begins condensing into tiny water drops and ice crystals that create cumulus clouds, which often grow into thunderstorms.
In general, an extratropical cyclone's most violent surface weather is along the cold front, but this is not the storm's only area that pilots need to worry about. While the cold front is likely to bring a relatively narrow band of thunderstorms, the warm front often causes a widespread area of weather with solid clouds, likely below visual flight rules (VFR) minimums. If you were flying along the line from B to A, you would first see high clouds, and then run into more solid clouds at lower and lower altitudes as you continued toward A, as seen in Figure 2. The distance from Point B to the warm front at the surface could be 200 or 300 miles, maybe farther.
The winds around low air pressure in the Northern Hemisphere flow generally counterclockwise, but near the surface they curve more sharply to the left to flow across the isobars. One consequence is that winds converge, or come together, in areas of low pressure at the surface, including along the fronts--especially along the cold front. Such convergence pushes the air up to create clouds and precipitation.
The terms cold front and warm front can be misleading. A cold front refers to air that's cold in terms of the air that it's replacing. During winter across the northern United States, temperatures could drop 30 or 40 degrees within a few hours of a cold front's passage. In the summer, however, a cold front could bring little temperature decrease, but it will bring lower humidity with relief from sticky heat.
In the same way, a winter warm front in the North is likely to bring a day or two of snow, and maybe sleet and freezing rain from the clouds ahead of the front. When the surface front moves past, temperatures might rise only by a degree or two. In other words, a warm front can be very bad news in winter.
The fronts pivot counterclockwise around the system's low-pressure center, with the cold front usually moving faster than the warm front. At the same time, the entire system is traveling across the country with the low-pressure center and its associated fronts heading to the east or northeast, sometimes even toward the southeast.
Since television forecasters are more likely to focus on the fronts than other parts of the storm, you might think that the weather to the north of the low center, where there are no fronts, would be easy to handle even if it is cloudy.
Such thoughts could fly you into trouble. The generally counterclockwise winds around the low pressure center bring warm, humid air over the warm front and wrap it around to the north side of the storm's center. Atmospheric sciences refer to this air motion as a "warm conveyor belt." Like a warm front, the air here is warm only in relation to its surroundings, and the air could be below freezing with enough humidity to deposit thick ice on any aircraft that flies into this part of the system.
The World-War-I-era Norwegian meteorologists saw extratropical storms with their low-pressure centers and fronts as being somewhat like individual battles in the ongoing "war" between cold air in the north and warm air to the south. (In the Southern Hemisphere the cold air is in the south and warm air to the north.)
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| Figure 3 shows the symbol for a stationary front, a boundary between cold air and warm air. |
| Figure 4 is a simplified upper air chart of a trough, which is an elongated aera of low air pressure. |
These battles break out when contrasting masses of air move away from the areas where they form. Cold air masses, for instance, grow when light winds allow air to sit over a frigid region, such as Siberia or Arctic Canada, for a few days. In winter these regions see very little sun and the air grows colder and more dense until it begins sliding south. While polar air masses born in the summertime aren't frigid, they are cooler and less humid than air masses that grow over the Gulf of Mexico and other warm waters.
No matter what season we're in, eventually the cold air moves into an area dominated by warmer air, maybe a mass of air that has sat for days over a warm ocean, becoming not only warm but also very humid before the winds push it toward the north. The boundary between the cold and warm air, which is often shown on surface weather maps with the stationary front symbol shown in Figure 3, is unstable because the colder air is denser than the warmer air, which--everything else being equal--means the air pressure at the surface is higher. The heavier, cold air begins pushing under the lighter warm air. As air begins moving, the Earth's rotation combined with other forces causes it to turn to the left.
This alone isn't enough to give birth to an extratropical cyclone, which is one reason that waves, which look like a kink in a front, form and then move along a stationary front or a slow-moving cold front, creating low clouds and precipitation. A wave's weather can ground a VFR trip and is something to watch out for if your flight plan takes you across a stationary front.
Surface winds spiral in toward the center of low pressure at the surface. If nothing else happens, the air flowing into the low-pressure area will "fill" it. That is, the pressures will equalize and the winds will die down.
The upper atmosphere needs to cooperate in order for a surface wave on a stationary front to grow into a storm.
Figure 4 is a simplified upper air chart of a trough, which is an elongated area of low air pressure. It shows lines of equal air pressure in black and the winds, which blow parallel to lines of equal pressure aloft, in dark blue. At Point 1, on the left, the winds are converging, or coming together. This tends to force air down to build an area of high air pressure at the surface. At Point 2, on the right, the winds are diverging, or flowing apart as the lines of equal pressure diverge. This gives air coming up from the surface a place to go; it allows the pressure to fall in a developing low-pressure storm center at the surface.
This picture, of course, is greatly simplified. Still, it should help as you watch television weather reports trying to determine what the fronts and storms the weathercaster is talking about will mean for your flight. Just don't forget that each storm is different and your general picture is just that, general. Before taking off you need a preflight weather briefing to fill in the details of how the weather is likely to treat you and your airplane.
Jack Williams is coordinator of public outreach for the American Meteorological Society. An instrument-rated private pilot, he is the author of The USA Today Weather Book and The Complete Idiot's Guide to the Arctic and Antarctic, and co-author with Bob Sheets of Hurricane Watch: Forecasting the Deadliest Storms on Earth.
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