Visualize what the maps don't show
To see how you go about doing this, let's begin with the weather map in Figure 1, a snapshot of the weather across the central United States one morning in May.
Figure 1 is a surface analysis chart. "Surface" means it shows what's happening at ground level. "Analysis" tells you that it shows the weather observed at a particular time -- in this case 10 a.m. Central Daylight Time. It's from AOPA's Meteorlogix weather service, which is available in the members-only section of AOPA Online. You will find similar charts on the National Weather Service's Aviation Weather Center Web site.
The blue, red, and purple lines on the map are weather fronts, which are boundaries between large masses of air with different characteristics, mainly temperature. For example, the red line running east from the junction of blue and purple lines over south-central North Dakota identifies a warm front.
It indicates that warm air is moving toward the north, replacing cooler air. The red circles on the north side of red line show which way the warm air is moving. "Warm" and "cool" are relative. During the winter, a warm front might bring 40-degree air, but it's warmer than the air it's replacing. A summer cold front might bring 60-degree air to push away 80-degree temperatures.
Figure 2, which is a cross-section of a typical warm front, gives you some idea of what you might run into on a flight from northern Iowa to northern Minnesota. Your flight would begin in warm air, or at least air that's warmer than the air over northern Minnesota. As that warm air rides over the cold air to the north, it will cool, probably cooling enough for its humidity to condense into an area of widespread clouds beginning somewhere to the north of the warm front line on the map.
Before taking off, you need to find out how widespread the clouds are and whether they are creating low ceilings that would require flying under instrument flight rules.
In addition to the fronts and other weather features, Figure 1 also shows weather radar images. The green area north of the warm front indicates rain. (Aviation Weather Center surface charts do not show radar images.)
The blue line from North Dakota to the "L" over Kansas represents a cold front, in which cool air is replacing warmer air as it pushes east. The blue triangles on the line show which way the cold air is moving. Figure 3 is a cross-section of a typical cold front. It shows cold air pushing into warm air much like a plow. A cold front is likely to vigorously shove warm air up, creating thunderstorms. Often, as the green and yellow radar images over Iowa in Figure 3 show, thunderstorms can be far ahead of a cold front.
The "L" over Kansas represents an area of low air pressure; that is, the air pressure at the surface here is lower than in the surrounding area.
Air rises from the ground in areas of surface low pressure. As the rising air cools, clouds and precipitation form unless the low pressure is weak, the air is dry, or both. That seems to be the case in Figure 1 because weather radar indicates no precipitation anywhere near the low pressure over Oklahoma.
The line that looks like a warm front and then a cold front running south and west from Kansas into New Mexico represents a stationary front where neither cold nor warm air is advancing -- it's a standoff. A cross-section would look much like the cold front in Figure 3. Stationary fronts can often bring a few days of low clouds and precipitation, especially in the winter. At the time of the map in Figure 1, however, no precipitation had formed along the stationary front.
The purple line across North Dakota represents an occluded front, which differs from other kinds of fronts because three air masses, not two, are involved.
Figure 4 shows how the three air masses of an occluded front can be arranged. The line on the map shows where the boundary between the cold and cool air touch the ground. In some occluded fronts the warm air would be at the ground on the right with the cool air above it.
Many textbooks say that occluded fronts form when a cold front catches up with a warm front. Meteorologists who figured out early in the twentieth century how fronts work gave this explanation of how occluded fronts form. But the detailed weather observations that became available late in the century showed that this rarely, if ever, happens. Instead, as the American Meteorological Society's Glossary of Meteorology notes, occluded fronts form when the low-pressure center of a storm re-forms deeper into the colder air than its previous location.
A cold and warm front often meet at a storm's low-pressure center. The low center over the U.S.-Canadian border in Figure 1 could have been located where the cold and warm fronts meet over North Dakota before it redeveloped farther north, creating the occluded front.
If the weather had been more active than it was on the day the Figure 1 map represents, the occluded front could have produced a mix of the widespread clouds with steady rain you expect to find ahead of a warm front, plus thunderstorms, like those you'd find with a cold front, embedded in the widespread clouds.
Creating your own three-dimensional picture of the weather shown on a map is only the beginning of a good preflight weather briefing, of course. To complete the picture, you need to study reports from weather stations along your proposed flight route, and also look at forecasts of how meteorologists expect the day's weather to play out.
For example, if you know the kind of weather an occluded front can cause, you'd check very carefully before taking off for a flight from west to east across southern North Dakota if the map looks like Figure 1.
As it happened, the occluded front wasn't stirring up dangerous weather at the time of the Figure 1 map. Bismarck, North Dakota, for instance, which was close to the occluded front, was reporting 10-mph winds and visibility of more than 10 miles with the lowest layer of clouds covering more than half of the sky at 1,500 feet above the ground. It would have been a good day for a flight instructor to have a student practice maneuvers such as turns around a point, which wouldn't have required flying much more than 1,000 feet above the ground. It would not have been a good day to send the student on a solo cross-country flight in North Dakota.
At the time shown in the Figure 1 map, weather across the north-central United States was calmer than on many spring days because the air a hundred or so miles west of the cold front was no more than about 10 degrees cooler than the air to the east. A front with a large temperature contrast between its two sides is more likely to stir up dangerous weather than a front with little contrast.
But, as the sun heats the ground during the day, the weather can turn more dangerous. Figure 5, which is a forecast for 7 p.m. CDT the day of the Figure 1 map, shows that forecasters expected this to happen. The map is what meteorologists call a prog (prognosis) map showing weather expected at a particular time.
It shows that forecasters expected thunderstorms to the east of the fronts. The map's solid red area shows where forecasters expect thunderstorms to cover more than 50 percent of the area around 7 p.m.
The area enclosed by the red line circling from the Nebraska-South Dakota border south into Oklahoma and back toward the north to Lake Michigan should have thunderstorms covering less than 50 percent of the area.
A three-dimensional image of what's happening along the cold front helps you to understand why thunderstorms are expected: the cold front will be shoving warm air upward, creating one of the key conditions for thunderstorms -- rising air.
Taking a few minutes to paint your own three-dimensional mental picture of what a weather map tells you, even when you're not planning a flight, will help you make sense of what you see or hear during a preflight briefing. Like practicing maneuvers in the airplane, the time you spend practicing reading and making sense of weather maps will help you to become an accomplished pilot.
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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