Oversimplification is one disadvantage of the traditional highs, lows, and fronts picture. But, a realistic picture would require brutally complicated, three-dimensional weather maps that you'd need a doctorate in meteorology to understand. For pilots who recognize the limitations of this type of weather depiction, the traditional extratropical cyclone picture of a low with fronts attached will help to make sense of the weather predicted along a route of flight.
To meteorologists, a cyclone is any large-scale weather system with air swirling into an area of low air pressure at the surface and rising - a storm. Extratropical means that the cyclone didn't form over the warm waters of the Earth's tropical regions. Hurricanes and typhoons, by contrast, are tropical cyclones. The big difference between the two is that extratropical cyclones involve both cold and warm air, while tropical cyclones are embedded in a mass of warm, humid air.
For most of the year, weather in the middle latitudes is dominated by the push and shove of masses of cold air from the polar regions and warm air from the tropical or subtropical parts of the Earth. Fronts are the dividing lines between battling air masses. In fact, the word front came to be used right after World War I because the boundaries between air masses were seen as somewhat like the fronts along which nations had battled during the war.
Air masses are huge-sometimes half the size of the United States. An air mass that forms over cold land, such as in Alaska or northern Canada, will be cold and dry as it begins moving south. As it moves over warmer ground or into areas with more sunlight, it will begin warming up, but will stay dry.
Figure 1 shows two air masses dominating the contiguous 48 states. The H over where Idaho, Montana, and Wyoming meet is the high air pressure in the center of a mass of cold air that is moving east across the country. To the west and south, there's no sharp boundary between it and air masses over the Pacific and western Mexico. In other words, it and the air masses to the south pretty much fade into each other.
The blue and red lines running from northeastern Mexico north and then east across the United States to the Atlantic represent the boundary between the cold air mass and a warm, humid air mass centered off the Carolina Coast.
The white lines are isobars, which connect areas of equal air pressure. The air pressure falls as you move from either of the high pressure centers toward the boundary between the air masses.
The blue and red lines are fronts. Blue is a cold front where cooler air is advancing in the directions in which the blue triangles along the blue line are pointing. Red represents a warm front where warm air is advancing in the direction of the half circles. Alternating red and blue segments, such as those from northeastern Missouri to Lake Michigan and in Canada between the two low-pressure centers, are stationary fronts, where neither warm nor cold air is advancing - it's a standoff for now.
The L over northern Texas and Oklahoma is the low-pressure center of this particular system. The L by itself on the northeastern coast of Mexico is a low-pressure area that's forming and should be watched by people across the South. The two lows over Canada are waves on the front and could be potential sources of clouds and precipitation. Also, either could develop into a new storm center.
The green, yellow, and red shaded areas on the map represent weather radar images. Red shows the strongest precipitation, green the weakest. Red and yellow on such maps are often bad news for pilots because turbulence, even extreme turbulence, is likely. You can see that the strongest precipitation is around the low-pressure center. Later in the day that this map was made, severe thunderstorms, including a few with tornadoes, formed near the low center.
It's obvious that this real weather map, from December 4, 1999, is different from the idealized extratropical system shown in Figure 2. The fronts represent the same kinds of boundaries that are shown on the real map, but the low center is not along a frontal boundary as it is on the real map. You'll see both cases and other variations in the real world.
The black arrows in Figure 2 represent the general flow of the wind, which is counterclockwise, but not a perfect spiral, around the low-pressure center. The winds converge along the fronts, especially the cold front.
Converging air streams force air to rise, which cools it and condenses its moisture into clouds and precipitation. The fronts are also in troughs of low pressure. You can think of air masses as mounds of air and the boundaries between them-the fronts-as the valleys between the mounds.
When a surface front moves across a region, the barometric pressure will fall and rise and the wind will shift direction. These changes are more pronounced with cold fronts than warm fronts. If you fly across a front, your altimeter will read lower than your true altitude because of the air pressure drop at the surface. It also means that you should be ready to change your wind correction angle to stay on course.
If you look at Figure 2, you'll see that if you were flying from the warm to the cold sector of the storm, you'd have a left wind correction angle in the cold air, but a right correction angle in the warm air.
In an extratropical cyclone, the cold and warm fronts are advancing, with one end usually anchored to the low-pressure center. At the same time, the entire system is moving, generally to the east or northeast, sometimes the southeast.
Figure 3 is the weather map for a period about 24 hours after Figure 1. You can see that the main low center, now over Missouri, has become the dominant part of the system. We now have one low center instead of two in Canada, and the low off the Mexican Coast has disappeared.
During this period, heavy snow fell behind the storm in some places, such as parts of Kansas, and strong thunderstorms moved across Texas with the cold front and ahead of it. As shown in Figure 4, after another 24 hours, the storm had moved to the East Coast, heading toward Canada and over the Atlantic.
If you can figure out where the low center is going to move in the next 24 to 48 hours, and whether it will strengthen or weaken, you can have a pretty good handle on what the weather is going to be like for places in the storm's path for the next two days. Where a storm goes makes a big difference in the kind of weather a place sees.
Even though storms tend to follow a few general storm tracks across the United States, forecasting where a particular one is going to go is harder than it might seem. Fairly reliable forecasts have become possible only in the last couple of decades, thanks to computer models.
If the storm shown in Figures 1, 3, and 4 had continued to move eastward from Texas to the Atlantic Ocean, it probably would have gained strength off the North Carolina Coast and then moved toward the northeast along the East Coast. If this had happened, the major cities of the East could have had a big snowstorm, because the storm's cold air would have made it to the Atlantic.
From fall through spring, extratropical storms generally move to the east across the United States, following a variety of paths. In the summer, storms tend to stay to the north, across Canada. But at times, summer storms will move across the United States and fall or winter storms will stay in Canada. This happened during the last couple of weeks in November 1999, when the centers of storms moving into the Pacific Northwest moved to the northeast into Canada, staying 200 or more miles north of the U.S. border as they crossed the continent. This is one reason that Chicago had its first November on record with no snow.
Much of the weather action in extratropical storms takes place along the fronts, with the cold front's weather usually being more dramatic than what's happening along the warm front. Along the cold front, advancing cold air is shoving warm, humid air upward. The shoving can be vigorous, which helps to trigger thunderstorms and gusty winds. These can occur not only along the cold front, but also ahead of the cold front.
In fact, pre-frontal squall lines usually produce stronger storms than lines of thunderstorms along the front itself. Upper air features associated with the storm system, such as cold fronts aloft-the warm-cold boundary doesn't reach the ground-help to create such squall lines.
The warm sector, the area of the storm ahead of the cold front and behind the warm front, generally the southeast quadrant, is usually where the strongest thunderstorms occur. This is especially true in the spring and early summer when strong contrasts between cold and warm air supply energy for thunderstorms that can bring tornado outbreaks across the South, the Plains, the Midwest, and sometimes into the East.
With the warm front, the main action is usually ahead of the surface front shown on the map. Warm air is riding over the denser, cool air ahead of the front. As the air rises, it cools and its humidity condenses into widespread, generally flat clouds that produce rain or snow and poor visibility over an area a few hundred miles ahead of the front.
Dangers in this area are icing and thunderstorms that might be embedded in the generally flat clouds. In fact, during the winter, the clouds ahead of a warm front are a prime location for icing.
A glance at a diagram of an extratropical storm might lead you to think that the weather should be pretty calm north of the low-pressure center where there are no fronts on the map. This can be misleading. Some of the air that is riding up over cool air ahead of the warm front is wrapping around the low. This can make for widespread poor visibility and icing in clouds.
More often than not, the best flying weather will be in the cool, high-pressure area to the west of an east-moving storm. This might last only a couple of days, however, before the warm front associated with the next storm begins pushing in warm, humid air with clouds and precipitation followed, maybe, by a brief clearing before the cold front arrives.
Watching the parade of extratropical storms across the United States in the winter and into the spring, noting the different paths they take and the succession of clouds, precipitation, and wind changes they bring is a good way to begin learning to read the weather. Seeing the big picture of highs, lows, and fronts on weather maps can help you to understand what you see when you look at the sky from the ground or an airplane.
