Weather

As North America moves into late fall and winter, pilots must monitor the multiple effects of large, organized weather systems with various hazards, ranging from widespread areas of low clouds to icy precipitation to or long lines of thunderstorms that sweep across the South.

These systems are called extratropical cyclones. “Extratropical” means these storms form and thrive in the middle latitudes, away from the warm tropics where hurricanes form. Meteorologists define “cyclone” as a system a few hundred miles across with a low-pressure center surrounded by winds that flow generally counterclockwise around the center in the Northern Hemisphere and clockwise south of the equator.

Figure 1 is a typical weather map overview of a mature extratropical cyclone.

The “L” is the storm’s low-pressure center. The blue line with blue triangles is a cold front where colder air is replacing warmer air at the surface. The red line with red circles is a warm front where warm air is replacing cooler air at the surface. At the right end of the warm front you see a blue line and triangle, which shows the warm front has become a stationary front with neither cold nor warm air advancing.

The black lines show the general, counterclockwise flow of winds around the system. The green area indicates precipitation, which could be rain, snow, or a mixture of rain, snow, and ice. The darker green area shows where heavier precipitation is falling.

A STORM IS BORN

Extratropical storms often form along stationary fronts—where a kink or wave forms along the front, often with the help of an upper-air disturbance that encourages air to rise from the surface. As air rises, some of the surrounding air flows in to replace it. Earth’s rotation causes this moving air to begin turning in a counter-clockwise direction in the Northern Hemisphere; the opposite way south of the equator. As air flows into the low-pressure center, it rises, and more air flows in to replace it.

Figure 2 is a three-dimensional view showing how an extratropical cyclone begins forming as the warm air on the right flows over the heavier cold air. At the same time the cold air begins to wedge under the warm air, lifting it.

As all of this is going on, the entire system is moving across the countryside with the atmosphere’s general west-to-east flow.

At its height an extratropical cyclone has distinct cold and warm fronts, which can make the storm quite large. If the storm’s center is over the Midwest, the cold front can stretch south across Kentucky, western Tennessee, and Mississippi to the Gulf of Mexico. Strong thunderstorms can occur in the South while snow showers, followed by bitter cold air, are falling on the Midwest. Figure 3 represents a storm at this stage.

The warm front can stretch eastward to the Appalachians with widespread low clouds, rain, freezing rain, sleet, and snow stretching to the north across the Great Lakes.

Most of the time an extratropical cyclone’s most violent surface weather occurs along the cold front, but the warm front often causes a widespread area of weather with solid clouds with a widespread area of minimum visibilities.

An extratropical cyclone reaches its final phase and begins to fade away when an occluded front forms. Figure 4 shows an occluded front with the violet line with triangles and half circles between the cold front and the storm’s center.

MORE TO STORMS THAN YOU MIGHT THINK

Unless you take college-level courses in meteorology, you could end up thinking that a storm’s fronts present the greatest concern for pilots.

This is because the FAA’s sources of training materials, such as the “Weather Theory” chapter in The Pilot’s Handbook of Aviation Knowledge, focus on the fronts with little or no mention of other parts of extratropical cyclones.

An ordinary weather briefing would tell you that widespread clouds are occurring and expected to continue over the large area north of a storm’s center, and you might think it should be easy to handle, especially if you have an instrument rating.

Don’t count on it. 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.

When an extratropical cyclone approaches, meteorologists might be looking forward to a couple of days of “interesting” weather. On the other hand, the last thing a pilot wants to hear from a weather briefer is, “Looks like you’ll have some interesting weather.”