Hurricane-like weather can happen anywhere
From late fall, through the winter, and into early spring, pilots can become connoisseurs of television weather as they try to figure out whether they should plan on flying in the next day or so.
They quickly learn that when a weather system like the one centered over Illinois on the surface chart below is nearby, the chances are high for precipitation--rain, snow, ice, or a mixture of the three--poor visibility, and dangerous winds.
Atmospheric scientists call such systems extratropical cyclones. TV meteorologists often call them lows, storm systems, or storms. A cyclone is any large-scale weather system with air swirling into an area of low air pressure at the surface and rising to create precipitation. (In some parts of the United States people used to call a tornado a "cyclone," but that use has pretty much died out.)
Extratropical means that the cyclone formed outside of the tropics, over either an ocean or land. Density differences between warm and cold air supply most of the energy for an extratropical cyclone. The denser cold air moves under the lighter warm air and warm air moves over cold air as the earth's rotation gives the moving air a spinning motion.
Tropical cyclones, such as hurricanes, form over the warm water of a tropical or subtropical ocean. Because they draw their energy from warm ocean water, they weaken and die when they move over land or a cold ocean. Unlike extratropical cyclones, tropical cyclones don't have fronts because they are made of warm, humid air. While the air aloft in a tropical cyclone is colder than the air near the surface, it's warmer than the surrounding air at the same altitude.
Since temperature contrasts supply most of the energy for extratropical cyclones, these are strongest in winter because the contrasts are greater. For example, in July the difference between the monthly average temperatures in Churchill, Manitoba, in northern Canada, and Brownsville in southern Texas is 30 degrees Fahrenheit. In January the average difference between the two cities is 77 degrees. In addition to the increased energy available to extratropical cyclones, winter weather also pushes the zone of the greatest temperature contrast further south, which means extratropical storms follow more southerly paths.
The map shows the surface weather at 7 a.m. Eastern on December 19, 2008. The system's center is an area of relatively low atmospheric pressure, labeled with a hard-to-read, red "LOW" over Illinois. It's surrounded by a thin red line, called an isobar, marked 1004--the air pressure in millibars of every point along the line. The pressure inside the circle is lower than 1004 millibars. Between the 1004 line and the next one, the 1012 line, the pressure is between 1004 and 1012 millibars.
The red line with red half circles extending to the east from the storm's center marks the surface location of a warm front where warmer air is moving north, replacing cooler air at the surface. The blue line with blue triangles stretching to the southwest is a cold front where colder air is replacing warmer air at the surface. The cold front over northern New York and central New England is from a similar system that moved across Canada during the previous two days.
The dashed blue line going from the northeast to the southwest across the center of the low-pressure area shows the southernmost extent of freezing temperatures at the surface. The dash-dot line near the Canadian-U.S. border is the southern extent of below-zero surface temperatures. The green areas show where precipitation was falling at the map's time.
Extratropical cyclones travel generally from west to east but they sometimes move almost directly north or south. They also strengthen or weaken, and the fronts can fade away or new fronts form. This means that where a storm was heading and what it was doing yesterday isn't a reliable guide to what it will do today. Fortunately, today's computer-assisted forecasts usually do a good job of predicting where a storm will travel and how strong, or weak, it will become over a few days. Even so, pilots shouldn't take such forecasts as a script for what a storm will do when they plan a flight. Instead they should treat the forecast as a general outline of what the storm is likely to do.
Also, although the main features of such storms are the same, their details can differ greatly. The general picture of most extra-tropical storms includes:
- The storm's center, where air that is spiraling in toward the center, is rising and cooling to form clouds and precipitation.
- A cold front, where advancing cold air is vigorously shoving up the warm air that it's replacing, creating cumulus clouds, showers, and thunderstorms.
- A warm front, where warm air is flowing over colder air, up a gradual slope to form a widespread area of stratus clouds and precipitation.
- Sometimes an occluded front separates warm, cool, and cold air with one of its two boundaries aloft.
The trickiest weather, especially for visual-flight-rules pilots, is in the warm sector--between the cold and warm fronts--where squall lines of thunderstorms can form and quickly grow along or several miles ahead of the cold front. Thunderstorms aren't always found along or ahead of the cold front, however.
For example, as the map shows, no precipitation was falling along the cold front at 7 a.m. But in Paducah, Kentucky, ahead of the front, the wind was blowing at 24 mph and gusting to 40 mph. By the time the cold front moved across Paducah under clear skies at around 3 p.m. the 30-mph winds were gusting to 42 mph.
Weather ahead of a storm's warm front often precludes visual flight rules flying and can threaten instrument-rated pilots with icing and even thunderstorms that are embedded, or hidden, by the surrounding stratus clouds. This weather can last for days. For example, at the time of the map, Washington Dulles International Airport, which the warm front was approaching, had only two miles' visibility in mist, broken clouds 500 feet above the ground, and an overcast at 900 feet. The rest of the day was misty and foggy with on and off light rain. At the same time, Lincoln, Illinois--inside the 1004-millibar circle--had calm wind, an overcast ceiling only 300 feet above the ground, and four miles visibility in mist.
Most discussions of the weather associated with extratropical cyclones focus on what's happening in the frontal zones, which can lead pilots to think they don't have to worry about what's happening on the opposite side of the storm from the fronts. It's not this simple. Some of the air that is riding up over cool air ahead of the warm front is wrapping around the low toward the west, which can create widespread poor visibility and icing in clouds as well as precipitation, as shown by the large green area north of the storm center on the map.
It can also lead to weather surprises. At Chicago O'Hare International Airport at the time of the map, the visibility was less than a mile in light snow and mist, with broken clouds only 1,200 feet above the airport. A half hour later the light snow was falling from a thunderstorm. Such "thundersnow" is relatively rare.
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 for your flight."
Jack Williams, a freelance science writer specializing in weather and climate, is an instrument-rated private pilot. The latest of his six books is The AMS Weather Book: The Ultimate Guide to America's Weather. He answers questions about weather on his Web site.
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