March 25, 2013
BY JACK WILLIAMS (From Flight Training, May 1996.)
Meteorologists use the war-like term fronts for one of the atmosphere's key weather makers, because Norwegian scientists developed the theory of fronts during World War I when headlines screamed about battles along the Western Front. The name seemed appropriate for the zone between contrasting air masses.
As one air mass displaces another, the "battle" creates clouds, precipitation, changes in air pressure, and shifts in wind direction. This battle is sometimes mild and other times ferocious. Either way, pilots who expect to fly across the lines should find out more about what's going on now and what's likely to be going on later.
Figuring out what might happen requires an understanding of what normally goes on across various kinds of fronts. It also requires an understanding of how fronts fit into the big weather picture. First, let's clear up a common misunderstanding.
When arctic air pushes southward into the United States during January and February, you might hear about a "cold front bringing bitter weather." Actually, it's not the cold front that brings the bitter weather; it's masses of cold air. The cold front is merely the boundary between that bitter air and the milder air it pushed out.
Similarly, a warm front is the boundary between warm air and the colder air it's replacing. A stationary front is the unmoving boundary between cold and warm air, a standoff in the battle of air masses. An occluded front is a zone where warm, cool, and cold air come together to form boundaries at the surface and aloft.
Air masses are characterized by their densities. A warm, humid air mass is less dense than the cold, dry mass. Fronts form because air masses of different densities don't easily mix; one will displace the other, with the lighter air flowing over the heavier air.
Air masses form when air stays over polar or tropical regions-either land or sea-for several days, allowing the air to take on the characteristics of the surface. When the air stays over polar land, especially in winter, the air becomes cold and dry. Air masses from polar oceans are warmer and more humid. Tropical oceans create warm, humid air masses, while those from deserts are hot and dry. Global wind patterns eventually set the air masses in motion, with polar air moving toward the tropics and tropical air moving toward the poles. They come into conflict, creating fronts over the middle latitudes, which include the United States except Hawaii in the tropics and northern Alaska in the Arctic.
Weather fronts are characterized by: relatively large temperature changes over short distances; differences in humidity, most easily seen on weather maps by the dew point; shifts in wind direction on either side of the front; a trough of low air pressure along the front; and clouds and precipitation, unless the air on both sides of the front is very dry.
The temperature changes that define a front are relative. A winter cold front could be replacing 40? air at the surface with 15? air. A 75? summer cold front might displace 90? air. In summer the humidity change might be the most noticeable indication of cold-front movement. Polar air in July is not all that cool, but it is much less humid than tropical air from the Gulf of Mexico, so the "cold front" can really be a "lower humidity front."
The winds ahead of a cold front generally blow from the southwest. As the front passes, they switch to the northwest, and then to a more northerly direction. Cold fronts often create thunderstorms, which have their own wind hazards. Cold-front winds can be a major aviation hazard with turbulence and sudden changes in direction and speed. Warm-front winds are usually weaker. As a front moves through an area, air pressure falls and then rises as the front passes. This means the altimeter setting is changing, and pilots who don't use the current local setting could be closer to the ground than the altimeter says.
Clouds and precipitation are also frontal hazards. Cold fronts have steeper slopes and tend to push warm air up more violently than warm fronts. This helps create thunderstorms and showers ahead of and along the front. Pre-frontal squall lines are lines of strong thunderstorms that can be 100 or more miles ahead of cold fronts. Sometimes the sky will quickly clear after a cold front passes; other times the clouds linger behind the front. Pilots need to find out what's going on ahead of and behind the front, not just along the front itself.
If you plan to fly from the warm-air side of the front to the cold-air side, your best bet might be to stay on the ground until the front passes, if it's creating violent weather. If the front's weather is relatively calm, you should be ready to compensate for the southwest to northwest wind shift when you cross the front. While warm fronts usually do not produce thunderstorms, they often bring widespread low visibility that can last for a day or two.
If you are planning to cross the front, into the cold air, you and your airplane had better be instrument rated. Even then, the front might offer too much of a challenge.
Weather maps indicate that the front is located at the surface warm-cold boundary, shown with the warm front symbol. The frontal weather extends more than 700 miles ahead of the frontal boundary. On a winter flight, you might see cirrus clouds at 25,000 feet in the warmer air aloft. At the surface, the air is below freezing.
As you fly southward, the clouds become thicker and lower. After flying about 300 miles, the clouds have thickened to block the sun, their bottoms are only 5,000 or 6,000 feet above the ground, and it's snowing. As the clouds ahead become lower and thicker you should start worrying about icing.
Knowledgeable instrument-rated pilots flying properly equipped airplanes might continue at this point. As the aircraft approaches the surface front, the altimeter setting becomes lower as air pressure at the surface drops. South of the surface front, the sky begins to clear and the air is warmer. The dew point is higher than it was north of the front, and the wind is from the south or southwest.
In spring and summer, the icing danger decreases, but thunderstorms can be hidden in the widespread clouds north of the front. Such embedded thunderstorms make instrument flight hazardous because pilots can unknowingly fly into them-until the storm's turbulence begins tossing the aircraft around. Stationary fronts can cause widespread low ceilings and visibilities with precipitation, like that caused by warm fronts. But the foul weather can last for days as the front stays in place instead of moving along.
Fronts are parts of large extratropical cyclones for part or all of their lives. These cyclones are the storms that move across the U.S. In addition to the potentially dangerous frontal weather, the storm stirs up a large area of clouds and precipitation around it. The storm's center is an area of low atmospheric pressure where air is slowly rising, condensing the humidity into clouds and precipitation.
The fronts are in troughs of low pressure extending from the central low. (Air masses are like mountains, and fronts are valleys of low pressure.) The storm system is moving generally eastward across the country, and the fronts are rotating around the center. As a storm moves into its final stages, an occluded front often forms in the position shown in Figure 4, between the storm center and the cold and warm fronts.
Some textbooks say that occluded fronts form when "fast-moving cold fronts catch up with slower warm fronts." Researchers have now concluded that this rarely happens. Occluded fronts seem to form when the cold and warm fronts break off from the low center, and the center redevelops in the colder air to the north. For pilots, the key consideration is that occluded fronts can bring the bad weather associated with cold and warm fronts-thunderstorms can be embedded in widespread areas of clouds.
The features of fronts and storms described here apply best to weather east of the Rocky Mountains. Air masses moving into the West Coast from over the Pacific Ocean have traveled for hundreds of miles over the relatively mild ocean. As the leading edge of the air mass moves ashore, the temperature contrast might be small. Also, the mountains in the western third of the United States distort wind patterns, which means frontal-passage wind shifts might not be as clear-cut as they are east of the Rockies.
Having a general idea of what's going on along a front will help any pilot make better decisions about flying into the battle zone. The general picture will help pilots mentally organize briefing information about actual and forecast weather. The aim is to avoid blundering into the weather's war zone because you didn't know or understand that a battle is going on along your intended route of flight.
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