The Weather Never Sleeps

What They Never Told You About Storms And Fronts

Unlike textbook flying, our actual flying experience finds that storms and fronts exist in a three-dimensional atmosphere and are much more complex than those shown on TV. Without all the facts, you could be surprised by turbulent winds and dangerous thunderstorms in places you wouldn't expect based on textbook knowledge.

Weather training videos and books, TV newscasts, and many newspaper weather maps make storms look pretty simple. They'll show a low-pressure center marked with an L, with a warm front and a cold front extending from the center. Pilots learn to watch for thunderstorms, gusty winds, and turbulence along cold fronts. Ahead of warm fronts, you should expect IFR conditions with low clouds, haze, and widespread rain or, in the colder months, snow. If you're away from the low's center and fronts, it appears you probably won't have to worry too much about the weather.

The "Old School"

Chapters on weather in most aviation manuals, and even many college-level meteorology textbooks, use the "Norwegian cyclone model" to explain mid-latitude storms. The model places the storm's center where its lowest air pressure is found. A warm front and cold front pinwheel outward from this center.

Scientists in Bergen, Norway, created this basic storm model during World War I, when fighting was in the news. They likened the "fronts" to battle lines because they marked the edges of advancing and retreating cold air. Fronts are typically described as wedges that extend continuously from the ground to lofty altitudes, separating warm air from cold. These moving wedges boost surface air upward, forming clouds and thunderstorms that can drop rain and snow.

The Norwegian cyclone model is a good start to recognizing the players in mid-latitude storms. The basis of this model is the spin-up of a wave of low pressure along a stationary frontal boundary. Cold air north of this boundary pushes southward as warm air to the south tries to plow north. As the air masses battle, the Earth's rotation sets them into a slow dance counterclockwise (in the Northern Hemisphere) about a central point. Air from both sides swirls into the developing system's center where it rises.

Strong, high-altitude winds blowing over the surface center whisk the rising air away, leaving room near the ground for more air to swirl in and rise. But, the winds aloft are much stronger than those near the ground which pulls more air away from the whirling center than enters it from all sides, causing the surface pressure to fall. A storm is born.

The storm feeds off the temperature difference between the two contrasting air masses. The entire process is known as "polar front theory" in which cold polar air will not sit comfortably next to warm air for very long without a storm spinning up.

What's Really Going On

Many books and training manuals describing weather limit their discussions to these basics. The focus for pilots flying near and around mid-latitude storms is "along" the cold front and "along and ahead" of the warm front, where hazardous conditions are said to occur.

As many pilots have discovered, sometimes to their sorrow, thunderstorms often form hundreds of miles ahead of a cold front. Marching in long lines that can stretch 500 miles or more, they can force pilots to seek alternate routes or land to wait out the weather.

It doesn't stop there. Strong winds blowing into the larger storm system can create severe turbulence in clear air behind the lines of thunderstorms. Ahead of the storm's warm front, pilots are routinely told they can fly smoothly above low-topped stratus clouds. But mid-latitude storms also can spawn thunder clouds rising to high altitudes above the stratiform cloud deck. Turbulence and dangerous icing are likely in this area that's supposed to offer smooth cruising.

During the last decade, meteorologists in the U.S. began looking more closely at the different pieces that come together to form large storms capable of stirring up these "unusual" weather hazards. They found patterns within the mix of air masses and winds that are leading them to question the basics of polar front theory.

So, why were the Norwegians so far off base? From what these early scientists observed, they really weren't. The Norwegians based their cyclone model on observations of North Atlantic storms as they moved into northern Europe. Their ideas translate well to swirling systems that barrel across the ocean before coming ashore. But the storms that blow up over the central United States east of the Rocky Mountains take on entirely new characteristics that make them more complex.

For starters, it's becoming increasingly clear that cold fronts are not simply wedges that plow along as solid walls. Instead, they often become detached aloft from what's going on near the ground. The upper-level front can outrun its surface partner, racing away to trigger clouds and precipitation, typically in the form of big thunderstorms, on its own.

Scientists studying these recently identified weather features call them "cold fronts aloft." To understand how these and other features of mid-latitude storms work, we'll look closely at a storm system's evolution.

Let's introduce an area of low pressure developing east of the Rockies. Warm air from the south is pushing northward while cold air drives southward, just as in textbook polar front theory. Dry air sweeping down the eastern slope of the Rockies, however, introduces a third player: A front separating dry air to the west from humid air to the east forms between the warm front and cold front. Many meteorology books call this boundary a dry front or "dryline." Meteorologists using polar front theory often mislabel it as a cold front.

Warm, humid winds plow north at low levels in the atmosphere ahead of the dryline. A moist, low-level jet stream of fast-flowing air typically forms at about 5,000 feet, helping to drive warmth and humidity into the growing storm.

These low-level winds can create a bumpy ride for aircraft climbing into them. When they reach the warm front, the humid winds lift, forming low-topped stratus clouds that drop rain and snow north of the warm front. Some of the humid air wraps around the low center, stirring up freezing rain and snow behind the surface cold front. Icing can be a major problem northwest of the low center. Dry air from the Rockies between 5,000 feet and 10,000 feet blows ahead of the surface dryline and rises above the moist, low-level winds. The dry air over humid air creates instability in the atmosphere, and bands of rain and thunderstorms blossom along the dry front aloft. The heaviest rain band forms right along the upper-air dry front and moves steadily east and north, catching up to the warm front and overtaking it. These towering storm clouds riding above the warm front's stratus deck produce bursts of rain and snow, with lightning and thunder.

Pilots flying above the stratus deck are likely to encounter moderate to heavy turbulence and icing in air that training manuals say should be stable. While the dry front aloft spreads storms ahead of the surface dry front, the storm's cold front begins sweeping around the storm on its western side. The surface cold front tries to keep up with the cold front aloft as both move out of the Rockies. But the air behind the surface front compresses and warms as it blows down the mountains, washing out this boundary near the ground. The cold front aloft, meanwhile, moves steadily eastward and catches up with the surface dry front.

A combination of forces then sets the stage for violent thunderstorms. Air converging along the surface dry front is forced to rise. As it does this, it moves into the nose of the cold front aloft where it is now warmer than its environment and becomes buoyant and unstable. As the warm air bubbles into the colder air it gets an extra hard shove upward by convergence along the cold front aloft. Cloud towers rapidly form and grow into lines of thunderstorms within the nose of the cold front aloft, which has pushed ahead of the surface dry front. The resulting squall lines can grow several hundred miles long while racing east with the cold front aloft.

Flying Around Storm Systems Safely

With the idea in mind that models of storms might be further refined as scientists learn more about how these mammoth weather systems evolve, pilots can use the current knowledge to fly more safely.

The key to deciding whether you should apply the new model to the latest weather situation is in finding the cold front aloft. If it exists, then it's likely the storm system will have features much different from what the Norwegians described more than 80 years ago. A look at a standard surface weather map and a satellite or radar image will probably work best for locating the cold front aloft.

Find the cold front labeled at the surface. Then check the satellite photo or radar image to see whether the main area of rain and thunderstorms is well ahead of the surface cold front, or a cloud band is located ahead of the surface front shown on the map. If either of these is apparent, it's possible that a cold front aloft is responsible. This could alert you to a storm system you should be wary of since it may not behave the way your training has led you to expect.

Models of the weather, like the one the Norwegians created 80 years ago, help us make sense of the complicated atmosphere. Because you fly in the real atmosphere and not a model, you have to be aware of when the model could be misleading. Understanding some of the ways your mental model could go wrong will help you avoid trouble.

One of the biggest challenges of flying is navigating safely around and through storms. Clearly understanding what's going on inside these sprawling weather systems can help you avoid their hazards.