The Weather Never Sleeps

Weather On A Mesoscale

Over the last couple of decades scientists have put a lot of effort into learning more about what they call mesoscale weather, which turns out to be the kind of weather that can cause pilots a lot of trouble. Their findings have added terms such as mesoscale convective systems, mesoscale convective complexes, and derecho to the language of meteorologists.

Pilots who learned about weather features such as airmass thunderstorms and occluded fronts, but not meso anythings, during their training are likely to hear some of the new terms during weather broadcasts or briefings. When this happens, they might wonder if the weather being described to them is occurring on the same planet as the weather they studied back in flight school. It is. The problem is that a lot of weather training hasn't caught up with what meteorologists are talking about as we move into a new century.

You can think of mesoscale weather as referring to mid-sized events that affect an area from a few miles to 200 or 300 miles across.

Before meteorologists begin turning more attention to mesoscale weather in the 1970s and 1980s, the focus was mainly on what's called synoptic scale weather. This is the weather reflected on weather maps that show all or most of the United States with areas of high and low pressure and fronts of various kinds. At the other end of the scale, we have microscale events, such as a single shower or ordinary thunderstorm, although the term microscale is not used very much.

Synoptic scale weather is important because it determines the general kind of weather that large areas are going to experience. For instance, the warm sector of a storm system that's moving across the country could be as large as the entire Southeast. (A warm sector is defined by a warm front - where warm air is advancing - to the north and a cold front - where cold air is moving in - to the west or northwest.) In summer, the area within the warm sector will be hot, humid, and probably hazy. The part of the country to the north and west of the cold front, on the other hand, is likely to be cooler and much less humid. You can think of synoptic scale events as those that determine what kind of weather is possible. For instance, places in the warm sector could have thunderstorms, while showers or thunder are unlikely to exist in the region to the west and north of the cold front.

Microscale weather is also important, especially when the microscale event is a thunderstorm on the approach path to the runway where you want to land.

But, if that single thunderstorm is part of a 300-mile-long squall line, it's part of a mesoscale phenomenon. In that case, the mesoscale weather will affect lots of pilots, including those who are forced to make long detours around the line of thunderstorms.

Of course there's nothing new about mesoscale weather. Squall lines and other mesoscale events have been occurring since weather first began forming on a young Earth. And pilots have had to cope with mesoscale weather since they began flying in anything but clear, calm weather early in the twentieth century.

But from the beginning of organized weather forecasting and the production of daily weather maps in the last quarter of the nineteenth century, the focus was primarily on synoptic scale weather because the network of weather stations wasn't fine enough to catch smaller weather events. That is, weather stations that observed the weather were generally so far apart that their reports didn't capture the details of the weather.

Until weather satellites began sending back images in the 1960s, meteorologists had no way of seeing what was going on between weather stations, except for the relatively small amount of information provided by aviators who radioed in pilot reports of the weather.

Along with weather satellites, the 1970s and 1980s brought improved weather radar; relatively inexpensive, automated weather stations; and computers that kept the flood of new data from drowning weather scientists and forecasters.

As more data became available, the realization grew that middle size weather events, that is mesoscale events, were the ones that generally had the most dramatic effects on people, including pilots.

Of course, most people, including pilots, can avoid the most dangerous weather by heeding warnings and alerts without ever having a clue what a mesoscale event is. Still, knowing some of the new mesoscale meteorology terms will help pilots better understand what forecasts really mean. Besides, when the weather is doing its best to frustrate your plans or make your life more exciting than you really want it to be, there's some comfort in understanding what's happening.

During the warm season, from the spring through summer and into the fall, most middle-sized weather events fall into the general classification of mesoscale convective systems. As we have seen, the term mesoscale tells us that the phenomenon is from a few miles to maybe a few hundred miles across. Convective tells us that the up-and-down air motions associated with thunderstorms and known as convection are involved. System is used because the thunderstorms that make up the event aren't scattered willy nilly across the countryside but are organized into a system of some kind.

Squall lines are the most common kind of mesoscale convective system, and they are also the kind that over the years has caused as much pilot grief as any other type of warm season weather.

In simple terms, a squall line is a line of thunderstorms; the strongest are usually well ahead of an advancing cold front and can draw in the warm, humid air needed to feed thunderstorms at both the front and the rear of the advancing line. All of the hazards associated with thunderstorms are found in squall lines, of course. These include severe turbulence, icing, hail, and lightning. One danger is that what appear to be gaps between storms in the line can catch unwary pilots who try to slip in between individual thunderstorms, rather than going around the entire line. A new storm can grow so quickly that the airplane is no longer in a clear area between storms but in the middle of a growing thunderstorm.

Since squall line thunderstorms can grow taller than 50,000 feet, especially in the central United States, flying over them isn't an option, even for jet air carriers. Landing to wait out the storms or flying around the end - staying far away from the storm on the end of the line - are often the only options. Sometimes, even flying around a squall line isn't a real option for a small general aviation airplane, since a squall line may stretch from the Ohio Valley well into the Gulf Coast states.

Sometimes a television meteorologist or a briefer might refer to a squall line having a bow echo. This refers to a pattern seen on weather radar that shows a squall line that is arched or bowed out in one area. Such bow echoes are usually a sign of strong winds, including downbursts, which are winds that blast down from thunderstorms.

If you do land to wait out a storm, finding a hanger to shelter your airplane or at least making sure you tie it down well are good ideas. While squall lines rarely produce strong tornadoes, they often blast the ground with strong straight-line winds. In fact, meteorologists have a special name for squall lines that produce especially damaging and widespread winds - derecho. The term, which is pronounced day-RAY-cho, was coined in the 1880s by Gustavus Hinrichs, who was director of the Iowa Weather Service, from a Spanish word for straight ahead or direct. He wanted the term to contrast with tornado, which comes from a word that means, among other things, to turn. To qualify as a derecho, a storm must have winds stronger than 58 mph, and its damage has to cover an area at least 280 miles long.

Assuming a derecho or the winds of a bow echo don't rip your airplane from its tiedowns, your problems might not be over even after the thunderstorms have moved past. Squall lines often produce wide areas of rain and poor visibility in low, flat clouds behind the thunderstorms. While the air might now be relatively smooth and thunderstorm hazards far away, you might not have VFR weather conditions for a few hours.

Squall lines aren't the only type of mesoscale convective systems; meteorologists classify tropical storms and hurricanes as mesoscale convective systems as well, but these storms have their own forecasting center - the National Hurricane Center, which is part of the National Weather Service. This center, with the help of the news media in places where hurricanes hit, does a good job of making sure these storms don't sneak up on anyone. The most likely way for a pilot to be caught by a tropical storm or hurricane is by waiting until it's too late before trying to fly an airplane to safety as a storm approaches.

The time when a hurricane is forecast to hit usually refers to when the storm's eye, or center, is expected to move ashore. But heavy rain and strong, gusty winds that only grow stronger are likely to hit hours before the center arrives. Today, Hurricane Center forecasters and local news media try to make this point clear: The weather is likely to be too dangerous to try to drive, much less fly, long before the storm center arrives.

One kind of mesoscale weather that is likely to catch pilots unaware is the mesoscale convective complex, or MCC. By definition, an MCC is a nearly circular cluster of thunderstorms with an area of cloud top temperatures colder than minus 26 degrees F that covers an area of about 39,000 square miles - almost the size of Ohio. Infrared images from satellites measure the cloud-top temperatures. The colder the cloud tops, the higher the clouds, and cloud height in thunderstorms is a good measure of the strength of the thunderstorms.

MCCs usually begin forming from disorganized clusters of thunderstorms in the late afternoon and reach their peak in the middle of the night. They then normally die down sometime around dawn, but by then the system has generally developed a swirl of wind in the upper atmosphere, which continues long after individual thunderstorms in the system weaken and die. This swirl, or vortex, continues moving generally toward the east and can trigger a new MCC the following evening.

MCCs are most common on the Plains, often forming just east of the Rockies and moving generally eastward. But they do occur in other parts of the country. As a rule, MCCs don't bring large tornadoes or exceptionally strong winds, although just about any thunderstorm can spin out a weak tornado or kick up dangerous winds.

But MCCs do tend to bring heavy rain, which can cause flooding. Scientists estimate that MCCs bring the Plains more than half of the rain that falls there in the summer, which is good news for farmers but not always for pilots. MCCs are also noted for their widespread, vivid displays of lightning.

One of the major threats they pose to pilots is widespread areas of poor visibility and low ceilings, especially at night when flying in such conditions is most dangerous. Today, with the growing understanding of how MCCs form, move, and grow, aviation weather forecasters take them into account. In fact, you will sometimes see MCCs mentioned in the synopsis part of area forecasts.

Entire books have been written on mesoscale weather, but don't go looking for one with the hope of learning more about the subject unless you did well in college physics and calculus and have a pretty good knowledge of academic meteorology. Pilots don't need to delve too deeply into this topic. Learning the basics about mid-sized weather, however, will make it easier to understand what's going on in the sky around you when you fly.