On the move

Let’s look at the bigger picture and approach weather formation from a different viewpoint. Think of the weather as a pyramid of powerful forces. At the top are the planetary, or global, forces that initiate all weather. This includes the rotation of the Earth’s axis, which is inclined at 23 1/2 degrees. It’s this inclination that gives us the seasons as the Earth orbits the sun.

Jet streams are another global-scale factor. These can be thousands of miles long and their patterns can last as long as a day or more. These undulating rivers of high-altitude, high-speed winds (think 100 to 200 knots) divide polar air from the warmer air at lower altitudes. Where those undulations dip southward, heights of pressure levels between cold and warmer air can create large-scale troughs—areas of lower atmospheric pressure. As parcels of air round a trough, the pressure differential decreases. This creates divergent motions at high altitude, which in turn causes air to converge near the surface—and diverge at altitude. Presto! A surface low is born.

Surface low pressure systems act on what’s called a synoptic scale, and their associated cold and warm fronts can be as long as 500 to 6,000 nautical miles on average and last for days. Hurricanes and tropical storms also fit in the synoptic category.

Of course, the circulations within and around fronts and other synoptic-scale scale events can create other, smaller events. Temperature and pressure differences create turbulence as cold air wedges itself under warm air (cold fronts) or warm air overrides colder air (warm front). The deeper the low-pressure systems’ central pressure, the stronger the winds as they flow counterclockwise around them. And the results don’t end there. They create the next-lower scale of atmospheric motions, which can cover 1 to 500 nautical miles and have life spans lasting from minutes to several hours.

Called mesoscale motions, they include thunderstorms in lines or clusters, gust fronts, and squall lines—as well as widespread low-IMC and icing conditions.

Next in the pyramid of motions is the microscale. As the name implies, this would include localized breezes or turbulence. Think of that abrupt jolt you always seem to get on final approach to a familiar airport or when crossing a specific hill. Microscale events are fleeting and cover areas as small as a baseball diamond. Even so, don’t discount them. Tornadoes also count as microscale events.

By now you’re probably wondering what these scales of motion have to do with that curveball in tomorrow’s flying plans.

Here’s the thing: The smaller the scale of motion, the more difficult its weather is to forecast. The global motions are large, last a long time, and thus are easier to track. Synoptic motions change more rapidly and can be less predictable. Mesoscale motions are even more subject to change because their internal circulations are smaller and often put differing, churning air masses in conflict. Microscale motions are so small that pinning their locations down becomes a challenging art.

Numerical weather prediction, with its many computer-generated models, are becoming more accurate. Forecasters compare various models to see what degrees of consensus emerge, examine weather satellite imagery, and discuss trends that signal trouble. It’s said that today’s 10-day forecast accuracy is as good as the five-day accuracy from just a few years ago. Even so, forecasters hedge their bets, knowing that chaotic changes could be right around the corner. That’s why you often see large watch or warning boxes, even though the weather inside them may not appear threatening—for now.

Thomas A. Horne is a former editor at large for AOPA media and the author of Flying America’s Weather.