WX BRIEF

Clues to changing weather

Pilots can stack the odds in their favor by watching simple signs.

By Karsten Shein
Comm-Inst, Climate Scientist


NetJets First Officer Randall Henderson looks up at the sky to get a reality check of the weather before departing ANB (Anniston AL). Spending a moment to take in and interpret the weather around you can be just as important as looking at weather maps.

You'd be hard pressed to find a pilot who isn't the least bit interested in the weather. In fact, most pilots know quite a bit about the atmosphere and some even know enough that they'd qualify as meteorologists.

The weather is such a part of our jobs that we spend a fair bit of time learning how to interpret what all the symbols on the weather maps mean, and we can track our way around a developing cell on our radar like nobody's business.

Most pilots would probably categorize themselves as fairly weather-savvy. However, there's a lot more to aviation weather than maps and instruments. An equally important part of flying the weather is the ability to interpret the clues the sky tosses your way.

Back in the day of the open cockpit and silk scarf, all a pilot had to rely on was a few rudimentary weather instruments at the airport and a solid intuition of what the current conditions were telling him.

Those pilots relied on a lot of the same weather knowledge that generations before them either on the high seas or herding sheep in a meadow had gleaned from experience. Unfortunately, while we inevitably glance up at the sky before takeoff (it's pilot genetics we can't avoid it), we tend to have replaced these ancient traditions of atmospheric understanding with a head down reliance on technology to tell us what we need to know.

Reasonably, one can't expect to be able to look at the sky and estimate what the weather 1000 miles away will be doing. But, likewise, a weather map will give you only a vague generalization of the weather in your vicinity.

In the cockpit, a lightning detector or radar scope will do a great job of showing you where you might find active thunderstorms embedded in a thick cloud bank, but most pilots would probably agree that seeing a windscreen full of blackish green cumuli with anvil tops illuminated by frequent lightning flashes is a far more direct indication of bad weather ahead than any alert your onboard technology can throw at you.

You and your environment

Our ancestors often derived their knowledge of the weather without the benefit of any basis in atmospheric physics. Take for example the oft-repeated weather saying (and its variants), "Red sky in morning, take warning-red sky at night, take delight."

Today we know that the middle latitudes, where this saying developed, sit beneath a westerly flow of the atmosphere. Weather systems tend to move from west to east, which means that a red sky in the morning is due to the long (red) wavelengths of sunlight at sunrise traveling through clear easterly skies to reflect off westerly clouds of an approaching storm system.

Conversely, the reds of a setting sun in the west pass through clear air to bounce off the clouds of a departing system to the east.

Folks in the 17th century only knew that most of the time when they had a very red sunrise, they got a storm later that day, and if they had a red sunset, the next day's weather was sunny.

Such simple cause-and-effect generalizations don't make them any less valid than the scientific explanation. Some other nuggets of meteorological knowledge are embedded in weather folklore that every pilot should know.

"Ring around the moon, rain comes soon" is another common saying. Halos and coronae around the moon are common when high-level cirrus or cirrostratus clouds are present. These clouds indicate the presence of high-altitude moisture and are also common precursors of frontal systems.

Scoping things out

Growing up, I had a weather rock. It was an amazing piece of meteorological instrumentation. The instructions directed me to hang it from a string on the branch of a tree outside my window and then observe it.

While this fog shrouding the Space Shuttle Challenger on its way toward its launch pad at Cape Canaveral FL is thick enough to be visible from the air, most fog is too shallow to perceive directly. However, its presence can be identified by muted colors and distant objects that appear slightly out of focus.

If it was swinging back and forth, it was windy. If the rock was wet, it was raining. If it was white, it had snowed. If you couldn't see the rock, it was night. And, if the rock and the tree were gone, you'd had a tornado.

In all the time I had that weather rock, it was never wrong. The next time you preflight your aircraft, take a moment to look up at the sky. Scan it in a slow 360° panoramic view. Stand for just a minute and turn to feel the wind, or lack thereof, on your face.

Become the weather rock. Of course, the weather rock is a tongue in cheek novelty. While technically it does tell you something about the weather at least in a comedic sense it doesn't forecast anything.

Taking a look at your atmospheric environment, on the other hand, not only gives you a good sense of the current conditions it may also reveal a great deal of how the weather is shaping up. In short, unlike the rock, you can interpret the weather signs to come up with relatively accurate short-term forecasting.

For example, let's start with clouds. They represent a visible manifestation of moisture and convection in the atmosphere, and are a great indicator of weather conditions. Low cumulus clouds generally indicate light thermal turbulence with the surface layer.

High cirrus normally accompanies strong winds aloft and the potential for turbulence at cruise altitudes. In addition, the wispier the cirrus, the more likely the shear aloft will be more than a light chop.

In the middle latitudes, a high cloud deck that is lower to the west is a pretty good announcement of an approaching weather system, while a solid stratus deck is an indicator that you are in close proximity to either a warm or stationary front.

This is especially true if the clouds to your south have lower bases. Heavy weather is even more likely if it is spring or fall and you are sitting in a warm and muggy atmosphere with a cloud bank approaching from the west.

An absence of clouds is just as significant. Clear skies are generally found beneath high pressure systems, which can mean good flying conditions for the next few days, although surface heating may lead to convective turbulence.

In the tropics, the tradewinds dictate that you should look to the east for clues about approaching weather. Normally, conditions in these latitudes are relatively benign, but steady heating of the ubiquitous ocean surface quite often gives way to towering cumuli by noon.

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