April 1, 2003
By Thomas A. Horne
Sure, you've been briefed by flight service, and yes, you checked your favorite Internet sites for preflight weather information. But it's only when you step outside and immerse yourself in the feel of the weather that your preflight data feed meets reality. Briefers may have mentioned a cold frontal passage, for example. But standing on the ramp, can you tell? Do you have the weather smarts to confirm or deny the official word? Do you understand when forecasts are likely to be the most accurate — and the most iffy? Being able to answer yes means you're a savvier pilot — one who can think ahead of the weather. Here are a few kernels of wisdom that have stood the test of time:
Strong winds equal turbulence. Nobody likes turbulence, but you can expect it if surface winds are up beyond 20 knots or so. Gusty, shifting winds almost guarantee a bout with moderate turbulence in the lower levels of the atmosphere — say, below 7,000 or 8,000 feet msl. There's also the challenge of wind shear and crosswinds on final approach, so be prepared if you decide to launch for some practice in the pattern.
Flat cloud bases mean a wild ride. After a cold front moves through you'll typically see two markers. One is a shift in the wind direction from the southerly quadrants of the compass to west or northwesterly directions. The presence of a shallow layer of flat-based cumulus clouds at about 4,000 feet agl is another. Those flat bases are another turbulence tip-off. They're caused by the shearing forces of strong, low-level winds. Below the bases, you'll be in the bumps, but if you can safely climb to on-top conditions a smooth ride awaits.
Beware a strong southerly flow. If surface winds are out of the southerly points of the compass and temperatures and dew points are high, be on guard. You know the setup — fast-moving cumulus clouds ripping along at low altitude, bringing remarkably warm and humid conditions with them. This is a danger sign! A violent, fast-moving cold front may well be on the way. Thunderstorms — in single cells or squall lines — could occur in this warm flow of air. The speed of those clouds could even indicate that a low-level jet stream is at work, with wind speeds of 50 knots or more at altitudes as low as 3,000 feet agl. Whether you're a VFR-only pilot or a seasoned pro, this is a time to sit it out and wait until the pre-frontal winds, storms, and turbulence have passed.
No sun, no morning fog burn off. Radiation fog occurs when nighttime temperatures fall to the dew point. The fog won't burn off until the heat of the day raises temperatures and widens the temperature-dew point spread. If there's little in the way of cloud cover, the fog should burn off by mid-morning. But if the fog is especially dense, or trapped beneath a cloud deck, the sun can't as efficiently shine through and warm the surface. This means the fog won't burn off until later in the day — perhaps much later.
Slow-moving means widespread. If flight service mentions marginal VFR weather or instrument meteorological conditions (IMC) associated with a stationary front along your route, or a slow-moving cold or warm front, don't expect conditions to improve anytime soon. Slow-moving fronts won't budge until a more powerful front or upper-air disturbance comes along, and that may be a day or two. In the meantime, adverse weather can spread across an ever-widening area as slow-moving systems continue to draw in moisture.
Low IMC can be slow to lift. How slow depends on a wide range of variables, so a precise time doesn't come easily. If a frontal passage involving drier air is imminent, then forecasters can predict with fair accuracy when ceilings and visibilities will lift. But if your departure or destination airport is near a large body of water, or surrounded by a slow-moving weather system and no fronts are near, it's a tougher shot to call. Small changes in temperature or cloud moisture can make ceilings fluctuate within the IMC range (ceilings to 1,000 feet agl, visibilities to three statute miles), but predict a clean break to good VFR weather? That would make the most experienced meteorologist nervous.
Thunderstorm forecasts are general. Flight service has predicted thunderstorms, but everything looks fine as you perform preflight checks on your airplane. Is it time to rant against forecasters? I don't think so. Thunderstorms are often predicted in generalities — areas that might experience convective activity (the convective outlook, or AC), and areas defined as zones where severe thunderstorms may occur in the next 48 hours (the severe-weather outlook chart). It's impossible for a morning prediction to pinpoint where storm cells pill form, let alone describe how intense or widespread they may become, or predict their direction of movement.
Later in the day it's a different story. Puffy cumulus can grow to towering cumulus, then graduate to full-blown cumulonimbus status. As soon as the first convective activity occurs, the nation's network of weather radars (plus some new forecast tools) can give very accurate descriptions of when and where storms will pop up, and where they are headed. This short-term accuracy is one big reason to stay in touch with flight watch (122.0 MHz); its whole purpose is to warn of short-term threats.
En route, keep your eyes peeled. Now that we're moving into thunderstorm season, remember that a good lookout is your best defense against an unwelcome weather encounter. I like to tell pilots (VFR and IFR) to climb to on-top conditions for the en route phase of a long cross-country flight. It's smoother up there (though you may pay a groundspeed penalty for headwinds), and you can spot buildups long before they show up on anyone's weather radar. If cells grow, you can come up with an avoidance plan quickly.
What's that? You say you can't top the clouds? Then that's a warning all by itself. It means that your locale is coming under convective influences already. It's time to check with flight watch for the latest radar returns, and perhaps make a 180-degree turn and descend to VFR conditions below the clouds — where you can see and avoid any rain shafts or other signs (e.g., dark cloud bases) of thunderstorm cells.
There are plenty of other rules of thumb like these, but as we move out of the cooler months and into the thunderstorm season the ones listed above should hold you in good stead in the changeable weeks ahead.
E-mail the author at firstname.lastname@example.org.
AOPA Pilot Editor at Large Tom Horne has worked at AOPA since the early 1980s. He began flying in 1975 and has an airline transport pilot and flight instructor certificates. He’s flown everything from ultralights to Gulfstreams and ferried numerous piston airplanes across the Atlantic.
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