It’s been said—and confirmed, in a conference I attended at the FAA’s Oklahoma City complex a couple of years ago—that you can miss every weather-related question on every FAA Knowledge Test (“written”), from Sport Pilot all the way through and including the ATP, and still pass each test…and ultimately, pass every checkride. Our instructors and aviation periodicals implore us to become students of aviation weather, but only on rare occasions are we actually given the tools we need to make weather-related go/no-go decisions. Certainly one of the most common requests I get from my recurrent flight students is for help in understanding weather well enough to make informed choices that protect their families when they fly. So how can we quickly and methodically sift through page after Internet page of aviation weather data to make informed decisions?
There are six primary aviation weather hazards: thunderstorms, turbulence, reduced visibility near the surface, strong surface winds, carburetor ice and airframe ice. All but one (carburetor ice) affect all aircraft. Including carb icing, all six hazards are especially dangerous to instrument pilots flying on an IFR clearance. Let’s develop a quick, systematic way to make a preflight go/no-go decision to manage the risk of IFR weather.
The primary flight hazard of thunderstorms is turbulence aloft, and low-level wind shear. Secondary but still critical hazards are hail, reduced visibility near the surface from heavy precipitation, and airframe ice. Lightning is the defining characteristic of a thunderstorm, but lightning strikes are at the bottom of the list of threats. To adequately prepare for an IFR flight, evaluate the following:
Radar is probably the first place most pilots look when planning a trip. Radar depicts precipitation. There is a strong correlation between heavy rain and thunderstorms, but you can have thunderstorm activity away from precipitation echoes—one-third of a thunderstorm’s life, the development or updraft stage, occurs by definition before rain begins to fall to the surface. And sometimes heavy radar echoes appear without thunderstorm activity, especially in maritime environments and when the precipitation occurs as snow. Recall also that thunderstorm charts are history…what happened at the time of the observation or the end of the observation loop. Radar charts are a good trend indicator, but only show what has happened, not what might be happening by the time you’re flying over a specific location. Radar, then, is one place to look to get the thunderstorm-hazards picture, but unlike what many may think, it is not the only place.
Convective Sigmets, or significant meteorological statements, report actual thunderstorm activity with a forecast of movement and development.
Weather watchers talk about stability charts and lifted indices, but we can see the result of this specialized knowledge in the plain-language convective outlook. This tells us the likelihood of thunderstorm development for a given geographical area through the forecast period.
When the FAA’s Aviation Weather Center has not issued a Convective Sigmet, but its air route traffic control centers get pilot reports or radar indications of thunderstorms, the center meteorologist may issue a center weather advisory (CWA) giving essentially the same type of warning. The center meteorologist program is succumbing to funding issues, so we may see fewer CWAs than we used to.
The surface analysis is the “weather map” we are used to seeing, depicting the location of pressure systems and fronts. Look for cold fronts and occluded fronts (when a cold front has overtaken a warm front) as likely locations for thunderstorm development at any hour, although frontal storms generally form as the surface heats, meaning they tend to develop late in the day and continue into the evening hours.
Large areas of high pressure also may see thunderstorm development in the heat of the late afternoon, if there is enough moisture present. Knowing (in the northern hemisphere) air flows clockwise around high pressure areas and counterclockwise around lows, look at whether the air is blowing from a large body of water into your route of flight. Hot days and abundant moisture may cause “air mass” thunderstorms to pop up away from frontal zones.
Use the prog charts the same as you do the surface analysis. A “prognostication” is a prediction, or forecast, so the prog chart is a forecast of expected conditions at identified times in the future (actually, the prog contains four future forecast times).
A Metar, or meteorological report, is an observation of conditions at a reporting point, usually an airport. Two things to think about: First, Metars only report conditions in the immediate vicinity of the reporting point. Second, and especially in the Great Plains, it’s not unusual for conditions to remain marginal VFR or even VFR and include a thunderstorm in the Metar reporting area.
The Metar’s counterpart is the TAF, or terminal aerodrome forecast, providing information similar to the Metar but forecast for various times in the future.
There are other aviation weather products providing cues to possible thunderstorm development (for example, the area forecast), but those listed above are mostly pictorial, not textual descriptions (I certainly get much more information out of weather pictures). Together, they provide a good idea of the location of thunderstorms and, more importantly, the possibility thunderstorms may appear where you expect to be, when you expect to be there.
Turbulence aloft can occur in or out of thunderstorm activity, so it’s a weather hazard in its own right. To predict non-thunderstorm turbulence, several weather products may be helpful.
One is the prog chart. Frontal zones tend to create turbulence, especially when close to the low pressure center that spawns them. If the isogonic lines (the lines of equal air pressure between highs and lows) on the chart are bunched up and close together, expect turbulence aloft.
Meanwhile, Sigmet reports not identified as “convective” are issued for non-thunderstorm hazards that currently exist, along with a forecast of expected movement. Sigmets identified by the letter “T” are for turbulence.
Pireps are the most accurate and usable indicators of the intensity of turbulence. Pirep turbulence intensities are defined by the effect the bumps have on the aircraft. Generally the larger the aircraft, the less perceived effect for a given amount of bumpiness. Therefore, when evaluating turbulence Pireps, consider whether the reporting aircraft is larger or smaller than yours. If a Boeing is reporting “moderate” turbulence, for example, the effect on your Cessna may be “severe.”
Instrument flight is all about obscured visibility and flying without reference to the natural horizon, right? Yet in virtually all cases you need to see out for at least the first moments of a departure, and the purpose of an instrument approach for all but the biggest airliners is to safely arrive at a point where you can visually see the runway environment and land. To anticipate weather delays, the likelihood of a missed approach and diversions, some products we’ve already discussed may be useful.
Look not only at the current Metar, but the last few as well. Get an idea of the temperature/dew point spread and whether that spread has been increasing (improving conditions) or decreasing (worsening visibility).
Do the same with the TAFs into the forecast future.
Aviation meteorological reports, or Airmets report widespread existing and forecast reductions in visibility aimed primarily at conditions likely to affect flight planning for pilots of light aircraft.
The prog chart provides a quick pictorial view of areas of widespread VMC, marginal VFR and IMC conditions at the surface. Note that localized conditions can be significantly different.
Just as you did with thunderstorms, look at the surface analysis to determine the direction from which the wind is blowing en route and at your destination. Wind off a body of water tends to reduce visibility; wind from dry land tends to improve it.
Strong Surface Winds
If you were to look at the AOPA Air Safety Institute’s Nall Report, which analyzes NTSB data to find trends in mishap causation, you’d find that about half of all weather-related accidents involve loss of directional control on takeoff or landing in crosswind conditions. We don’t usually think about runway excursions as weather-related mishaps, but given the history we certainly want to pay better attention to potential crosswinds as part of our go/no-go (or destination-change) decision-making.
Using Metars and TAFs, see what winds exist, and what are forecast. Compare the wind direction and speed to the runways available.
The area forecast, or FA, is the best information available on likely conditions at airports lacking on-site weather reporting.
Many wind-related takeoff and landing accidents occur in the vicinity of thunderstorms. Give storm cells the wide berth they deserve (20 miles, according to most sources), especially when you’re slow and flying close to the ground.
We don’t usually think of carburetor ice as an aviation weather hazard. If your aircraft has a carbureted engine, carb ice can occur in much higher temperatures than we usually associate with frozen water. Carb ice only requires humid air to form—unlike airframe ice, you don’t have to be in visible moisture to get carburetor icing. That’s because the venturi in a conventional carburetor reduces the air’s temperature significantly, potentially cooling it to the freezing point. Humidity can condense and then freeze in the venturi, even when the outside air temperature is well above freezing. Use a carburetor ice probability chart, available in your AFM/POH or other sources, to help predict when carb heat may be needed while en route.
Also look at the observed temperature and the temperature/dew point spread found in a Metar. A narrow (2 deg. C or less) gap means the air mass is moisture laden, making carb ice more likely. Check the TAF’s forecast temperature/dew point spread for your projected arrival.
Airframe ice accumulation, of course, alters the lifting capability of the wing, the aerodynamics of the tail, the ability of control surfaces to move and/or be effective, the efficiency of propellers, the need for engine inlet ice protection in turbines, and the ability to see out of the aircraft if a windscreen ices over. Icing generally forms in temperatures from about +2 deg. C to -10 deg. C in stratiform clouds, and +2 deg. C to -40 deg. C in cumulus.
The Compleat Briefing
We learn a lot about weather (or at least we should learn a lot about weather) before being turned loose with an instrument rating. At least in my experience, most personal/business-use pilots do not have a great deal of confidence in making a thorough evaluation of actual and forecast weather conditions.
Obviously, you can use many more weather products than those mentioned in this article. But follow this basic outline for methodically investigating thunderstorms, turbulence, reduced visibility, strong surface winds, carburetor icing as applicable and airframe icing to get a good idea of conditions as they exist and as they’re expected to change over the duration of your planned IFR flight.
Tom Turner is a CFII-MEI who frequently writes and lectures on aviation safety.