3 QUESTIONS TO ASK ABOUT WEATHER REPORTS
Acknowledgements: AIR FACTS (John Zimmerman)
Many discussions of aviation weather focus on tactical concerns, like how to interpret specific radar images, what an SLD icing chart shows, or why an MOS forecast is limited. Such topics are important because weather flying is fundamentally a very practical exercise, but they shouldn’t come at the expense of broader thinking. That starts with the familiar advice to get the big picture (fronts, lows, upper air charts) before diving into the particulars of a weather briefing.
Instead of saying, “the current METAR shows marginal VFR,” a wise pilot would suggest, “a strengthening low-pressure system is moving in quickly from the west, which is causing visibility to drop faster than forecast.” When you consider the big picture, you’re really creating a weather hypothesis – an overarching narrative that ties together the various weather reports.
Once you have a theory about the weather, it’s time to evaluate it, both before take-off and in flight. That means continually comparing your expectations to all available data. For better or for worse, “all available data” can be quite a lot these days, so it’s important to be structured about your research. Weather reports are not all developed the same way, and not all deserve equal attention.
Here are three questions to consider when comparing different weather products:
· What is an observation and what is a forecast?
This may sound obvious, but many pilots trip up here. I’ve heard more than one pilot refer to the turbulence charts as if they were actual observations of in-flight turbulence, instead of the computer-generated models that they really are. The same goes for convection, icing, and visibility – understand whether you’re looking at actual conditions or a prediction about what might happen. This isn’t to suggest that forecasts are worthless; in fact, they’re getting noticeably better in recent years. When in doubt, though, an observation is generally worth more than a forecast. Don’t launch in the hopes that the TAF will magically come true; it may be time to update your weather hypothesis given the new data.
· What is real time and what is delayed?
Especially when considering fast-moving weather events like a squall line, it’s critical to understand which weather products are in real-time, and which ones are delayed.Datalink radar, which most of us fly with these days, is incredibly valuable for long range planning. However, it is delayed by 5-15 minutes. That’s probably not an issue with lazy afternoon build-ups, but it could be a significant limitation in the middle of Kansas on a summer afternoon. Satellite imagery is even worse, with some maps onlyupdating every 30 minutes, an eternity in most weather systems. It’s also worth noting the age of METARs. While technically not “real time,” some AWOS/ASOS systems report new weather every minute, compared to every hour for other airports. That’s a significant difference to consider, so be sure to check the time stamp of every METAR you read or listen to. What is truly real time? Onboard radar, if you’re lucky enough to have it on your airplane. This equipment can be finicky, but it offers priceless insights into the convective weather just off the nose. Other than radar, your eyes are the best real time weather detector – and they’re always on board. No matter what the XM Weather map shows, if it looks ugly it should be avoided. Just like an observation should probably get more weight than a forecast, a real time data source should override a delayed one.
· What is objective and what is subjective?
Finally, separate objective weather products that report “just the facts” from more subjective ones that are subject to biases. For example, visibility is an objective value that is measured by calibrated instruments. While sensors can (and do) lie, most of the time 1 mile of visibility really means 1 mile. A PIREP, on the other hand, is far from objective. What feels like moderate or severe turbulence in a Cessna 152 may be reported as light turbulence by a Boeing 767 crew. Even among similar airplanes, the phase of flight can have a significant effect on what gets reported: icing often seems heavier during a slow climb than during a rapid descent.
I find PIREPs to be quite valuable, especially for determining where the ice and cloud tops are, but they must be understood in context. Consider the type of airplane, the phase of flight, the age of the report, and the broader weather picture before you accept it as fact. The number of PIREPS can also be a key indicator of accuracy (nine reports of moderate turbulence probably mean more than a single one), but don’t fall into the “no PIREPs means good weather” trap either. There are usually no PIREPs in a hurricane; that’s not because there is no weather to report!
THE GOLDEN RULE:
The ever-increasing variety of weather forecasts, from icing probability charts to forecast radar images, is a real benefit for pilots. They aren’t to be feared or ignored simply because a computer spit out the result. Just remember to keep the right perspective about our job as pilots: while weather forecasting is a science, weather flying is not. Online weather maps and datalink radar are part of the in-flight decision-making process, but the most powerful tools are probably your eyes and your gut.
In flight, Richard Collins’s familiar rule is the one to remember: What you see is what you get. No matter what your theory was before take-off, and no matter what the forecast suggests, you have to deal with the weather you find in the atmosphere around your airplane. That argues for using observations, real time weather sources and objective data whenever possible.