12 Steps to Understanding Weather Information
Tidbits on the Basics
By Richard L. Collins
March 2007
A lot of pilots are satisfied to fly away with the terminal forecasts and metars (in plain language, please) and I suppose that might meet the letter of the law on weather information. But there is so much more than that to weather and the pilot who puts some effort into understanding weather, and how it affects his flying, will find less weather-related surprises as a flight unfolds. Let's look at some of the basics that are available to us and see what we learn from them.
The Synopsis:
Maybe the word is misunderstood, but as it appears at the beginning of an area forecast it is basically a description of the weather map. It is followed by a clouds and weather forecast. Do pay attention to the note "Non MSL hgts denoted by AGL or CIG." That means cloud bases are generally above sea level except where noted. Big difference in the mountains.
Why is this basic item important? The location of high and low pressure areas and fronts tells a lot about the conditions that will be available for flying. If a trip is headed toward a low or a front, conditions will likely deteriorate the closer the airplane gets to the condition. Tops will probably be higher and bottoms lower. At lower altitudes, there will likely be turbulence in the frontal zone. This may not be wing-bending but it will make the flying more difficult and any passengers less comfortable.
We have to be careful with weather maps. The ones on TV in the morning generally show weather for the whole day. For flying, we need to know what is expected to exist at a specified time. Official prog (for prognostic) charts are available on the web at adds.aviationweather.noaa.gov. There is a lot of other good stuff there, including a useful icing forecast.
Lows:
Low pressure areas are more important to flying weather than high pressure areas because the lows make the inclemencies that challenge us. The circulation around a low is counter-clockwise in the northern hemisphere and air flows around and into a low and then circulates upward. Low pressure areas almost always move, though they can become stationary or erratic in their movement. The normal movement of a surface low is roughly with the wind flow at the 500-millibar level, or about 18,000 feet. The prog charts will show the projected movement of lows, or you can look at the 18,000-foot wind forecast and visualize the movement.
Upper lows are just that: complete circulations aloft. They are complex, it is difficult to forecast the formation, movement or dissipation of an upper low, and they can make nasty weather over a wide area. Called cut-off or closed lows aloft, if one is overhead nearby, the best surface forecast is for continuous clag. Any surface low might not be directly below the low aloft, and the air swirling into the surface low and then up into the low aloft can be quite bothered. Bad place to fly.
Fronts:
They come in cold, warm, stationary or occluded versions. The cold variety can spawn serious thunderstorms, especially when the low is strong and the temperature difference on the warm and cold side of the front is great. Warm fronts make for inclement weather over a wider area and can harbor embedded thunderstorms.
Occluded fronts, generally found where a cold front overtakes a warm front because of an exceptionally strong circulation around a low, can make for nasty, bumpy flying. This is especially true just as the fronts are starting to occlude. Stay as far away from the driving low as possible.
Stationary fronts come when a low peters out or moves so far away that the circulation is no longer strong enough to move the fronts. The weather might stay bad for days in a stationary frontal zone and only the development of a strong new low will change things. If all the parameters for a front are not met, the condition might be called a trough and, to a pilot, it might seem for all the world like a real front.
Source Regions:
That's simply where the air is coming from. Visualize the flow around the low and if the source of the low-level air along your route is moist, as in that coming from over the oceans or the Gulf of Mexico or California, then there will be plenty of ingredients for clouds and rain. The stronger the flow, the more important this becomes.
Wind:
A complete pilot studies wind, especially surface and wind aloft forecasts. If the forecasts are bang-on, that means the model of the atmosphere is accurate and the other forecasts should be pretty good. If the wind forecast is incorrect, then the other forecasts might be too. Generally, a wind that is more southerly or easterly than forecast means the surface weather will be worse than forecast. A stronger wind than forecast means that the low pressure area causing the wind is stronger than forecast.
Wind shear is important, too. Defined as a change in direction and/or velocity over distance or height, wind shear can result in enthusiastic turbulence and can create low-altitude hazards around airports. There is always wind shear in frontal zones and where areas of strong wind (jet streams, streaks or cores) aloft interact with areas of lighter winds.
Temperature & Dewpoint:
This is really basic, but these items give clues to a lot of things. We learn early and often that if they are close, the flying weather suffers. They are especially important as the day is ending and beginning, when a small spread can result in rapidly deteriorating weather after sunset, or fog at and just after sunrise. If there is a big difference in the temperature and dewpoint ahead of and behind a cold front, there is probably a lot of action in the frontal zone. Also, if the temperature aloft is warmer than forecast, that means more moisture. The ability of the atmosphere to hold moisture doubles with every 11 degrees Celsius rise in temperature. That is why, when the talking heads say a heavy rain would have been so much snow, they don't know what they are talking about.
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