With the advent of computer graphics and easy access to a variety of weather information on new forms of avionics and handheld devices, there is no need for an extensive textual forecast—the FA.
While there are several excellent weather sources on the internet, we’ll use the NOAA suite (www.aviationweather.gov) for our illustrations. It may be argued that for the proficient instrument pilot, in a well-equipped airplane there are three “show-stoppers”; 1) below minima ceiling and visibility, 2) thunderstorms, and 3) Icing.
From the home display a variety of graphical presentations can be selected to address these critical flight conditions. Their presence (or forecast) can be quickly grasped with just a few clicks on a tablet or smartphone before departure. Color (not available in our illustrations) enhances the displays. As they say, “There’s an app for that.”
While a positive move, sometimes when we simplify things we may miss some essentials. There is no doubt that a competent pilot needs a good grasp of basic meteorology and be able to mentally analyze the weather as in the old Area Forecast.
Recall that a forecast is a fancy word for an educated guess. Any pilot with a few of hours of IMC has probably experienced their well laid plans going to the dogs. When this happens, you may have to become your own meteorologist, so let’s review the basics of understanding the weather—you could say that the weather boils down to water and heat (no pun intended).
Solar Heating
The earth experiences different amounts of solar heating based on daily and seasonal patterns. This results in irregular heating, generating variations in pressure. Since nature is said to abhor a vacuum (and indeed any differences) this causes wind. Throw in varying degrees of humidity and you have what we generally call weather.
That portion of the atmosphere in which we fly is called the troposphere (which varies from the surface to a height from 35,000 to 55,000 feet—depending on a polar or tropical location respectively). It is in the lower half of this region that most of our weather is generated.
Traditionally, an analysis of the weather is accomplished by plotting various data from a multitude of stations on a chart of the area of interest—pressure, temperature, humidity, wind, visibility and cloudiness. This was done manually by an observer at a fixed point, and (for practical purposes) often at an airport at certain standardized times.
Symbols were hand drawn on the map depicting the current observations and the meteorologist then put on his thinking cap and sharpened his crayons. Or at least that’s the way it was 30 years ago when I was in the Air Force. Much of this work is now automated (and augmented by satellites and powerful weather doppler radar) with computers doing the graphical magic—but the basics of an analysis remain the same.
Stations reporting equal air pressure (when reduced to the common denominator of Sea Level Altimeter Setting, known internationally as QNH) are connected to form isobars. Areas of High and Lows then clearly emerge. In the northern hemisphere wind flows clockwise around a High and vice versa, and with a higher velocity with a significant pressure gradient. Add to this the temperature and humidity characteristics of the air mass being looked at and you have a weather system.
Air Mass Characteristics
We cannot determine if it’s a “warm” or “cold” air mass by simply observing the temperature. The important aspect is the air temperature in relation to the underlying ground temperature.
A warm air mass will (by definition and logic) be cooled from beneath by the underlying ground.
Since cold air is heavier (higher density) than warm, it will not easily move vertically—hence the air is stable. This may sound like a good thing, and can sometimes be, since the air is often smooth. However, this also results in humidity and cloudiness being “locked” in various layers. Visibility will be poor, and with layered clouds over large areas (stratiform).
Stratus clouds are a good example, spread out like enormous wet blankets with fog and or widespread rain or snow making flight challenging for pilots.
The reverse is the cold air mass. Since it gets heated from below its properties are also opposite. Turbulent, though with usually with good visibility, and clouds of the cumulus type. If the vertical development is significant, and sufficient moisture is present, precipitation may occur in localized areas.
Because the skies are usually mostly clear, the daily variation in weather is large, while in the warm air mass the sun will not have much effect through the thick and spread out cloudiness.
Frontal Conditions
When we have air mass characteristics on our weather chart we can draw a line between the different air masses. This is known as a front. Depending on which air temperature is replacing the other (driven by the wind as depicted by drawing the isobars) either a cold or a warm front is defined. The “front” is the point at which the blue (cold) or red (warm) air on the map intersects the ground. What we really have is a frontal area where the air masses meet. This causes them to have different characteristics.
Warm air forcing away cold air produces a warm front and since the former has lower density, it has greater buoyancy. It therefore tends to gradually slide on top of the cold air over hundreds of miles. As it does, it forms the typical high-altitude cirrus clouds—little hook shaped wisps that can (since they’re far ahead of the front itself) often be visible days in advance—a handy forecast tool.
As the front approaches (or of course, if a pilot flies into it) we can expect gradually thickening, and lowering clouds. At some point the cloud may be dense enough to form precipitation (rain or snow). This can cause lowering visibility—an all too often occurrence at our destination airport.
The cold front (for the same reason) has relatively little depth to it and so the cumulonimbus may turn up quite quickly and cause a sudden deterioration with showery activity and localized strong winds. Depending on the strength of the front, airborne radar or ground derived data through Nexrad can sometimes be used for the pilot to safely find a passage through.
If the option chosen is to simply stick it out on the ground, remember that the cold front is not wide and intense activity often moves quickly, so the wait for it to clear may not be more than a few hours.
Some Final Thoughts
A professional meteorologist goes through years of higher education. As instrument pilots we have the advantage of firsthand real-time knowledge of the atmosphere they cannot possibly achieve. It’s therefore important that we share unusual or unforecast weather we experience by reporting it as PIREPs to ATC or FSS.
There are many excellent books on meteorology as well as online material. For those of us who still enjoy the touch of paper, I recommend The Handy Weather Answer Book by Walter Lyons, which is easy QA reading. A more in-depth treatment is Case Studies in Meteorology by Patrick S Market et al. Used copies of either book can be found on Amazon for a few dollars.
Bo Henriksson is a captain with a regional carrier and has more than 15,000 flight hours.
This article originally appeared in the March 2018 issue ofIFR Refreshermagazine.
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