Winter Weather Simplified

So you’ve got the instrument rating, a few hundred hours in the clouds, and a good sense of when to cancel. You know freezing rain means warm air aloft, that you climb or descend to get out of ice, and that stratus usually burns off by mid-morning. Except sometimes none of that is true.

Most of what is taught comes from primary flight training. Simplified rules are built around a textbook atmosphere. But as an experienced forecaster I can attest to the fact that we often work with a much messier situation. Spend some time on pilot forums and the knowledge gaps show up quickly. A given weather situation doesn’t seem to fit what’s in pilot training.

This article picks up where the training left off as we go through the some of the more troublesome areas of winter aviation weather. This doesn’t require a meteorology degree, but a simple willingness to tackle more difficult weather subjects and to get acquainted with a diagram that looks about as fun as filling out a 1040 Schedule C.

This diagram doesn’t need to be part of a regular weather briefing, but I can guarantee that adding this skill to your toolbox will make you more well rounded in understanding aviation meteorology concepts and will help push you beyond simplistic guidelines and rules of thumb. When you’ve been burned by a weather forecast and need to see what went wrong, check a Skew-T once you’re on the ground and often it will all make sense.

Breaking Down The Skew-T

The Skew-T diagram is a graph of height versus temperature, with height on the Y axis and temperature on the X axis. We take a series of temperature readings in the atmosphere above a given station or airfield (usually from a radiosonde but model predictions can be used) and plot it on this diagram to construct a temperature profile or sounding.

That’s explanation is about as simple as it gets. While we could load up Excel, set up a graph, and draw a simple X/Y graph of these numbers, there are many advantages to using the Skew-T format and shifting, or “skewing” the Y-axis so that it leans to the right (thus “Skew-T“). This makes the temperature profile stand up straighter instead of leaning far over to one side, making every little irregularity and quirk stand out.

Up until the 1980s, the Skew-T was a sort of mysterious, arcane tool used by forecasters. But now that we’re in the era of high-tech meteorology, it has become extremely common on internet map sites, YouTube videos, and even in the appendix of NTSB crash reports. It’s really the only meaningful way to understand how temperature and moisture quantities are distributed above a given station. So it pays for pilots to have at least a rudimentary understanding of how they work.

To help make these diagrams less confusing, I have calibrated the examples shown here to show altitude in hundreds of feet rather than pressure in millibars. This is very unusual, but it should help the information sink in better. Normally these charts show pressure, based on the fact that pressure decreases with height. On normal Skew-T’s, a value of 1000 mb is close to sea level, with 500 mb at 18,000 feet and 200 mb at about 39,000 feet.

The isotherms slope up and to the right. To plot a data point, all you need to do is locate it according to altitude and temperature. The lines sloping up and to the left are dry adiabats. That’s not important here, but I’ve drawn them just to get you accustomed to how they look. Finally I have shaded the diagram in different color bands: tan for above 0° C, light blue for 0 to -20° C, and darker bands below -20° C and -40° C. Skew-T’s aren’t usually colored this way but I think it makes them easier to read.

Normally a line representing temperature is drawn on the chart to indicate the vertical temperature profile at a given location. Dew point can also be drawn on the chart to represent moisture. It is usually drawn in green and the leftmost of a pair of lines. To keep things simple I have not drawn dew point here.

Winter Storm

To help put these skills into practice, we can take a look at a winter storm across the Central and Midwest states (see graphic below). Point “D” is in the warm air mass over Mississippi. The Skew-T diagram shows lots of warm air, with the sounding being entirely above freezing up to 9,000 feet MSL. So all of your flying here will be free of icing hazards below that level, with the possibility of icing from 9,000 feet to about 18,000 ft depending on whether there is cloud and precipitation. The Skew-T makes it a snap to visualize where these hazards are. There is plenty of moisture here and the temperature plot leans somewhat to the left, suggesting an unstable air mass, so showers and storms are likely.

Heading up to the warm front we end up near Evansville, Indiana at point “B.” The map shows freezing rain here. Looking at the Skew-T diagram we find freezing rain and severe clear ice close to the surface up to a few hundred feet. Above that is warm air all the way up to about 4,000 feet. This is part of the warm conveyor belt transporting warm air from south to north over the warm front. So even with freezing rain on the ASOS report, you’re perfectly good in the 1,000- to 4,000-feet band and can likely find a good alternate to the south as long as you avoid the storms and are IFR equipped.

Going farther north to about Peoria, Illinois, at point “A,” the atmospheric column is entirely subfreezing and the dominant precipitation form is likely to be snow. Supercooled large droplets can still exist, especially below 5,000 feet where the temperatures approach 0° C and there is a hint of the warm conveyor belt bringing more moisture into this area. Icing potential diminishes above 5,000 feet.

Finally near Joplin, Missouri, at point “C,” we are firmly within the cold advection zone. The profile leans further over to the left, indicating deep instability, and this favors the development of snow showers. Only a thin sliver near the ground is above freezing, but snow is likely even at the surface. As the lower levels are still close to 0° C we cannot rule out icing, and the shallow convective towers can be a hotbed for large supercooled drops and significant icing until convection subsides or the column cools below -5 to -10° C.

If you want to try this out with real world weather, a good place to go is PivotalWeather.com and look at the model charts. When you click on a map, a Skew-T will pop up, letting you see the atmospheric profile at any given point instantly.

Surprise Inversions

Many rules of thumb for predicting freezing levels and temperatures aloft follow the ISA (International Standard Atmosphere) lapse rate. This can quickly get you into trouble. The rule states that you take the surface temperature and decrease it by 2° C per thousand feet of altitude. Stagnant summer weather is the best time to use this rule of thumb, and I would certainly use it myself for quick ballpark estimates.

But around major winter weather systems, things get complicated. On the cold side of surface fronts, we find inversions. By its very definition, a front is where an air mass displaces another air mass, not only horizontally but vertically. The more buoyant air mass, the warm one, is displaced aloft. When this happens, we find the unusual situation of cold air at the surface and warm air aloft. To help you picture this better, we’re usually talking about the top of this layer being in the range of hundreds to several thousand feet.

Whenever the temperature at the top of a layer exceeds the temperature at the bottom of a layer, that layer is said to have a temperature inversion. These are normally based at the surface, such as with radiation inversions. But we often find inversions suspended aloft along frontal surfaces, leading to unexpected heavy icing. A common situation is to be above the overcast with a 2° C OAT, but when you descend into the overcast it drops to -2° C with heavy icing. This cloud material, often stratus or stratocumulus, is embedded within the frontal inversion and marks the top of the cold air mass which is often invading the region as part of a cold advection pattern.

Sometimes there’s no inversion but just significantly diminished cooling with height. The effect is the same: We get unexpected temperature changes and suppression of vertical motion. All of this is quite effective at destroying assumptions based on the ISA lapse rate.

Icing In Snow

When we’re talking about snow, it is commonly assumed we’re safe from icing problems. This assumption is based on the fact that snow consists of ice crystals, and if there’s ice crystals it’s all solid stuff. If the air underneath is above freezing, then it’s too warm for icing there. No problem, right?

One complication is that supercooled drops can still exist within a cloud producing snow, enough to produce rime icing. This is especially true in showery precipitation, where there are significant updrafts entraining warmer low-level air and generating supercooled water, which can exist at temperatures far below freezing.

This is a significant problem in lake effect snow, such as that on the Great Lakes and along coastal waters. These snow showers can produce moderate to severe clear icing in the lowest 10,000 feet of the atmosphere. Since snow showers don’t look very impressive on airborne radar and look like an amorphous gray mass visually, they can catch pilots off guard.

There’s also the problem of complex frontal setups, especially along warm and occluded fronts. Sure, you might be descending and see snow outside, but down below is a warm low-level jet and a subfreezing surface layer. Snow gives way to mixed snow and rain on the descent and becomes freezing rain on the final approach. This is one reason us forecasters rely heavily on the Skew-T diagram and at least a conceptual knowledge of temperature profiles around a forecast area.

METAR and TAF Gotchas

Using METAR reports during winter weather does require a bit of diligence. Unless you’re at a major airport, the METAR is generated entirely from automated systems. During complex mixed phase precipitation, ASOS sites can generate unexpected or incorrect reports of precipitation type. Blowing snow and ice pellets are common culprits that further degrade the ability of ASOS to discriminate precipitation type. When the system is completely confused, it will indicate UP, or unknown precipitation. Comparing reports from neighboring ASOS is good practice to accurately identify precipitation type.

A common trap is to anticipate flight level hazards based on what’s being reported on the METAR or TAF. These reports strictly identify precipitation types occurring at the surface. It is entirely possible for the ASOS to be reporting rain with a temperature of 2° C, while freezing rain and severe icing is occurring just 2,000 feet above the surface. A lack of FZRA in the METAR or TAF does not mean an absence of freezing rain or icing on the arrival or approach.

Finally on METAR reports be sure to check the remarks for FZRANO and PWINO (freezing rain sensor or present weather sensor not operational). They’re easy to overlook, meaning things might look fine at the airfield when freezing rain is in progress.

Freezing Fog

Freezing fog (FZFG) is another problem you might run across. It is fog that occurs at temperatures of below 0° C, composed of supercooled droplets that freeze on contact with any exposed surface. Freezing fog is not a cause of major structural icing but causes a thin, frosty buildup of rime ice on aircraft surfaces that needs to be removed. Freezing fog can ice up propellers, pitot tubes, windshields, and induction systems while on the ground, and if that sounds unpleasant, you might be piling real icing on top of that during the climb.

Ice fog is a different type of fog usually at temperatures below -30° C comprised of ice crystals; in other words, it’s all solid. Oddly enough there is no METAR code for it. In the U.S. it’s treated as freezing fog and carries the FZFG abbreviation. Ice fog carries no significant icing hazard, but the METAR observation does not make it clear which type you are dealing with so only assume ice fog if it’s below -30 to -40° C.

Ice fog is well known for producing clouds of persistent fog around power plants, factories, and industrial parts of airfields. It hangs near the surface in dense layers that don’t cause much slantwise visibility restriction, such as on the approach, but can significantly block visibility horizontally, behaving somewhat like shallow fog. In other words, you can see the runway fine on the final approach, but over the numbers visibility drops to near zero. It becomes essential to not just wing it but to take note of the reported visibility and get reports from any aircraft ahead of you. Just the mere presence of running vehicles and other aircraft can temporarily worsen ice fog, and this is a common problem at airports like Fairbanks, Alaska.

Stratus Behavior

An old habit that carries over from summer into winter is expecting fog and stratus to break up by late morning. Low sun angles, reduced surface heating, and the appearance of dynamically enhanced inversions slow this process down and make it more difficult for these weather phenomena to dissipate. The stratus and fog can be further reinforced at night by thermal radiation into space from the top of the cloud layers, which intensifies the inversion, creating a feedback loop.

In the western states, valleys easily trap air masses whenever the pressure gradients diminish over an area and patterns stagnate. This frequently affects areas as large as the San Joaquin Valley, the Salt Lake Valley, the Columbia River basin, and the Snake River Valley. Stratus and fog develop from ground moisture from recent rains, and once developed can hang on for days, or in extreme cases for weeks. It takes a major upper-level pattern shift and intensifying pressure gradients to break up these events. Always have your alternates lined up even if it looks like the fog or stratus is going to break up.

Such events can also occur in the Appalachians, the Piedmont region, and coastal plains of the Carolinas and Georgia due to cold air damming. This occurs when cold air east of the Appalachians stagnates and is blocked by the mountain range, even when a new weather system approaches from the west. Unlike West Coast events, the fog and stratus hang around for days instead of weeks, but can be compounded by periods of rain, snow, or mixed precipitation as these systems override the cold air mass. A period of strong westerly winds is usually needed to break up these kinds of patterns.

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