WX BRIEF

Winter weather refresher

Learning synoptic scale charting for storms can eliminate many frigid air headaches.


Fronts

A low, solid cloud deck is characteristic of winter warm fronts. Fog and freezing drizzle often accompany them.

Since the surface low forms along the surface front, as the surface air spirals in toward the center (counterclockwise in the Northern Hemis­phere) it draws cold air in from above and behind, while pulling warmer air poleward from ahead and below. This motion sets up the classic warm and cold fronts that create various unwanted winter weather challenges for flyers.

Each kind of front contains its own set of adverse weather. Cold fronts, which during warmer months are often the bringers of squall lines and severe thunderstorms, lose a bit of their punch in the winter due to the decreased energy and instability in the air.

Thunderstorms can and do occur during winter cold fronts, and when they produce snow showers are sometimes termed thundersnow. However, what winter cold fronts lose in extreme convection they more than compensate for with strong winds that can whip light snows into blinding blizzards.

The strong winter cold front winds are largely due to the increased intensity of winter lows and highs. Remember that behind the upper air trough is an area of upper air convergence. The presence of this area means that a surface high tends to develop behind (in the Northern Hemisphere, west to northwest of) the surface low.

With both the low and high being generally stronger during the winter, the pressure gradient between the two is much larger and the resulting winds are that much faster.
One look at the surface pressure isobars will bear this out. In wintertime lows, the isobars are much more closely spaced behind the low—and the closeness of the isobars is directly related to wind speed.

The closely spaced isobars around winter lows also hint at a less visible foe. These well-packed isobars indicate a very strong pressure change over a short distance. If you happen to be flying across the isobars toward the low, you will need to readjust your pressure altimeter frequently, as you will quickly find yourself flying at much lower altitudes than indicated otherwise.

In addition to the strong winds and pressure changes, icing is a consideration in cold fronts. Ahead of the front, warmer air, often holding copious moisture, is lofted and cools as it ascends. Unfortunately, the surface air, even in the warm sector, is not usually all that warm, meaning it will likely reach the freezing point at a relatively low altitude.

Beneath the clouds, the air will cool at a rate of about 5°F/1000 ft (10°C/1000 m), and inside the clouds it cools at about half the rate it did below the cloud base. So, if you know the surface temperature is 50°F and the cloud base is at 3000 ft, the freezing level—and your best chances for icing—should be around 4000 ft.

Trans States Airlines ERJ145 taxis at IAD during a Jan 2010 snowstorm. Strong pressure gradients mean high winds that can often turn ordinary snowfall into blinding blizzards.

As it happens, in the convective clouds of the cold front, supercooled liquid droplets may occur well above the freezing level, meaning that climbing may not be much of an option. And, since the freezing level will tend to occur relatively close to the ground, you may find that air warm enough to shed the ice is below the MSA.

It is best to have a good understanding of where the freezing level resides in the cold front clouds. If you must penetrate them and cannot do so in above-freezing air, you can reduce your chance of accumulating ice by staying well above the freezing level—preferably in air that is at 0°F (–18°C) or colder—as this is around the temperature at which most water in the atmosphere will be solid rather than liquid (although some liquid droplets may still exist to temperatures around –40°F [–40°C]).

In the earlier scenario, that means penetrating the front at roughly 17,000 ft—not too difficult to reach in most business aircraft. If, however, you must climb or descend beneath cold frontal clouds, you may be able to avoid most icing by remaining in the warmer air beneath the clouds—assuming you have adequate terrain and obstacle clearance—until you have traversed the cold front.

Fortunately, ahead of the cold front, the freezing level should be several thousand feet higher and in relatively clear air. Likewise, behind the front, the freezing level will likely be close to, if not at, the surface, and although you may find yourself flying in strong gusty winds, most precipitation falling behind the front will be coming as snow.

Winter warm fronts are more insidious than cold fronts. Not only do they tend to move far more slowly—cold fronts can move upward of 40 kts and usually at least twice the speed of typical warm fronts—they can cover vast regions of airspace with thick icy stratus clouds and freezing rain that may extend all the way to the surface.

Warm fronts are the result of the counterclockwise flow around the low drawing in the cool air from ahead of the low, and moving the warm sector air from the southeast up and over that air as it too is drawn inwards. The result is a shallow, inverted wedge with the upper regions of the front extending well to the north of the surface front, often by hundreds of miles.

The same cooling rates apply to the air lifted along warm fronts as to air along cold fronts, so that 50°F air in the warm sector will still have a freezing level of around 4000 ft. But, unlike the cold front, the air below the freezing level beneath a warm front may not be cold enough to change any freezing rain over to snow or ice pellets.

As a result, pilots often find themselves picking up ice at all levels between the surface and the cloud tops. When flying a warm front, the best chance of avoiding icing is to remain above the cloud cover that contains the freezing level—usually the one closest to the surface. On climbs or descents, it is important to minimize your time in these lower levels in close proximity to the surface front.

Another thing happens in the vicinity of a warm front. As the air lifts and clouds and precipitation form, the slow-moving nature of the front ensures that lower and lower levels are able to saturate. Eventually, clouds at the frontal boundary may be very close to the surface.

Behind the front, precipitation continues to evaporate as it falls and, in the light winds, surface fog may form. The fog itself may be very near freezing or be an ice fog, capable of producing thin rime on your airfoils and certainly further reducing already deteriorated visibility on takeoff or landing.

Fortunately, with most winter warm fronts the majority of adverse weather occurs below a few thousand feet. While numerous cloud decks may exist above the primary warm front cloud cover, these will usually tend to be thinner alto or cirrostratus and not adversely affect a flight. So, when being iced in a warm front, your best exit will usually be a climb.

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