WX BRIEF

Winter weather woes

Cold season flying includes preparing for both severe and subtle adverse conditions.


Winter fronts

Graphical Airmet and Sigmet maps such as this from a late October storm system show clearly how many of the significant meteorological hazards pilots face can be found in and around a midlatitude cyclone.

In general, conditions associated with a winter cold front are slightly less intense than what might be expected in the spring or fall. With rare exceptions, winter cold fronts do not spawn too many thunderstorms, although convection can generate a great deal of snowfall, and the high winds that often fill in behind these fronts can reduce visibility to zero in blowing snow.

The pressure gradient—the difference between low and high pressure over a given distance—tends to be strongest just behind the center of the low, where there is the greatest change in temperature and thus air density.

In this region, sustained winds can easily exceed 50 kts as they flow in behind the cold front, and can gust upwards of 90 kts in severe instances. In addition, while most cold fronts originating over a land mass will bring relatively dry cold air in behind them, when a cold front sweeps onto a western coast­line the cold air will also contain relatively high humidity.

As the cold humid air is forced to rise over the coastal terrain, it can create snow, freezing drizzle, ice pellets and low ceilings of supercooled cloud droplets that may obscure the terrain beneath. Winter warm fronts are perhaps more insidious to pilots.

Like all warm fronts, they are the line on the surface map representing the boundary between cooler air and warmer air. In many instances, such as along eastern coast lines, the inflow of both warm and cool air originates over the ocean, and thus both air masses are laden with moisture, meaning generally much lower ceilings ahead of the front than would be expected in the other seasons.

Unfortunately, in the winter, even the warm air is cooler than it would be during the other 3 seasons. Therefore, it doesn’t generally take much cooling of the warm air as it lifts over the cooler air mass before the warm air reaches the freezing level.

Chances are the cool air below is also near the freezing level. This means that it is generally unlikely that a pilot will find air warm enough to break off any accreted ice that might form within the warm front clouds or underlying freezing rain.

However, most winter cold fronts are relatively shallow. The smaller temperature difference between warm and cool air, and the lower amount of moisture held by the warm air often mean that the condensed moisture is spent sooner.

As a result, it’s far quicker to climb through (and easier to remain above) the cloud deck over a winter warm front than a summer one. However, chances are that any climb through a winter warm front will include flight in known icing conditions, so a pilot must ensure that their aircraft is so certified and that they have a pretty solid idea of where the cloud tops can be found.

Warm fronts—and, in general, any moderate onshore flow of air along the higher-latitude west coasts of the world—can generate another problem for pilots. If winds are gentle—say, less than about 5 kts—the cool humid air can quickly saturate as it begins to rise along the coastal terrain. If the freezing level is near the surface, freezing fogs can form.

These fogs are simply fog where the liquid droplets exist in subfreezing air. In freezing fog, a thin layer of rime ice can build up quickly on any object moving through it. Although accretion in freezing fog tends to be relatively light, it can still add significant weight to the airframe and distort the lifting properties of the airfoils.

Radar image showing tops of precipitation echoes in association with a late fall cold front moving across the eastern US. Colder winter air depresses the top of the troposphere, keeping echo tops well lower than they might be in the summer.

Occlusions tend to be where the worst winter weather is found. Because the cold dense air behind the low is able to displace the warm air ahead of the cold front more easily than the warm air is capable of displacing the cooler air ahead of the warm front, the cold front inevitably moves faster than the warm front.

This means that, eventually, the chevron shape (cold front and warm front with the low at the apex) occludes as the cold front overtakes the warm front. The occlusion begins nearest to the center of the low because the distance between cold and warm fronts is at a minimum there. When this occurs, the warm air at the surface is cut off from entering the low.

This is good news, because it means that the low will likely begin to weaken. The bad news, however, is that the occlusion enhances the lifting of the warm air around the low. The lifted air is now able to flow at mid levels quite a way around the top side of the low, bringing precipitation to the bitterly cold, windy upper back quadrant of the system.

On a satellite image or weather map, an occluded system begins to look a bit like a comma, with a large, cloudy head surrounding the low, and the cold front comprising the comma’s tail. Beneath the comma head, precipitation is most likely going to be falling as snow.

However, at higher levels, where the warm-sector air may still be at or just slightly below freezing, rime and mixed icing are distinct dangers. Closer to the surface, however, the strong winds that often reside behind the low are likely to kick up the falling and fallen snow into a blinding blizzard.

Many on-the-ground, takeoff and approach accidents have occurred in this sector.

Forecasting and winter weather products

Assessing the winter weather situation is not much different than looking at the weather at any other time of the year.

A pilot’s primary weather products are satellite and radar imagery, surface and upper air charts, Metars, TAFs and prog charts, Airmets/Sigmets and Pireps. A review of this basic meteorological arsenal, followed by an approved weather briefing, should give a well-rounded scope of weather conditions.

Most large-scale winter weather systems show up beautifully in surface and upper air charts. On upper air maps, look for sharp troughs in the polar jet stream. These will be where cold air intrusions, high winds and potential winter cyclones will develop.

 

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