In general, the atmosphere is unstable when the temperature contrast between warm air near the ground and cold air aloft is the greatest. In the spring, the sun begins heating the ground, warming the lowest layer of air while air aloft retains its winter chill longer. The result is the bumpy air and thunderstorms that are typical of spring. As nights grow longer in the fall, the ground begins rapidly cooling while the air aloft is slower to give up its summer warmth. The resulting decrease in temperature contrast makes the air more stable, giving us fall's calmer days.
Winter Storms
Winter brings larger, more frequent storms that cross the country, blowing away fall's calm. The worst storms can ground flights - even airline flights - all across the country. A winter storm is like a huge mixer with winds spiraling in toward its low-pressure center.
A winter storm that moves from the Pacific Ocean into California, Oregon, or Washington brings with it humid ocean air that can condense to deliver heavy rain to lower elevations and deep snow to the mountains. As it moves inland, the storm pulls in cold air from western Canada, creating mixtures of rain, freezing rain, sleet, and snow across the West.
When a storm moves east of the Rockies, it often strengthens as it begins pulling in relatively warm, humid air from over the Gulf of Mexico. At the same time, no mountains slow the cold air flowing southward across the Plains from the Canadian Arctic. At this point, you can expect weather maps to show the storm's low pressure center somewhere over the Middle West, with a cold front stretching to the south and west to the Gulf of Mexico, and a warm front running east, perhaps roughly along the Ohio River and across the Appalachians to the Atlantic Ocean.
The cold front is the surface boundary where cold air is advancing, pushing warm air upward. The warm front is the boundary, also at the surface, where warm air is replacing cool air. While the word "warm" might sound good in the winter, a warm front often brings some of the season's most dangerous weather.
Ice And Snow
The illustration shows a cross-section of the kind of weather that a winter warm front commonly brings. The danger for pilots is aircraft icing that occurs over large areas where cloud droplets are made of supercooled water - water that is colder than 32 degrees Fahrenheit but has not turned to ice. If sleet or freezing rain is hitting the ground, it's a good bet that there is supercooled water in the clouds overhead. It's also possible for storm clouds to contain supercooled water droplets even when no precipitation is falling. In fact, warm air flowing into the storm and upward along the slope between cold and warm air wraps around the storm's center, creating another area where icing is possible.
While falling snow, which is made of ice crystals, generally doesn't stick to airplanes in flight, you can't always be sure that "snow" is nothing but snow. At times freezing rain can be mixed with snow, and the raindrops are not always obvious. At other times, snow that is not mixed with freezing rain can still leave ice on an airplane. This is especially true of an airplane that is on the ground.
In the mid-1990s, scientists from the National Center for Atmospheric Research studied 10 takeoff crashes caused by ice. They found that snow which falls when the temperature is between about 25 F and 31 F is wet. When it falls onto an airplane, the water in the snow can freeze into ice. Small amounts of dry snow on an airplane's wings quickly blow off as the airplane picks up speed early in the takeoff run. But if the snow is wet, the ice it leaves on the wings won't blow off. In the crashes the scientists studied, ice on the wings kept the airplanes from developing enough lift to take off.
In several of the cases, the snow had appeared light because the crystals were small, meaning that visibility was relatively good despite the falling snow. Large, dry snow flakes that are likely to fall when the air is colder than 25 degrees block more light than smaller, wet snow crystals, reducing visibility more. Poor visibility can make pilots more alert to the potential danger of snow than they when the visibility is better.
If snow is falling while the winds are blowing from around 9 to 15 mph, the researchers found there is an added danger. Normally, you taxi downwind to get to the end of the runway that heads into the wind for takeoff. While you're moving downwind on the taxiway, a 9- to 15-mph wind can pack snow on the top of the airplane's wings. If the snow is wet, you can pick up dangerous ice while taxiing.
As result of the study, devices are being installed at some large airports to measure the water content of snow. Airlines and others can use this data when they decide whether aircraft should be deiced on the ground and, if so, how much fluid should be used.
The lesson for pilots who don't have access to the deicing trucks that spray jets as they await takeoff should be never to assume that snow will blow off the wings when you start the takeoff roll. If snow has accumulated, you should always make sure that it's all snow and no ice.
As a storm moves on toward the east, cold air flows in behind it, usually clearing the sky and creating good flying weather, at least until the next storm moves in. Even after the sky begins clearing, you still might have to wait a while for comfortable flying. Cold air often arrives with strong, gusty winds that can create turbulence equal to that of the most unstable spring day. You should check the winds aloft because they can still be strong after surface winds have calmed down.
Frost
Even after the winds die down, clear skies can bring another hazard. Temperatures usually drop as the sky clears because the water drops in clouds absorb heat that's radiating away from the earth. The water drops, in turn, radiate heat back toward the ground. As a result, if everything else is equal, a clear night will be colder than a cloudy night.
As the air cools on a clear night, it can reach a temperature - called the frost point - at which water vapor in the air begins to sublimate onto objects, such as grass or an airplane that's sitting outdoors. Sublimation refers to water vapor turning directly into ice without first condensing into water.
The ice formed on objects like airplanes by sublimation is frost. Often you can hardly see frost on an airplane's wings, especially if they are white. Even if you can see some frost, it may look thin and harmless. It's not. If you rub your hand across the frost, it can feel like fine-grained sandpaper. This roughness is enough to disturb the smooth flow of air across the wing, reducing lift. Researchers have found that frost can reduce lift by 5 to 10 percent - enough to make a safe takeoff impossible. Parking the airplane so the sunlight falls on the wings can help to melt frost even when the air temperature is a little below freezing. Sometimes you see pilots and would-be passengers rubbing the wings vigorously with rags to melt the frost but this isn't always effective. The easiest way to remove frost is to push the airplane into a heated hangar, if one is available, and allow the frost to melt. Once the airplane is in a warm environment, you may be able to speed the process by rubbing the wings with rags. Make sure the melted water doesn't end up in places where it could refreeze and interfere with the movement of the flaps and ailerons.
Fog
In some parts of the country, fog becomes more of a problem in the winter than during other seasons. Valley fog is a common winter hazard in many parts of the West. When the winds are calm, cold air flows down the mountains into valleys where there can be enough moisture to condense into a thick fog. While the air next to the ground is cold, the air above is warm. This is known as an inversion, which keeps air near the ground from rising and air aloft from descending. Such mixing of the air would clear the fog; without it the fog stays. Winter's short days with the sun low in the sky mean that there is often not enough solar heat to burn off the fog. Many times valley fog will hang around for days until the winds of a new storm arrive to scour out the valley.
When the ground is cold, especially when it's snow-covered, warm, humid air moving in from the south will cause widespread advection fog. Meteorologists use the term advection to refer to the horizontal movement of air. As warm air is moved, or advected, over cold ground, the ground cools it to the point at which its water vapor begins condensing to form fog. This is a particular hazard in coastal areas.
Despite its hazards and hassles, winter can also bring beautiful flying. When the cold, clear air has settled in with only light breezes and you've cleared the frost off the wings - or even better kept the airplane in a warm hangar overnight so there's no frost and the seats are warm as you sit down - and pulled onto the runway for takeoff, you'll find that the cold air improves your airplane's performance. The airplane will seem to leap off the runway.
In the stable conditions that can follow a storm, the air is likely to be so clear that you see distant mountains you've never seen before from above your home airport. With the next storm a day or two or maybe longer away, you can turn to improving your flying skills or just enjoying being in the air on a sparkling day.
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