Be careful what you wish for
It's a good day for flying with a high ceiling, good visibility, and light wind, but the temperature is heading down from 20 degrees Fahrenheit. As you try to concentrate on examining the airplane, the cold seems to be sucking the warmth out of your body. "I wish a warm front would come in," you mutter through frozen lips.
Hold on. This is one of those wishes you really don't want to come true. While warmth might sound good, a winter warm front could shut down your flying with what television forecasters like to call the dreaded "wintry mix" of snow, sleet, and freezing rain.
Strange as it might seem, you have less concern about a wintry mix if you're on a freezing ramp in Minnesota or North Dakota in January than in, say, southern Ohio or even in Virginia. This is because the atmosphere needs relatively warm air to create freezing rain and sleet. During the coldest part of the winter, which is usually around the second or third week of January, seldom does enough warm air reach the north-central United States to cause mixed precipitation.
Let's take a look at two storms--one that brought an ice storm to the Southeast on December 15 and 16, 2005, and another the same day that spread snow across the north-central states--to understand how warm air causes winter trouble. These storms also serve to illustrate why, in order to really understand weather, you must grasp what's going on in three dimensions; not just the surface weather that is shown on almost all television weather maps. The surface weather picture works well enough if you're driving somewhere, but if you're flying high above the highways, you need to know what's happening aloft. Even more important, what happens aloft affects the surface weather.
Figure 1. A surface chart of a 2005 ice storm shows widespread precipitation, but more information is needed to plan a flight. |
The dashed blue line that extends south into Georgia, then north to the Great Lakes and back to the southwest into Texas, shows where freezing temperatures dip to the surface north of the line. The green areas show where precipitation was falling at the time. The brown lines are isobars, which connect areas of equal surface barometric pressure. The numbers on the isobars, such as the "1008" on the line surrounding the low pressure in Alabama, show the pressure in millibars everywhere on the line.
A weather-map cold front shows where colder air is replacing warmer air at the surface. The cold air flowing in behind the eastward-moving cold front in the South wasn't all that chilly. Places east of the front were in the high 50s at the time while those about 100 miles to its west were in the 40s. In the north temperatures were in the low 30s east of the cold front, and in the low 20s to its west.
Earlier in the week cold air had pushed southward over the eastern United States, and on the morning of December 15, temperatures in the higher elevations of the Southeast were only in the 20s and 30s.
A pilot looking at this map would know that the areas in green probably offer poor flying, but would have had no way to tell areas with good visibility in light rain or snow from areas of extremely dangerous weather. A surface map is a good way to begin a preflight weather briefing, but more is needed, including upper air information, about both what's happening now and what's forecast for the time of a planned flight.
Weather balloons launched around 7 a.m. Eastern Time on December 15, 2005, found above-freezing temperatures as high as 8,800 to 12,000 feet in the general area of the surface warm front in the Southeast. At the same time, the International Falls, Minnesota, weather balloon didn't find any above-freezing temperatures. In fact, at altitudes from 4,000 feet on up, temperatures were colder than 15 degrees F. This is important because when the air in clouds is warmer than approximately 15 degrees, precipitation generally forms as supercooled liquid water--that is, drops that aren't frozen even though they are colder than 32 degrees F. When the cloud is colder than around 15 degrees F, ice crystals form and fall as snow.
Ice forms on an airplane that flies into supercooled water drops. Supercooled drops that fall as rain or drizzle turn to ice when they hit, which is why they are called freezing rain or freezing drizzle. (Drizzle refers to drops that are less than 0.02 inch in diameter.)
The layer of air warmer than 15 degrees over International Falls wasn't a concern because most of the clouds over International Falls that day were above 4,000 feet with snow falling from them.
In contrast, the 7 a.m. December 15, 2005, weather balloon launched from Blacksburg, in western Virginia, found a cold-air/warm-air/cold-air sandwich aloft along with enough water vapor to create substantial precipitation. The surface temperature was 30 degrees F, and barely below-freezing temperatures continued up to 3,860 feet where the air was 33 degrees F. The weather balloon found temperatures between 33 and 38 degrees F up to 7,000 feet, where temperatures had cooled below freezing and grew progressively colder as the balloon ascended.
Such a layer of relatively warm air aloft is called an inversion. If freezing or near-freezing temperatures are below, and the air is humid enough for precipitation to form--as it definitely was on December 15--an inversion is also called an elevated warm layer or a melting layer. No matter what they call it, meteorologists know it's a sign of trouble.
Warm fronts create such layers, but the trouble develops far from the warm front that you see on a surface chart.
Figure 2 is a simplified cross-section of the December 15, 2005, warm front from Panama City, Florida, to Athens, Georgia, to Hickory, North Carolina, and Roanoke, Virginia. The surface location of the front is on the left where the wedge of warm air is on the ground north of Panama City.
Around noon on December 15, the surface weather at each of these cities was:
- Panama City: 68 degrees F, cloudy, no precipitation.
- Athens: 34 degrees F, rain.
- Hickory: 32 degrees F, freezing rain.
- Roanoke: 27 degrees F, ice pellets (sleet).
Figure 2. A cross-section of the December 15, 2005, warm front shows how layers of warm air affect the type of precipitation falling on varying locations. |
At Athens, snow could have been falling from high clouds, but it melted into raindrops in the relatively thick layer of warm air.
Farther north, over Hickory, snow melted as it fell into the layer of warm air aloft but began cooling as it fell through the lower layer of cold air, which was much thicker than the cold layer over Athens. The drops cooled below 32 degrees F and instantly turned to ice when they hit trees, power lines, roads, runways, and airplanes parked at airports.
Even farther north, over Roanoke, the layer of warm air was thinner and the layer of cold air thicker. Snow falling into the warm air didn't completely melt. As the partly melted ice crystals fell through the cold air, they froze into tiny pieces of ice about the size of the tip of a pencil lead. The National Weather Service reports such precipitation as ice pellets, but most people in the United States call it sleet. The important thing for pilots to know is that when the Weather Service reports ice pellets at the surface, freezing rain and dangerous ice are above.
The real weather, of course, is more complicated than the idealized picture in Figure 2. The Roanoke weather station, for example, reported light freezing rain from 5:35 until 6:23 a.m., when snow began mixing with the freezing rain; it turned to all snow two minutes later. Snow continued until 8:56 a.m. when freezing rain began mixing with it. Ice pellets (sleet) fell from 10:54 a.m. through 2:01 p.m., freezing rain began falling again; it continued until the precipitation ended at 7:54 p.m. These changes occurred as the elevated warm layer or the cold air below grew thicker and thinner.
During the day, as much as three-quarters of an inch of ice accumulated on parts of northern Georgia and South Carolina, western North Carolina, and southwestern Virginia. Temperatures over the next couple of days barely rose above freezing, and any pilots who weren't busy coping with power outages or downed tree limbs would have found unhangared airplanes coated with ice.
The freezing rain, of course, delayed airline flights in the area, including at Atlanta's busy airport. Flying was extremely dangerous for general aviation airplanes that couldn't quickly climb through the freezing rain and clouds of supercooled water drops, as jets can.
When freezing rain lasts several hours and deposits more than a quarter-inch of ice, the National Weather Service calls such events ice storms. Forecasts of such events prompt ice storm warnings.
While the northern storm was much colder, it wasn't nearly as disruptive. At International Falls, Minnesota, on the Canadian border, light snow fell almost all day December 15 as the temperature ranged from 22 to 29 degrees F. But only 1.8 inches of snow--with no freezing rain or ice pellets--fell that day, bringing the amount of snow on the ground to seven inches. Snow plows easily kept highways and runways clear. Ceilings were low and visibility was poor in intermittent snow showers in northern Minnesota, and winds were gusty, but experienced pilots with instrument ratings could have safely flown because the icing danger was low in the frigid air aloft.
On December 16 the low temperature at International Falls was minus 8 degrees F and the high only 22 degrees F. At 1 p.m. it was only 12 degrees F. But a pilot freezing during a preflight inspection there would have been better off than a pilot in Hickory, North Carolina, contemplating an airplane covered by a quarter-inch of ice.
Jack Williams is coordinator of public outreach for the American Meteorological Society. An instrument-rated private pilot, he is the author of The USA Today Weather Book and The Complete Idiot's Guide to the Arctic and Antarctic, and co-author with Bob Sheets of Hurricane Watch: Forecasting the Deadliest Storms on Earth.
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