Icing and the 500-M Line

Author: Karsten Shein

During colder months, airports and flight ops can be at the mercy of snow, sleet, and freezing rain. Not only do they make for a miserable pre-flight walk around, these conditions produce a significant increase in expenses as runways need to be cleared and aircraft receive a coating of de-icing fluid. Whether the expenses are borne directly and immediately by the aircraft owners or passed on to them through increases in landing and tie-down fees or hangar rentals, the costs of dealing with winter weather increase the overall tab for operating into and out of these airports.

While we can do little about airports embedding snow removal costs into their operational fees, we can sometimes avoid the hazard of ice accretion and the expense of de-icing by making informed, weather-based decisions about flight timing or destinations.

While there is no substitute for a proper weather briefing before any flight, much of this information is available to us in the form of weather maps and local forecasts. Larger urban areas will normally have a local television station that maintains the current forecast on its website. Often these are more accurate than what you might get from a national agency or weather company, simply because they are made by meteorologists who are very familiar with the nuances of the local weather. At smaller locations, forecasts from the agencies or big weather companies usually provide good short-lead outlooks for the type and timing of any snow, sleet, or freezing rain.

A very useful option for pilots, however, is a type of weather map known as the 1000-500 thickness map. This map shows the distance in meters between the 1000-mb (or hPa) pressure plane and the 500-mb plane in the atmosphere. The 1000-mb level averages around 100 m (~300 ft) MSL, and 500-mb occurs around 5,600 m (~18,000 ft) MSL, meaning they are separated (on average, by around 5,500 m).

Because pressure and temperature are linked through air density, the colder the air, the denser it becomes, meaning the difference in altitude between the two pressure levels (thickness) decreases. As expected, as the air in the lower atmosphere becomes colder, there is an increasing likelihood of any precipitation falling as supercooled or frozen.  Adding in the moisture content of the lower atmosphere, meteorologists have determined that a 1000-500 mb thickness of around 5,400-m is the threshold for differentiating between frozen and non-frozen precipitation. At smaller thicknesses, the lower atmosphere is cold enough to support unmelted snow reaching the ground, while at greater thicknesses, any frozen precipitation will likely melt during its descent.

Things get interesting around 5,400 m of thickness. Give or take a few hundred feet, this thickness indicates that snow (most precipitation begins life aloft as ice) will at least partially melt before it reaches the surface. Depending on the surface temperature, this can mean that sleet or freezing rain may be reaching the ground and any parked aircraft.

Thickness maps are not commonly provided in briefing materials, but they are a routine product of forecast models and are easily located on the Internet. They are generally presented as contour maps, and thicknesses may be presented in decameters (5,400 m = 540 dm). Contours are normally drawn every 40 m. Regardless of the display, you should estimate that any precipitation falling in the region between 5,360 and 5,440 m thickness (+/- 1 contour line) will be a rain/snow mix, sleet, or freezing rain.

If a thickness map is unavailable, a 500-mb map can be used instead, but recognize that the rain-snow line is closer to 5,500 m and is not as precise. Also, bear in mind that this relationship works best at low elevation locations. In higher terrain, precipitation is less likely to have sufficient melting time before reaching the ground in the vicinity of the 5,400 m thickness, and you’ll want to use a larger thickness (perhaps 5,500-5,600 m), depending on elevation.

Karsten Shein, PhD, is a climatologist with the US National Oceanic and Atmospheric Administration (NOAA), and TrainingPort.net’s Subject Matter Expert for Meteorology.