Warm fronts

These large-scale weather makers can create widespread problems for aviation activities.

By Karsten Shein
Comm-Inst
Climate Scientist

The bleed air was keeping the leading edges free of ice, but a bad breaker took the windscreen heaters out of operation as they entered the area of freezing rain beneath the warm front.

Fortunately, because of increasing fog at the airport, they were already shooting the Cat III ILS approach and the auto-land was locked firmly onto the signal. “I just hope it gets warm enough to melt this ice before the tires hit,” said the copilot. “Yeah,” the pilot replied with a straight face. “Otherwise, you’ll have to get out and scrape it off for taxi.”

Freezing rain is likely to occur during spring and fall time warm fronts, so slippery runways and taxiways are a possibility at your airport.

Since the dawn of aviation, weather fronts have contributed to millions of delays, deviations, and even a few accidents.

Some fronts are relatively weak and produce little more than a shift in wind or temperature, while others can disrupt traffic over hundreds or even thousands of miles.

When pilots think of fronts, it is often the sight of a line of towering storm cells lighting up the sky along a swiftly-moving cold front. Experienced pilots, however, understand that warm fronts can be every bit as hazardous – or even more so.

A bit about fronts

From an atmospheric standpoint, fronts are the boundary between 2 regions of air that have different properties. When those properties, such as temperature or humidity, are starkly different, so is the air density on either side of the front. The greater the difference, the more likely it is that the denser (colder/drier) air will be able to flow beneath the less dense (warmer/wetter) air, forcing it aloft.

In the middle latitudes – home to many of the world’s flight movements – we often experience a major frontal boundary between the cold air of the Arctic and warmer subtropical air. Much of the time, the polar front is a stationary front – that is, it doesn’t move appreciably in any given 3-hour period.

However, when wave troughs in the overlying polar jet stream foster the development of surface low pressure centers beneath them, the cyclonic circulation (anticyclonic in the southern hemisphere) of surface air being drawn into the low initiates a warping of the polar front.

Behind the low, the arctic air advances as a cold front into the space occupied by the subtropical air, forcing the warmer air aloft – often explosively. Ahead of the low, the warm air also advances, but being less dense than the cold air, cannot displace it. Instead, it glides over the colder air and the forward movement of the surface warm front is more correctly a retreat of the cold air as the low’s circulation draws it inward around the top of the low.

Of course, there are other types of fronts – ones that delineate humid and dry air, and fronts tied to local geography, such as sea breeze fronts. But most such fronts are smaller in scale than the fronts associated with mid-latitude cyclone systems.

The wedge

Lengthwise, a warm front may stretch for hundreds of miles, with clouds stretching far ahead of it. But the profile of a typical warm front is that of a narrow wedge of cold air over which relatively stable warm air rises as it flows poleward. This wedge results in a set of conditions that can be very problematic for aviation. These can include lowering ceilings, freezing rain, and widespread fog.

Mid-latitude cyclonic weather system over the eastern US in April 2020. Warm fronts are normally indicated by red lines with bumps facing the direction of movement. They often produce a widespread shield of light to moderate precipitation and are likely areas to encounter low ceilings and freezing rain at low altitudes.

If a pilot is flying toward a warm front from the cold sector, or if he/she observes an approaching warm front from the FBO window, the first thing he/she will notice is a deck of cirrus and cirrostratus clouds.

Occasionally, high decks can include cirrocumuli, which indicate the presence of instability aloft and potentially heavier showers and even thunderstorms embedded in the approaching clouds – although far less likely than with a cold front.

Timing can vary widely, but the high clouds often precede the onset of precipitation by around 6 to 10 hours.

As one approaches the warm front, the ceiling lowers and thickens into altostratus and altocumulus. When the deck has descended to around 8000 ft or so, pilots can expect an increased likelihood of light precipitation if the deck is thick enough.

Because of the gradual slope of the front, this can occur many miles ahead of the position of the front at the surface. Although nimbostratus are most common, cumulonimbus clouds bringing heavier rain can appear in more unstable environments, such as is often the case during summer.

Close to where the front intersects the surface, clouds are thickest and decks may extend nearly all the way to the ground. Steady and prolonged moderate to occasionally heavy precipitation can saturate the underlying air, which may produce thick fog when coupled with weak surface winds, effectively reducing the ceiling to zero and measuring visibility in feet of RVR.

Summertime warm fronts tend to be largely benign IFR affairs, owing to the normally small temperature differences between the 2 air masses and the fact that the freezing level will be far above the precipitation region, removing any chance of aircraft icing.

From fall to winter, the cold air cools far more quickly than the warm air, and similarly is slower to warm in spring. This can lower the freezing level into the precipitating clouds – and often all the way to the surface.

Freezing rain

Most precipitation – even warm front precipitation – starts life frozen. But as it falls through the warm air layer, it may partially or fully melt before falling into the colder surface layer. If the air beneath the front is above freezing, the precipitation will reach the surface as rain or a mix of water and ice. However, depending on the thickness of the subfreezing air beneath the front, the melted precipitation may not have a chance to refreeze, creating a region of freezing rain ahead of the front.

Warm fronts form a shallow wedge as warm air rises gradually over slowly retreating colder air at the surface. They can be hundreds of miles long and produce widespread cloud coverage and precipitation.

Although most business and commercial aircraft have adequate anti-ice systems, beneath a warm front is one of the more dangerous places to encounter ice. This is because, given the shallow nature of warm fronts, the aircraft will already be at a low altitude.

It can take some time to recognize ice accretion, particularly if focused on the instruments in IFR conditions. Any altitude lost to ice accretion at these already low altitudes can become critical. If the aircraft is climbing out, it is already headed toward warmer air, but if it is making an approach, pilots may become focused on the descent and apply inadvertently the normal response of descending toward warmer air, when in fact the frontal profile means warmer air is above them.

Similarly, the propensity for freezing rain beneath fall or spring warm fronts means that both airport personnel and pilots should be vigilant about possibly slick runways, taxiways, and tarmacs.

Fortunately, cyclonic weather systems and the fronts they produce are normally forecast with sufficient accuracy, so pilots can anticipate likely weather conditions well in advance of their departure or arrival.

However, even in more remote locations where good forecasts are difficult to come by, pilots can estimate the movement of a warm front relative to their position by reading the clouds or by looking at surface weather charts from the previous few hours (warm fronts are shown as red lines with semicircular ‘humps’ on the side of the line facing the direction of movement) to get an idea of the speed it may be traveling.

Warm fronts tend to move at about half the speed of cold fronts, generally about 20 to 25 kts. Radar will often show a wide shield of light to moderate returns as well.

Naturally, if you encounter conditions in a warm front that your fellow pilots should be aware of, file a pirep.