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Smoke and ash


Lengthening wildfire seasons create challenging conditions for pilots.

A C-130 Hercules drops fire retardant on a wildfire. Non-firefighting aircraft flying too close to fires can create a collision hazard in the reduced visibility created by the smoke.
By Karsten Shein
Comm-Inst Climate Scientist

The pilots had gotten the word from Anchorage Center as they crossed over the Aleutians enroute to STS (Santa Rosa CA). The airport had closed an hour earlier and would likely not reopen that day due to being in a wildfire evacuation zone. Crew acknowledged and dialed in their alternate – APC (Napa CA).

Descending from cruise level, they could see the widespread blanket of smoke, and a large swath of charred landscape where a major wildfire raged. Although the fire was still well north of the airport, their entire approach would have to be on instruments – there was no way they’d be able to pick up a visual until at least the inner marker.

Complicating things, the wind whipping those fires was strong. A ridge-top station near the fire recorded a sustained 50-kt wind with gusts reaching 80. Winds at APC were relatively lighter – 20 kts gusting to 36. Although crosswind wouldn’t be too bad on Rwy 36, the gusts would make for an uncomfortable approach.

Descending through 8000 ft, slant range visibility began to deteriorate. Soon, the crew was fully on instruments as the pilots stuck to the glidepath. Outside, the sky was a murky whitish-brown. Passing through 2000 ft, the aircraft jolted off the glidepath as they hit a layer of stronger wind. The offshore windstorm that was fanning the flames was moving air above a stable marine layer.

That wind was slowly mixing in with the surface air, and producing moderate windshear as it did so. The aircraft was fast approaching the decision height when the copilot called out that he had the runway. It was just a vague dark rectangle in the windscreen, but they could see it, as well as the runway end lights. The FBO radioed that they’d meet the aircraft with face masks because the air quality was very bad.

The worst issue now was that both pilots’ homes, as well as that of their CEO, were in the evacuation area. They’d all have to wait out the fire with friends at a safe distance away.

Extreme Santa Ana winds blow smoke offshore from the Thomas fire in southern California in this satellite image from Dec 2017. Satellite imagery often reveals the source and extent of smoke that may impact aviation, as well as giving a hint of surface wind conditions.

Fire fuel

While wildfires are nothing new, recent years have seen longer droughts and stronger windstorms across many areas that are historically prone to conflagrations. This has extended the “wildfire season” in many of these places to a near-year-round menace. But what does this mean for aviation? Wildfires can form any place there is fuel and an ignition source.

That fuel is dry timber, grasses, and other dead or dried out organic matter. Those who fight wildfires refer to the fuel by time, as in 1-hr, 10-hr, 100-hr and 1000-hr fuels. These fuel ratings roughly correspond to the diameter of the fuel and refer to the approximate time it takes a fuel of that diameter to dry out as ambient humidity drops.

Accordingly, 1-hr fuels are thin grasses and scrub, generally under 1/4 inch diameter, which takes only an hour or so to dry to a moisture level equivalent to the surrounding air; 10-hr fuels are 1/4 to 1 inch diameter; 100-hr fuels range from 1 to 3 inches diameter; and 1000-hr fuels are 3 to 8 inches in diameter, meaning it would take a larger dead tree branch about 1000 hrs to dry out.

A reason that wildfire managers use this scale is that it makes it easier to track the availability of dry fuels and to understand how quickly the vegetation in a particular location might respond to a sudden shift toward favorable fire conditions. That information can help assess the likelihood of a fire igniting or estimating how rapidly it may spread.

Often, it is the 1 and 10-hr fuels that grow most quickly during brief wet seasons. They are adapted to taking advantage of a short period of rain, and the ecosystem may even be dependent on the frequent burning of those grasses and shrubs to provide nutrients to the soil. After their short growing period, this vegetation may become dormant or die.

Adequate humidity or further precipitation keeps the grasses and small brush from drying out completely, but, when warmer and drier air covers the area, the associated low humidity quickly saps any remaining moisture from the vegetation. At that point, all it takes is a carelessly tossed cigarette, an unattended campfire, a lightning stroke, or even a spark from a powerline brought down by high winds.

The latter is the reason that some utilities in fire-prone areas are now turning off power when winds exceed a threshold at which it could cause power lines to come down.

High-resolution visible satellite image from the Camp fire in California in Nov 2018. Larger wildland fires can disrupt aviation at many regional airports, even some distance from the fire itself.

Atmospheric conditions

In general, the atmospheric conditions that produce wildfires also keep most of the smoke and ash produced by those fires from rising too high into the free atmosphere.

There are some occasions where the heat of the fire is able to destabilize the lower level of the atmosphere and even generate convection. However, given the lack of humidity, what is carried aloft is not a vapor cloud, but rather a smoke cloud.

The rising ash and aerosols frequently carry a charge that can create lightning and make the smoke cloud appear as a cumulonimbus, but without rain.

Lightning from these pseudo-storms may, in turn, ignite more fires nearby. The primary challenges of wildfire smoke are a loss of visibility and thermal turbulence. Heat from the surface fires, even when not creating rising and billowing smoke plumes due to strong surface winds, will still produce convection that results in often moderate turbulence in the lower levels.

Surface heating is far greater in a fire zone than the normal daily cycle of heat that we expect will produce mild chop on climb-out.

As a result, the burning surface can maintain a strong convective cycle, drawing in cool air from its surroundings and quickly heating it to where it can rise. Strong winds above the surface layer can strengthen the turbulent eddies as they intercept the rising air.


As with smogs and fogs, the smoke layer may be relatively thin and even transparent when viewed from above. However, because it is concentrated near the ground, often trapped beneath a temperature inversion, it can severely reduce horizontal visibility. As a result, pilots will notice a distinct reduction in slant visibility as they descend toward an airport.

In addition, as with smog over major cities, the smoke particulates are effective at scattering light, resulting in muted colors and a deterioration of depth perception. Pilots traversing an area of fire activity at cruise levels do not normally experience any adverse conditions, and can often see the smoke plumes far below, which can provide some indication of the strength and direction of the low-level winds.

This lack of perceived danger can, however, be misleading. Caution must be exercised when climbing or descending through the smoke, which, although not inherently dangerous to aircraft, should be treated as flying through any other cloud. The smoke is likely to extend all the way to the surface, so pilots should be prepared to miss the approach if they cannot spot some part of the runway or its lights by the decision height.

Forecasters often issue wildfire danger maps to alert local emergency managers and help to
stage firefighting assets. Pilots can use this information to plan for potential fire disruptions at certain airports.

Special considerations

On takeoff, pilots need to treat smoke as they would fog. With horizontal visibility often reduced to a few hundred feet, the movements of other aircraft or vehicles on the runways or taxiways become less certain. If you can’t see far enough ahead to stop in time to avoid a collision, it may be prudent to wait for improved conditions, slow down on the taxiways, or even ask for a “Follow Me” truck.

While you may be certain of your position, if there are other aircraft or vehicles moving about, some of them may not be fully aware of their exact location, posing a collision hazard. Some of the most important considerations of operating in smoke are ones we may not think of.

In an active fire zone, there are often aerial firefighting operations underway involving multiple aircraft flying low and erratically, and their pilots are concentrating on delivering water or fire retardant.

While they may be in communication with ATC at nearby controlled airports, their paths may take them into or near the patterns of uncontrolled fields with little or no warning to approaching or departing pilots.

Normally, ATC will close the airspace around where aerial firefighting is taking place, but that is not guaranteed in more remote places, and it remains up to individual pilots to make themselves aware of any such advisory notams before they get anywhere near the region. Some countries have regulations in place prohibiting aircraft from operating too near to fires.

For example, Canadian regulations keep aircraft at least 5 nm from any active wildland fire. In the reduced visibility of a fire zone, it may be impossible to see a forest service helo or a low-flying, fast-moving McDonnell Douglas DC-10 tanker in time to avoid a collision as you approach some small airport downwind of the fire area.

A couple of other considerations are that, when you are on the ground, your passengers may need masks or even respirators before you open the cabin door. Every year, thousands of people die in structure fires from smoke inhalation. While outdoor smoke from wildfires tends to be less concentrated, it can still pose a significant health danger to you and your passengers, especially if you/they must spend significant time outside.

Once the smoke has cleared, you will also want to wash your aircraft to get rid of any residual ash that may have deposited, and change your cabin air filters because they will likely be clogged with smoke particulates. In extreme cases, you may need to arrange for a thorough interior cleaning to remove the smoke smell that will likely have permeated the aircraft.

Wildland fire forced the closure of YVC (La Ronge SK, Canada) in 2015. Airports in a fire zone may close due to the danger from the fire itself, or, more commonly, from the effects of smoke or presence of airborne firefighting activity.

Fire forecasting

While larger wildfires make the news, and pilots can reasonably prepare to operate in and out of affected airports, the vast majority of forest and brush fires go unreported. There are frequently hundreds of smaller wildfires in play on any given day of the year.

Airports in and downwind of places such as the Amazon basin, the Congo, or Indonesia, are nearly perpetually affected by the smoke from the many fires burning there year-round.

On some days, the shear number of small fires burning in those places collectively produces more smoke than single-source larger fires. The first hint of the presence of smoke at an airport can be found in the metar report. Look for the code “BR” in the remarks. BR is derived from the French brume, meaning smoke, though it is normally used to indicate any aerosol condition reducing visibility, including smog, haze, and even mist.

A next step is to examine any satellite photos over the area. Because smoke tends to be low level, thin, and warm, it will often not show up well on infrared satellite images, which are dependent on temperature differences to differentiate colder cloud from the warm ground beneath. But visible satellite images will normally show smoke plumes.

In an otherwise cloud-free airspace, imagery can quickly pinpoint the source of the smoke, and the direction and relative strength of the surface winds. Narrow and long plumes suggest the wind is fairly steady from a single direction, keeping the smoke confined to a distinct path. Plumes that spread out, on the other hand, indicate the presence of gusty winds or windshear turbulence.

Some plumes will even shift direction downwind from the source, giving pilots a clue about potentially strong directional shear. In mountainous areas, the plume can sometimes reveal mountain wave flow, helping pilots steer clear of the affected area. In situations where clouds are also present on the satellite image, it can sometimes be difficult to differentiate clouds from smoke.

This is where other satellite images can come in handy. Infrared images will show the clouds more clearly, while areas of smoke appear empty of cloud. Similarly, water vapor images will only show where there is water vapor. Although fog may be warm enough to not show on an infrared image, it will still likely register on a water vapor image.

So, if there appear to be clouds in the visible image that do not appear on either the infrared or the water vapor image, then it is likely that those are dry and warm smoke plumes.

Low-level smoke

If there is smoke beneath the clouds, satellite imagery is unlikely to be helpful. Instead, look to sources of wildfire information. Many countries provide this information and are able to track and map fire hotspots using infrared sensors on satellites.

In the US, a good source for active fire information is the National Interagency Fire Center (www.nifc.gov); in Canada, it’s the Canadian Wildland Fire Information System (cwfis.cfs.nrcan.gc.ca). Using their information and maps showing wind direction, it is not too difficult to estimate what airports might be affected by smoke plumes.

It takes time to control wildfires

Once you’ve established that smoke is likely affecting an airport at which you intend to operate, you can use TAFs, prog charts and other forecasting tools to generate a reasonably reliable estimate of whether the smoke will still be an issue when you’ll be departing or arriving.

In general, wildfires take several days to get under control when they are fought, so you can expect that, unless meteorological conditions significantly change in the hours leading up to your arrival or departure, the smoke affecting the airport now will still be present in a few hours or even a day or more. Things to look for that will indicate whether smoke will dissipate or get worse include a change in humidity and wind speed or direction.

The likelihood of precipitation is also a good indicator, although if thunderstorms are forecast, that can often lead to lightning igniting new fires and worsening the situation. On forecast maps, look for large-scale changes in the weather, normally brought about by an approaching or developing synoptic system such as a frontal passage or strengthening of a low over the area.

At the least, these larger systems will change the airflow patterns, moving the smoke in a different direction and perhaps clearing the air at your airport. At best, they will decrease the wind, bring in humid air and even precipitation that, combined, will help to mitigate the fire and smoke. TAFs will also often confirm these changes.

The meteorologists who provide your preflight briefings are a great source of knowledge about potential smoke impacts. When smoke is a factor, it is a good idea to speak with an actual briefer, rather than rely solely on a computerized weather briefing that may not be able to fully describe the behavior of smoke affecting your flight.

As always, if you encounter conditions that are not as forecast or expected, including smoke or the absence of it, or if you happen to spot a fire in a remote area, a pirep is always appreciated by meteorologists and your fellow pilots.

Karsten Shein is co­founder and science director at ExplorEiS. He was formerly an assistant professor at Shippensburg Univer­sity and a climatolo­gist with NOAA. Shein holds a commercial license with instrument rating.