September and early October this year resembled late summer more than autumn gray. However, for several days now, the weather has brought cooling, precipitation and cloudy skies. And even if fewer people are considering their next flight (because the flights themselves are also handled less in autumn and winter than in the summer season), the weather is much more likely to make it difficult and prolong, or even prevent, travel. Threats to aviation from autumn and winter weather are not just snowstorms and dense fog. With some of them the aviation services and technological achievements are coping better and better, against others they are still powerless and in dealing with them we have to acknowledge the superiority of the weather.
Fog and low cloud cover
Fogs, i.e. water droplets or ice crystals suspended in the air, causing a reduction in horizontal visibility to less than 1 km, are inevitably associated with autumn. And rightly so, since the frequency of their occurrence in Poland is highest in November, October and September (7.4 percent, 6.8 percent and 5.4 percent of meteorological observations per month, respectively). In the remaining months of the cool half of the year, the frequency of fog in our country does not exceed 5 percent, and 3 percent in the warm half of the year (Kotas, 2020). On a daily basis, fogs occur most often in the second half of the night and in the morning, but sometimes persist for the entire 24 hours.
Fogs can take many forms. They can only form near the ground and dribble over meadows, without limiting visibility at the observer’s eye level. Such fogs are associated with radiated heat from the ground and disappear quickly after sunrise or when the wind speed increases. However, if such fog forms very early at night and grows vertically, it may not disappear after sunrise and may persist throughout the short autumn or winter day.
Also, advection fogs, which form when warm and moist air masses flow over the cooled ground, may not subside after sunrise and persist for days. These are phenomena that sometimes cover the entire country and occur even at high wind speeds. Visibility will then improve only after a change in the air mass, such as the passage of an atmospheric front (Szewczak, 2014).
For air traffic, local airport conditions are also important in the context of fog. The location of airports close to river valleys or wet meadows means that dense fogs will be much more likely to occur there, thus increasing the danger to aviation. Wind direction is also important. Fog can form over meadows or bodies of water and be blown around the runway. There are times when local conditions adversely affect visibility at a particular airport, even though the neighboring airport in close proximity has beautiful and sunny weather.
Mists are often, but not always, accompanied by low stratus clouds – stratus. They can also occur regardless of visibility limitation, especially with the passage of warm fronts, intense rainfall or in warm areas of low and high pressure systems. We speak of stratiform clouds when their base is less than about 300 meters above ground level. However, for passenger aviation, those covering more than half the sky and located at an altitude of 100-150 meters or lower have a significant impact. The lag of low clouds will also delay the improvement of visibility and the cessation of fog by reducing the input of solar radiation.
The ability to carry out flight operations (takeoffs, landings), but also to move aircraft on the tarmac, as well as technical vehicles, baggage cars, etc., in conditions of reduced visibility and low cloud cover is dependent on the ILS (Instrument Landing System) category, the direction of approach at the airport and the arrangements of the airport managers (ICAO, 2018). ILS categories define the minimum visibility and cloud base height that must be present at an airport for a landing to be possible. In Poland, the airports in Gdansk and Warsaw currently have the III, the highest, category (visibility of at least 75 m in Gdansk and 200 m in Warsaw, with a base height of at least 15 m and 30 m, respectively).
The remaining airports are Category II and I: visibility is 300 m and 800 m, respectively, and altitude is 60 m. Regardless of the ILS category of the airport, decisions on the possibility of landing and takeoff are made by the pilot in each case, guided by the regulations for the type of aircraft, individual authority and the actual conditions found. Failure to land will result in having to wait in holding for improved conditions at the airport or, with prolonged persistent fog or cloud cover forecast, having to divert to an alternate airport. Dense fog and, to a lesser extent, stratus clouds can cause restrictions on the movement of aircraft and vehicles on the tarmac, which can also generate delays and endanger aviation.
Precipitation that impedes air traffic takes a variety of forms. In autumn and during warm winters, we have to deal with rain and drizzle. They can reduce visibility or cause a layer of water to build up on the runway and taxiways. This extends braking and takeoff distances by reducing the friction of the wheels against the ground or the formation of a water cushion. Intense precipitation can cause a significant reduction in visibility, but generally these phenomena are not so prolonged as to significantly delay takeoffs or landings.
A much more difficult situation is created by winter precipitation. We may have to deal with snowfall that causes, among other things. The need to clear snow from the runway and taxiways, resulting in their temporary closure. They can also reduce visibility as intensely as dense fogs. They also cause, despite the use of chemicals on the runway surface, a reduction in the coefficient of friction of the pavement and an increase in braking and takeoff distances (ULC, 2016).
During snowfall, it may also be necessary to de-ice the aircraft before takeoff. Although such measures may cause delays, they are necessary for safety reasons. Even a small layer of snow or ice on the aircraft reduces the lifting power of the wings and increases the weight of the machine, thus lengthening the takeoff path or making it impossible at all. Covering the aircraft with the appropriate chemicals causes the possible ice layer to detach from the wings during takeoff.
The most dangerous type of precipitation, occurring in autumn and winter, is freezing rain. This is when there is a negative temperature near the ground and a positive temperature at an altitude of about 100-600 meters. In such conditions, precipitation does not have time to freeze and is in the form of overcooled water droplets that, upon contact with the frozen ground, momentarily freeze, forming a layer of glassy, transparent ice – glaze. The occurrence of freezing precipitation is associated with a warm atmospheric front and is relatively rare, but the layer of ice so formed is much more difficult to remove than snow or ice formed as a result of a gradual drop in temperature.
There is also a risk that ice will be deposited on the wings of the aircraft during rainfall and despite the positive temperature at ground level – it is important to remember that the landing aircraft is severely chilled. In such a situation, a thin, transparent layer of ice invisible to pilots and ground services may be deposited on the wings. The ice formed by the ice from the ice firmly adheres to all surfaces and forms a transparent, hard coating causing reduced wing lift and friction on all surfaces and increasing the weight of the aircraft. Such icing requires special attention and thorough removal from the surface of the machine and the airport.
Flights, and other weather phenomena
Among other weather phenomena that can hinder or prevent flight operations during the fall and winter seasons are strong and very strong winds. Their incidence increases significantly in the cold semester. Strong wind leads to the occurrence of unpleasant turbulence, wind uplift and makes landing difficult, or even impossible in extreme cases. When attempting a landing, especially in strong crosswinds, pilots may have to abort the maneuver and attempt it again – a go-around. Such a landing can also be much more unpleasant for passengers.
During the occurrence of freezing fog and with negative air temperature and high humidity, the phenomena of frost and rime are formed, which, when intense, cause icing and the need to de-ice aircraft and airport surfaces. Frost is formed by tiny ice crystals, deposited at low wind speeds on horizontal surfaces. It causes a reduction in friction on the tarmac and can result in a reduction in the lifting power of the aircraft’s wings.
Rime or soot is deposited at higher wind speeds on vertical objects in the form of white needles. The phenomenon creates beautiful forms, but the mass of embedded ice can damage equipment and break cables.
Opportunities for counteraction and forecasting
To streamline air traffic in all weather conditions, airports are upgrading guidance and traffic control systems and developing equipment fleets to make snow removal and de-icing procedures more efficient. Traffic control services and duty officers can receive specialized weather forecasts and warnings covering meteorological parameters relevant to an airport (ICAO, 2018). They also have the opportunity to contact synopticians or meteorological observers directly to plan the intensity of air traffic and distribute winter equipment.
Despite the increasing ability to counteract the effects of unfavorable weather on aviation, we are often faced with flight delays or cancellations, especially in the autumn and winter. It also happens that the connection will be diverted to an alternate airport. It’s a good idea to keep track of weather reports and information posted on airport websites before you travel, so you won’t be surprised by long waits or even having to change your plans. Weather can pose a threat to aviation, but thanks to technology, the service is handling adverse events better and better.
In the article, I used, among others. From the works:
Kotas P. (2020). Forecasting selected meteorological elements, IMGW-PIB
Szewczak P. (2014), Meteorology for the airplane pilot, Avia-Test Lech Szutowski
Guideline No. 2 of the President of the Civil Aviation Authority dated January 25, 2016. On methods of assessing, measuring and reporting the condition of the runway surface, ULC 2016, OJLC.2016.6
Annex 3 to the Convention on International Civil Aviation, done at Chicago on December 7, 1944. Meteorological service for international air navigation, ICAO 2018, OJLC.2018.8
https://dlapilota.pl/ (Accessed 20.10.2023)
https://www.samoloty.pl/artykuly-lotnicze/15037-holding-czyli-procedura-oczekiwania (Accessed 20.10.2023)
https://pl.wikipedia.org/wiki/Instrument_landing_system (Accessed 24.10.2023)