Weather is the single largest cause of aviation delays (~70%) and a significant factor in aircraft accidents. Primary hazards include clear-air turbulence (increasing due to climate change), icing (supercooled water freezing on wings), wind shear and microbursts during takeoff/landing, and thunderstorms. Modern systems — onboard radar, ground-based detection, ILS landing guidance, and real-time turbulence reporting — have made flying the safest transport mode despite being the most weather-exposed.
Every commercial flight is a negotiation with the atmosphere — a journey through a medium that is never still, never uniform, and never entirely predictable. The weather that provides a pleasant breeze on the ground can produce invisible turbulence at cruising altitude that throws passengers from their seats. The fog that reduces visibility to a few hundred metres can close airports for hours, cascading delays across continents. The ice that forms silently on wing surfaces can alter an aircraft's aerodynamics enough to cause it to fall from the sky. Aviation weather — the science of understanding, predicting, and mitigating atmospheric hazards to flight — is one of the most critical and least visible elements of the air travel system that moves 4 billion passengers annually, and its sophistication is the reason that flying in weather conditions that would have been lethal a century ago is now routine.
TL;DR: Weather is the single largest cause of aviation delays and a significant factor in aircraft accidents. The primary weather hazards to aviation are: turbulence (clear-air turbulence is invisible and can cause injuries at cruising altitude), icing (supercooled water freezing on aircraft surfaces degrades aerodynamic performance), wind shear and microbursts (sudden changes in wind speed/direction during takeoff and landing), thunderstorms (containing turbulence, icing, hail, and lightning simultaneously), and low visibility (fog, rain, and snow that impede landing and takeoff). Modern aviation weather systems — including onboard radar, ground-based weather detection, and satellite monitoring — have made flying safer than ever, but weather remains the element that even the best technology cannot fully control.
~70%Of aviation delays caused or influenced by weather
65,000+Turbulence encounters reported by US airlines annually
58Passenger injuries from severe turbulence in a typical year (US airlines)
CAT IIIILS category allowing landing in near-zero visibility
Turbulence: The Invisible Hazard
Turbulence — irregular, unpredictable air motion that causes aircraft to pitch, roll, and bounce — is the weather hazard that passengers experience most frequently and fear most intensely. Most turbulence is merely uncomfortable; severe turbulence, which can produce vertical accelerations exceeding 1g (doubling or halving the apparent weight of everything in the aircraft), can throw unbelted passengers to the ceiling and cause serious injuries. In rare cases, extreme turbulence has caused fatalities — typically to passengers or crew who were not wearing seatbelts and were thrown against the aircraft structure.
Clear-air turbulence (CAT) — turbulence that occurs in clear sky, away from any visible cloud — is the most dangerous form because it is invisible. CAT is produced by wind shear (changes in wind speed or direction with altitude or horizontal distance) in the vicinity of jet streams, mountain waves, and temperature inversions. It occurs at cruising altitude (9,000–12,000 metres) where aircraft travel at their fastest and where the aerodynamic effects of turbulence are most significant. Unlike convective turbulence (associated with thunderstorms, which are visible on radar and can be avoided), CAT cannot be detected by current onboard weather radar and must be forecast using atmospheric models or reported in real time by other aircraft.
Climate change is increasing clear-air turbulence. Research published in Nature Climate Change has shown that CAT over the North Atlantic — one of the world's busiest air corridors — has increased in frequency by approximately 55 percent since 1979 and is projected to increase further as global warming strengthens the temperature gradients that drive jet stream wind shear. The implication is that flights will encounter turbulence more frequently and that the financial cost of turbulence (fuel burned in avoidance, injuries, and structural inspections after severe encounters) will increase. The development of forward-looking lidar systems that can detect CAT before the aircraft enters it is a priority for aviation research, but such systems are not yet widely deployed.
Icing: The Silent Killer
Aircraft icing occurs when supercooled water droplets — liquid water at temperatures below 0°C — impact the aircraft's surfaces and freeze on contact. The resulting ice accumulation alters the wing's aerodynamic profile, reducing lift, increasing drag, and potentially blocking engine air intakes and sensor ports. Icing is most dangerous during takeoff and landing, when the aircraft is flying at lower speeds (closer to the stall speed at which the wing can no longer generate enough lift) and at altitudes where supercooled water is most common (typically 0°C to -20°C, corresponding to altitudes of approximately 1,000–6,000 metres in winter).
The two main types of airframe icing — rime ice and clear ice — present different threats. Rime ice forms from small supercooled droplets that freeze instantly on impact, trapping air and producing a rough, opaque, white coating. While rime ice disrupts airflow, its rough surface makes it relatively easy to remove with deicing boots (inflatable rubber strips on the wing leading edge). Clear ice forms from large supercooled droplets that spread across the surface before freezing, creating a smooth, dense, transparent coating that can form beyond the protected area of the wing and is much more difficult to remove. The 1994 crash of ATR 72 flight American Eagle 4184, in which clear ice accumulated beyond the deicing boot coverage, remains one of the most studied icing accidents and prompted significant changes in certification requirements for flight in icing conditions.
Ground icing — frost, snow, or ice accumulation on the aircraft while it is parked or taxiing — is addressed through deicing procedures: the application of heated glycol-based fluid that removes existing contamination and provides temporary protection during taxi and takeoff. The "clean aircraft concept" — the regulatory requirement that all aircraft surfaces be free of frozen contamination at the moment of takeoff — is one of the most fundamental safety principles in winter aviation operations. Violations of this principle have contributed to numerous accidents, including the 1982 Air Florida crash in Washington, D.C., where inadequate deicing contributed to a wing stall immediately after takeoff, killing 78 people.
Wind Shear and Microbursts: The Low-Level Threat
Wind shear — a sudden change in wind speed or direction over a short distance — is most dangerous near the ground, where the aircraft's altitude provides little margin for recovery. A headwind that suddenly becomes a tailwind during approach causes an immediate loss of airspeed and lift; if the pilot does not respond quickly with additional power, the aircraft can drop below the glidepath and strike the ground short of the runway. This scenario has caused numerous fatal accidents, including the 1985 crash of Delta Flight 191 at Dallas-Fort Worth, which encountered a microburst during approach and crashed on the airport grounds, killing 137 people.
Microbursts — concentrated downdrafts that produce a pattern of intense wind shear as the descending air strikes the ground and spreads outward — are the most dangerous form of wind shear because they produce a characteristic sequence that is counterintuitive for pilots. An aircraft approaching through a microburst first encounters a headwind (increasing airspeed and lift, which the pilot may compensate for by reducing power), then passes through the downdraft core (where the vertical component pushes the aircraft downward), and finally encounters a tailwind (dramatically reducing airspeed and lift at the worst possible moment — when the aircraft is low, slow, and descending). The entire sequence can occur in 10–20 seconds, giving the pilot minimal time to recognise the situation and execute a go-around.
The deployment of Low-Level Wind Shear Alert Systems (LLWAS) and Terminal Doppler Weather Radar (TDWR) at major airports has dramatically reduced microburst-related accidents since the 1990s. These systems detect the wind patterns associated with microbursts and provide real-time warnings to pilots and air traffic controllers. The training of pilots in microburst recognition and escape manoeuvres — added to airline training programmes after the Delta 191 accident — has further reduced vulnerability. The combination of detection and training has made microburst encounters survivable events rather than the fatal traps they were before these systems existed.
Thunderstorms: The Multi-Hazard Environment
Thunderstorms are the most comprehensively dangerous weather environment for aviation because they contain multiple hazards simultaneously: severe turbulence (updrafts and downdrafts exceeding 30 m/s), icing (supercooled water at multiple altitudes), hail (ice stones that can damage or destroy engines and airframe components), lightning (which strikes aircraft regularly but rarely causes structural damage due to the aircraft's conductive skin), and wind shear (in the vicinity of storm downdrafts and outflow boundaries). The policy for all commercial aviation is simple and absolute: avoid thunderstorms. The minimum recommended clearance is 20 nautical miles (37 kilometres) from any thunderstorm identified as severe.
Onboard weather radar is the pilot's primary tool for detecting and avoiding thunderstorms. Modern weather radar systems display the precipitation intensity within clouds, colour-coded from green (light precipitation) through yellow (moderate) and red (heavy) to magenta (extreme — indicating hail, extreme turbulence, or both). Pilots route around areas of heavy and extreme precipitation, requesting deviations from air traffic control when the planned route passes through or near thunderstorm cells. In areas of widespread thunderstorm activity (such as the summer afternoons over the US Southeast or the tropical convergence zones), extensive deviations may be necessary, adding fuel consumption, flight time, and complexity to the flight.
For Greek aviation, thunderstorm hazards are most significant during the autumn and early spring, when Mediterranean depressions produce organised thunderstorm activity over the Aegean and Ionian Seas. Summer thunderstorms over the Greek mainland are typically localised over mountainous terrain and can usually be avoided by routing over the sea or through the clear air between cells. Athens International Airport, located in the Attica plain surrounded by mountains, occasionally experiences thunderstorm-related delays and diversions when storms develop over the surrounding terrain and move over the airport. Thessaloniki's Makedonia Airport, exposed to both mountain-generated and frontal thunderstorms, experiences more frequent weather-related disruptions.
Low Visibility Operations: When You Can't See the Runway
Fog, low cloud, rain, and snow can reduce visibility below the minimums required for safe approach and landing, triggering delays, diversions, and cancellations that cascade through the aviation network. The Instrument Landing System (ILS) — which provides electronic guidance for the final approach — is the primary technology for landing in reduced visibility, and its capability determines the minimum visibility at which an airport can accept traffic.
ILS operations are classified into categories based on the minimum visibility (Runway Visual Range, or RVR) and minimum decision altitude (the lowest altitude at which the pilot must see the runway to continue the approach). Category I ILS requires a minimum RVR of 550 metres and a decision altitude of 200 feet (61 metres) — suitable for moderate visibility reduction. Category II requires 300 metres RVR and 100 feet (30 metres) decision altitude. Category IIIa requires 200 metres RVR with no decision altitude (the aircraft can touch down using the ILS alone). Category IIIb requires only 50 metres RVR for taxi-out after landing. Category IIIc — theoretical zero-visibility landing — has been certified but is not operationally used because the taxi phase after landing still requires some visibility.
The capability of an airport to conduct low-visibility operations depends not only on the ILS equipment but on the airport infrastructure: precision approach lighting, taxiway lighting, ground radar for surface movement control, and the training and certification of both pilots and air traffic controllers for low-visibility procedures. Athens International Airport is equipped with Category IIIa ILS capability, allowing operations in visibility as low as 200 metres. Many smaller Greek airports, including island airports critical for domestic connectivity, have only Category I capability or visual approach procedures, making them significantly more vulnerable to weather-related closures from fog or low cloud.
The Future: Better Forecasts, Smarter Aircraft
Aviation weather science continues to advance on multiple fronts. Numerical weather prediction models — the computer simulations that produce the forecasts on which flight planning depends — are becoming more accurate, more detailed, and more frequently updated, providing pilots and dispatchers with better information about en-route conditions. Satellite-based weather monitoring, including the GOES-R series with its lightning mapping sensors, provides near-real-time information about thunderstorm development and movement that complements ground-based radar.
Onboard systems are also advancing. Forward-looking lidar (light detection and ranging) systems that can detect clear-air turbulence several kilometres ahead of the aircraft are in development and have demonstrated the ability to provide 1–2 minutes of warning before a turbulence encounter — enough time for the crew to turn on the fasten seatbelt sign and for passengers to secure themselves. Enhanced weather radar systems that can distinguish between hail and rain (critical for avoiding the most dangerous thunderstorm cores) are being deployed on newer aircraft. Automated wind shear detection systems that use the aircraft's own flight data to detect and respond to wind shear in real time have become standard equipment and have contributed to the near-elimination of wind shear accidents.
The single most significant improvement in aviation weather safety, however, may be the simplest: communication. Real-time turbulence reports from aircraft (transmitted automatically through data link systems that report the aircraft's vertical acceleration along with its position and altitude) create a continuously updated map of atmospheric conditions that is more detailed and more current than any forecast. When one aircraft encounters turbulence, every following aircraft on a similar route can be warned within minutes. This crowd-sourced weather reporting — thousands of aircraft acting as atmospheric sensors — is transforming aviation weather from a forecast-dependent discipline to a real-time monitoring system, reducing surprises and improving both safety and passenger comfort.
Aviation weather safety — from turbulence avoidance to low-visibility landing systems — is the invisible infrastructure that makes modern air travel safe in conditions that would have been lethal to earlier generations of flight.
Key insight: Weather is the only major threat to aviation safety that technology can mitigate but never eliminate. Mechanical failures can be prevented through maintenance, human errors reduced through training, and security threats deterred through screening — but the atmosphere cannot be controlled, only understood and avoided. The extraordinary safety record of modern aviation — with a fatal accident rate of approximately one per 10 million flights — is achieved not by conquering weather but by developing the forecasting, detection, and avoidance systems that allow aircraft to navigate through it safely. The atmosphere remains sovereign; aviation survives by respecting its authority.
The safety paradox: The safest form of transportation on Earth is also the one most vulnerable to weather. Aircraft operate in the atmosphere — the very medium that produces weather — and are affected by atmospheric conditions that ground transportation can ignore (clear-air turbulence, icing at altitude, jet stream wind shear). Yet flying is statistically safer than driving because the aviation industry has invested more in understanding, predicting, and avoiding weather hazards than any other sector of human activity. The mode of transport most threatened by weather is the safest because it takes the threat most seriously.
Weather and air travel:
Turbulence injuries happen almost exclusively to unbelted passengers — keep your seatbelt fastened whenever seated
Clear-air turbulence is increasing due to climate change — expect more bumpy flights in the future
Thunderstorms are the most dangerous weather for aircraft — all commercial flights avoid them by wide margins
Fog is the most common cause of weather-related airport closures — Athens can operate in 200 m visibility (CAT IIIa)
Greek island airports with basic ILS are more vulnerable to fog-related closures than mainland airports
Winter flights may be delayed by deicing — this is a safety procedure, not an optional convenience
In summary: Aviation and weather exist in a relationship of permanent negotiation — the atmosphere produces the hazards (turbulence, icing, wind shear, thunderstorms, low visibility) and aviation develops the technology, procedures, and forecasting systems to navigate through them safely. The result of this negotiation is the safest mode of transportation in human history, operating routinely in conditions that would have been considered impossible a century ago. The investment in weather understanding that the aviation industry has made — from satellite monitoring to onboard radar to automated turbulence reporting — represents the most sophisticated human effort to manage atmospheric risk, and its success is measured not in technology but in the approximately 100,000 flights that operate safely every day worldwide, each one a small victory of knowledge over uncertainty in the endlessly variable medium of the sky.
