Why Weather Matters at Airports

Weather is the single biggest external factor affecting aviation operations. It determines which runway is in use, whether aircraft can land, how much fuel a flight needs to carry, and whether you can see anything from the spotting mound. At Prestwick, the coastal location on Scotland's west coast creates a distinctive weather environment that every spotter and aviation enthusiast should understand.

How Weather Affects Flying

Wind

Wind is the most immediately important weather element at an airport:

• Runway selection — aircraft take off and land into the wind whenever possible. This reduces ground speed during takeoff and landing, which means shorter runway distances and safer operations. At Prestwick, the prevailing westerly to south-westerly wind means runway 12 (heading approximately 120 degrees, towards the south-east) is used most often, because aircraft approach from the north-west, into the wind
• Crosswind — wind blowing across the runway rather than straight down it. Pilots have crosswind limits for each aircraft type. In strong crosswinds, you will see spectacular crabbed approaches where the aircraft points into the wind at an angle to the runway, straightening up just before touchdown
• Wind shear — sudden changes in wind speed or direction, especially at low altitude. Wind shear is dangerous during takeoff and landing and is reported by pilots and detected by sensors
• Turbulence — gusty or variable winds, particularly around hills and buildings, can make approaches uncomfortable or unsafe

Visibility

Pilots and controllers must be able to see the runway environment for visual approaches, and even instrument approaches have minimum visibility requirements:

• RVR (Runway Visual Range) — measured in metres, this is the distance a pilot can see along the runway. Below certain RVR values, operations may be restricted or impossible
• Categories of approach:

- Cat I ILS — minimum decision height 200 feet, minimum RVR 550 metres

- Cat II ILS — minimum decision height 100 feet, minimum RVR 300 metres

- Cat III ILS — can land in very low visibility (RVR as low as 75 metres with Cat IIIc)

Prestwick has ILS approaches for both runway 12 and runway 30, supporting Cat I operations as standard.

Cloud Base (Ceiling)

The height of the lowest broken or overcast cloud layer determines the ceiling. If the ceiling is below a certain height, pilots cannot see the runway from their approach path and must either use instrument procedures or divert to an airport with better weather.

Rain, Snow, and Ice

• Rain — reduces visibility and creates a wet runway (longer stopping distances). Heavy rain can affect engine performance
• Snow — requires runway clearing, de-icing of aircraft, and may close the airport if accumulation is severe
• Ice — icing on the aircraft (in cloud at freezing temperatures) adds weight and degrades aerodynamic performance. On the ground, ice on the runway requires treatment
• De-icing — in freezing conditions, you will see aircraft being sprayed with de-icing fluid (typically orange or green) before departure. This is a fascinating process to watch and photograph

Prestwick's Weather Character

Prestwick sits on the Ayrshire coast at 20 metres above sea level, facing the Firth of Clyde and the open Atlantic beyond. This location gives it distinctive weather patterns:

Gulf Stream Influence

The North Atlantic Drift (an extension of the Gulf Stream) brings relatively mild water past the west coast of Scotland. This means:

• Mild winters — snow is relatively rare at Prestwick compared to airports further east or inland
• Cool summers — the sea keeps summer temperatures moderate
• High humidity — the maritime air mass carries a lot of moisture

Prevailing Westerlies

Scotland sits in the band of prevailing westerly winds. At Prestwick, the wind blows from between south-west and north-west on most days. This is why runway 12 (into the west/south-west wind) is the dominant runway direction.

When an easterly flow does establish — typically with high pressure to the north — runway 30 comes into play, and the weather character changes significantly.

Haar (Sea Fog)

Haar is the local name for advection fog — fog formed when warm, moist air moves over cold sea. At Prestwick, haar typically:

• Forms when mild, moist Atlantic air flows over the colder coastal waters of the Firth of Clyde
• Is most common in spring and early summer (April to June)
• Can roll in very quickly, reducing visibility from 10 kilometres to a few hundred metres within an hour
• May be patchy — the airfield can be clear while the approach path is foggy, or vice versa
• Usually clears when the wind picks up or shifts direction, or when the sun warms the land enough to burn it off

Haar is significant for spotters because it can end your photography session abruptly. It is also significant for operations — aircraft may have to hold, divert, or use lower-visibility approach categories.

Atlantic Fronts

Prestwick sits directly in the path of Atlantic weather systems — the low-pressure systems and their associated fronts that sweep across from the west. A typical frontal passage brings:

1. Warm front — high cirrus cloud, then lowering and thickening cloud, then steady rain. Gradual deterioration

2. Warm sector — low cloud, drizzle, poor visibility. Can persist for hours

3. Cold front — heavier rain, then rapid clearing with showers. Dramatic skies and excellent photography light as the front passes

The Prestwick Advantage: Diversions

Prestwick's coastal location gives it one significant advantage: it often has better visibility than Glasgow Airport (EGPF), which sits inland in the Clyde Valley where radiation fog forms readily.

When Glasgow closes due to fog — particularly in autumn and winter — flights divert to Prestwick. These diversions are exciting events for spotters, bringing aircraft and airlines that do not normally visit. You might see easyJet, Jet2, TUI, Loganair, and other operators on the Prestwick apron during a major diversion event.

Monitoring the METAR and TAF for both EGPK and EGPF helps you predict when diversions might occur.

Weather and Your Visit

Practical weather considerations for spotting at Prestwick:

• Check the METAR and TAF before you go — available on aviationweather.gov, the Met Office, or dedicated apps
• Layer your clothing — conditions can change rapidly on the coast. Base layer, mid layer, windproof outer layer
• Waterproof everything — your camera bag, your notebook, yourself
• Wind chill — on exposed positions like the Mound, wind off the Firth of Clyde cuts through inadequate clothing. Dress for the wind, not the thermometer
• Best photography weather — broken cloud with sunny intervals after a cold front passes. Blue sky with cumulus clouds makes the ideal backdrop