Once a departing aircraft has crossed the CAT 3 holding point, it is deemed to be on the runway until it is airborne. In addition to this, each aircraft needs to be given a longer 'runway occupancy' time. As a result, aircraft must wait farther back from the runway, at the CAT 3 holding point. However, when landing in fog, the integrity of the signal that the runway sends up to the aircraft (more on this later) is vital. In good weather conditions, aircraft waiting to enter the runway hold at the Category 1 (CAT 1) holding points. One of the main changes during LVPs is the holding points around the runways.

Under LVPs, the way in which ATC and aircraft operate changes significantly. When the visibility at an airport drops below a certain level, normally 600 meters (2,000 feet), the airfield switches to Low Visibility Procedures - or LVPs. Unlike valley or radiation fog, advection fog moves laterally, being blown in over the land by the wind.Īdvection fog over the Golden Gate Bridge, San Francisco. As the warm moist air off the Pacific passes over this colder water, it condenses and forms fog. The California Current brings cold water from Canadian waters down the Californian coast. A great example of this is the fog that forms around the San Francisco Bay. If this air mass cools to the dew point, fog will form. When warm, moist air moves over the top of a colder surface, the warm air cools. The fog is confined by the hills around it, and if the weather conditions are calm, it can last for several days. It's commonly the result of a temperature inversion that passes over the top of the valley or dip, keeping the cold air in.

Cold air settles in the troughs of a valley and as it condenses, fog forms. You may have noticed the effects of valley fog when driving into a dip of a road at night. Related reading: How the weather affects your flight - the atmosphere and winds Valley fog As the temperature moves away from the dew point, the air is able to hold more water vapor and the fog starts to evaporate. See privacy policy.Īs the sun rises, the surface starts to heat up. The Points Guy will not share or sell your email. I would like to subscribe to The Points Guy newsletters and special email promotions. This water vapor condenses around particles in the air and forms fog. When the temperature of this air reaches the dew point, it is unable to hold water as a gas. As the land cools, moist air close to the surface also cools. On nights where there is no cloud cover to keep the heat in, the land surface loses its heat to the atmosphere. Ever woken up in the morning to drive to work and there's a layer of fog sitting on the road? This is most likely to be radiation fog. Radiation fog is the type of fog you'll most commonly see. There are several different types of fog, each caused by different weather conditions. Mist is defined when the visibility is greater than 1 kilometer (.6 miles) and fog is when the visibility is less than 1 kilometer. What causes fog?įog and mist are, in effect, just clouds sitting on the ground. So why does fog cause delays, and how do pilots operate differently to keep our passengers safe in foggy conditions? Here's your guide. This results in aircraft backed up at the gates and other aircraft going round holding patterns in the air, waiting for their turn to land. While it's great for the photographers out there, we all know that fog combined with air travel normally results in delays and disruption.įogbound airports are forced to reduce the number of flights taking off and landing every hour. Combine the two and you've got the perfect recipe for fog. As winter closes in, the nights are getting colder and there's more moisture in the air.

Low visibility