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Common Weather Factors

The following headings relate to the most common weather conditions affecting airline operations. Each category considers the type of weather condition, effect on operations and the IOC responses for likely actions.


Fog is one of the most challenging weather conditions for airline operations. For departing flights, the crews must be able to taxi, then see a certain distance (by counting runway lights) down the runway or be informed by АТС of the measured RVR (runway visual range). For arriving aircraft, at most airports and for the vast majority of instrument approaches, they must be able to see the runway at a certain (decision) height before continuing the approach to land or execute a missed approach if not. Forecasting precise fog conditions and trends is extremely difficult, yet the IOC, Dispatch and the operating crews all depend on a high degree of accuracy and prediction to enable the creation of accurate flight plans, sufficient carriage of fuel and timely departures. Fog can form or dissipate rapidly, changing the circumstances markedly and making operations difficult to pre-empt. Sometimes, fog is covered by cloud, obscuring the sun, which slows the process of the fog burning away.

IOC response

Although fog can disrupt departures from an airport if it is sufficiently thick and there is little wind to move it, by far the main problem relates to approaches and landings. Thus, flights preparing to operate to fog- risk or fog-bound airports require careful consideration. In terms of the IOC involvement, several pieces of information are needed for the risk assessment. Some of this will be asked of the station or port itself as local knowledge is usually invaluable. The Meteorologist within the IOC plays a crucial role, as does the opinion of the Operating Captain(s).

  • a) What are the conditions now, and at the scheduled arrival time? (Ascertaining trends in the weather may indicate whether conditions are improving or deteriorating.)
  • b) Have other aircraft been landing, or are any at present?
  • c) Are aircraft holding at present? (This may encourage an IOC to dispatch a flight if other aircraft (or АТС) are anticipating landings in the short term.)
  • d) Are there other arrivals due at the same time as the flight in question? (Landings or missed approaches/diversions by other aircraft may help to inform the IOC’s, and the operating crew’s decisions appropriately.)
  • e) What is the forecast period for the fog? (If thick fog is forecast to continue for several hours with little change, a decision will be made either to hold at the departure port with a view to operating delayed or even consider the worth of operating the flight at all.)
  • f) How much fuel is/are the aircraft carrying? (If Controllers believe the fog will lift close to the arrival time, or that they are more willing to hold for a period rather than divert in a particular case, they may request additional fuel to be carried - this will delay the flight of course, as the refueller would probably need to be recalled to the aircraft.)
  • g) What alternates have been planned? (If several flights are operating to the fog-bound port, and their crews nominate the same alternate, congestion may arise at the alternate port if there are multiple diversions.)
  • h) How long can the aircraft hold before diverting?
  • i) Can the use of a different alternate increase holding time?
  • j) Has there been a history at that port over the last few days? If so, when did the fog dissipate previously, and were the circumstances similar to the current event?

Having gathered such information, the IOC then has options to consider:

  • a) The aircraft could depart (either on schedule or delayed) if there were a good chance of landing without excessive holding.
  • b) If other aircraft (other companies) have decided to depart according to a similar schedule, this might influence the IOC’s decision. (Others might have more current information, or worse - they could be working on obsolete advice, so the rigour of the information process needs to be both comprehensive and thorough.)
  • c) If it departs, should it hold close to the destination for a limited time (given that fuel is expensive!) with the expectation that АТС will let it execute an approach or two?
  • d) If the forecast is for the fog to dissipate at some estimate after the scheduled arrival time, should the flight be delayed to coincide with this later time?
  • e) If the fog is not expected to change for several hours up to and beyond the arrival time, should the flight be delayed substantially, or cancelled? (Cancellation may result in a simple ‘out and back’ cancellation, but if the aircraft is committed to continue on to a third port, should another option be considered?)
  • f) If, once the fog has lifted, the airport subsequently is inundated with arrivals, can the IOC start dispatching flights, and if so, should the arrivals be staggered to even the traffic flow (and keep delays to a minimum)?
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