Aviation Weather Tips: All You Need to Know about Ceilings
Ceilings at arrival and departure airports – measured cloud base height relative to the ground – impact business aircraft operations around the world. Conditions are most significant when sky is totally obscured and particularly when local terrain is also a factor. Ceilings impact visual flight rule (VFR) and instrument flight rule (IFR) operations differently, and minimum ceiling considerations will depend on type of flight, operator’s standard operating procedure (SOP), pilot experience, and operator comfort level. For the most accurate planning data it’s always important to work with your aviation meteorologist, both during the trip planning phase and closer to day of departure.
The following is an overview of what you need to know:
Ceiling requirements vary
Depending upon the crew, level of expertise and flight department SOPs, different requirements may be considered in terms of ceilings. In some cases, the crew may specify a minimum ceiling height, perhaps 3,000 ft, while in other cases they may require higher minimum ceiling heights.
Definition of ceiling
Ceiling at your intended destination may be defined as the lowest broken or overcast cloud layer. If the sky is totally obscured, height of vertical visibility is used as the ceiling. Your forecaster will provide insights regarding expected changes to ceilings, as well as other weather events such as fog is expected to move in/out of your destination.
Cloud cover and measurement of ceiling
Different types of cloud cover conditions include SKC (sky clear), FEW (trace), SCT (scattered), BKN (broken) and OVC (overcast). Cloud cover is reported in terms of 1/8th of sky cover with 1-2/8th being FEW, 3-4/8ths being SCT, 5-7/8th being BKN and 8/8 denoted at OVC. Automated Weather Observing Systems (AWOS) at larger airports routinely measure cloud heights and ceilings. In addition, many airports have a certified weather observer assist with and oversee AWOS data. We find that these weather observers are generally the most reliable source in determining accurate ceilings.
Part 135 charter (non-scheduled commercial) operators must, by regulatory requirement, use official government weather forecasts and not forecasts provided by any weather provider. This may affect the charter operator’s ability to operate to certain locations, or require a flight diversion, even though actual weather may differ from an official forecast. Private non-revenue operators have more flexibility in terms of sourcing weather data and providers. Certain airports may have minimum ceiling recommendations, based on local conditions and terrain. While minimum recommended ceiling/visibility conditions may be 800 ft vertical/2 miles horizontal (800/2), private operators, especially those very familiar with particular locations, may elect to operate even when ceilings are below recommended minimums.
Determining ceiling height
To make accurate determinations of ceiling height, your weather provider will first use weather forecasts and then other tools including satellite imagery, Pilot (weather) Reports (PIREPs) and local observations. Airport Meteorological Aerodrome Reports (METARs) provide current conditions while Terminal aerodrome forecasts (TAFs) indicate forecast conditions. Weather model data provides another source of useful information. However, the human factor is always important as forecasts can differ substantially from actual weather conditions. A pilot, for example, may call the tower for an observation, but the person he/she speaks with may not be qualified to provide the correct information. In other cases, weather information may be given in a language you’re not familiar with. In parts of South America, for example, weather information may only be available in Spanish or Portuguese.
Alternate airport considerations
When considering minimum ceiling requirements for destination airports, one must also consider available airport alternates. When operating to parts of China, Russia or Canada you may have few nearby alternate options, and this may mean specifying a higher minimum ceiling requirement for your destination airport. Additionally, equal time points (ETPs) along route of flight may impact minimum ceiling specifications. Whenever ceiling height is below minimum criteria, the airport should not be used as an alternate. When choosing alternates we often use 800/2 (based on Part 135 regs) as a minimum for planning purposes. It’s important to remember, however, that your service provider can only give you options. It’s the pilot in command’s responsibility to accept/reject minimum ceiling height conditions an airport alternates for any flight plan.
When operating to remote locations there may be limited TAFs, METARs, PIREPs and local weather data available. Your weather provider may need to estimate ceiling height based on weather models, satellite imagery, local terrain conditions and weather data from surrounding locations. Airport location, elevation, mountainous terrain, moisture levels and upper level soundings all help your weather professional to make the most accurate ceiling prediction.
Obscured visibility and seasonality
In some regions, and at certain times of year, ceiling heights may be habitually below minimums. Some locations in India, for example, may be under IFR conditions most of the year due to smoke and haze issues. The same is true in parts of China during the rice field burning season.
As weather conditions change rapidly, forecasts are most accurate when obtained no more than two to three days prior to operation. In determining predicted ceiling height it’s always best to rely on forecasts obtained within 24 hours of flight. The region you’re operating to is also a factor, as low ceiling conditions may be more likely during particular times of day or seasons of the year. Many tropical island locations, for example, are most susceptible to low ceilings during early morning, right before sunrise. Fog conditions in the London area can be notorious during particular times of year. Climatology research is always a useful tool in predicting low ceiling conditions. Your forecaster will access historical data to predict expected weather/ceiling conditions well ahead of day of flight.
To avoid potential flight delays and/or diversions we recommend obtaining preliminary weather briefs – to give you general information on destination weather – and following this up with detailed forecasts 48 hours, 24 hours and immediately prior to flight.
Depending on the type of flight, company SOPs, captain experience, and surrounding alternates, operators have differing requirements for ceilings for each destination. Always contact a certified meteorologist for the most accurate prediction of weather and ceilings for your trip. There are many factors that are used in predicting ceilings so it’s important to obtain preliminary forecasts and updates just prior to take off.