New research shows that the extent of weather-related airline flight delays and disruptions at any major U.S. airport in any given season can be predicted reliably.
Flight delays and cancellations in any given season grow in direct proportion to increases in the precipitation an airport receives or its outside air temperature, compared with the airport's 30-year average for that season, according to a study performed by San Francisco, Calif.-based weather-risk management company WeatherBill, Inc.
Airlines are affected in exactly the same predictable way, the research found.
"We can't control the weather, but now we can do a better job understanding the relationship between weather and flight delays," said David Friedberg, WeatherBill's CEO.
The company studied Department of Transportation records of the nearly 21 million scheduled flights operated by 18 leading U.S. airlines from June 2003 to April 2007. It also used long-term weather data to determine average temperatures and average amounts of precipitation (usually very low) for every day of the year for each of the 54 airports it studied.
WeatherBill found that not only were 3 million of the airlines' flights delayed or canceled by weather during the 46 months of the study, accounting for 55 percent of all flight disruptions during the period, but also that precipitation — either rain or snow — caused far more extensive delays and cancellations than did temperature variations.
However, WeatherBill — founded by former Google employees — also found that warmer temperatures than normal reduce winter delays and increase summer delays in a predictable fashion.
Disruptions proportional to rain or temperature increases
Employing regression analysis, the company was able to estimate the sensitivity of each major airport and airline to adverse weather in a statistically reliable way by showing that delays and cancellations are predictable in line with each inch of rain or each degree of temperature deviation above normal.
The relationships are completely linear and hold for each fraction of an inch of rain or fraction of a degree of temperature deviation, Friedberg said.
Using liquid-equivalent precipitation measurements (with 10 inches of snow being equivalent to 1 inch of rain), WeatherBill found that rain disruptions vary by season. During the study period, 16 percent of flights were affected in winter months, 15 percent in summer, 13 percent in spring and 12 percent in fall.
New York, ExpressJet worst
At two-thirds of the 54 major airports and 13 of the 18 airlines studied, more than half of all flight disruptions were caused by weather. The three major New York-area airports had the highest percentages of weather disruptions. Newark was worst, followed by JFK and then LaGuardia. Fourth-worst was Philadelphia, while Chicago O'Hare was fifth-worst.
ExpressJet — a regional airline which flies primarily on behalf of Continental Airlines, whose biggest hub is at Newark — had the highest percentage of weather disruptions. JetBlue Airways, based at JFK, was second-worst, with Continental third. Delta regional subsidiary Atlantic Southeast Airlines was fourth-worst and Northwest Airlines (two of whose hubs are at snow-affected Minneapolis/St Paul and Detroit) was fifth-worst.
The major airport least affected by weather delays was Salt Lake City. It was followed by Ontario, Calif., Albuquerque, N.M., and the California airports Oakland and San Jose. Regional carrier Skywest had the lowest percentage of weather-related flight disruptions, followed by Mesa Air, Southwest, United and Frontier.
Delay extent varies by airport and season
WeatherBill found that rain-related average delay times in a given season vary substantially from airport to airport. On days during spring months, Atlanta suffers 29 delay minutes for each inch of rain above the long-term average, but San Francisco averages 41 minutes of delay.
Also, at any given airport, there is seasonal variation in the duration of the average arrival or departure delay for every additional inch of precipitation or degree of temperature. At Chicago O'Hare, for instance, the average delay per inch of rain in winter is 75 minutes, but in spring it is 64 minutes. In summer it is 37 minutes, and in fall it is 52 minutes.
Similarly, seasonal delays per inch of rain at Phoenix range from 14 minutes in spring, 23 minutes in summer, and 13 minutes in fall. There is a simple explanation for this, according to Friedberg.
"If you think about what's normal for a particular airport, say San Francisco, it expects a lot of rain in spring but very little in summer," said Friedberg. If there is a lot of rain on a summer day, "The operation is not set up for it — the equipment might not be ready, the (dispatch) crew might not be prepared, and the ground crew might not be prepared. Delays become compounded. On a seasonal basis, the further you deviate from normal, the stronger the impact it has."
Regionals and smaller airlines more affected
Airlines, meanwhile, "are sensitive to the markets they serve — for instance, Mesa in the southwest U.S., which has mainly dry weather," said Friedberg. A regional airline that operates mainly in one area, or operates smaller aircraft, is more likely to be affected "on a raw dollar basis" by "abnormal" weather at its airports than "a larger national airline, even if it has a large number of flights in that area."
In line with this, WeatherBill's study shows that in each season of the year from 2003 to 2007, four out of the five airlines with the greatest numbers of arrival-delay minutes per additional inch of rain were regional carriers. In every season, also, all five airlines with the greatest numbers of departure-delay minutes for each additional inch of rain were regional carriers.
Similarly, in summer, the two airlines most affected in terms of both arrival- and departure-delay minutes by each additional degree of temperature above the daily norm were Comair and American Eagle, regional carriers that operate respectively for the Delta and American networks.
While temperature differences have much less of an effect on flight disruptions than rain, "Unusually warm or cold weather produces strong overcast and significant amounts of wind effect," said Friedberg.