NEW YORK (CNN/Money) - Picture a harried gate agent trying to explain to you why your flight is delayed: A blizzard at O'Hare. A hurricane in Miami.

Actually, truly horrific weather is rarely the reason your flight won't depart or arrive when you expect. But more modest climatic conditions affect the air traffic system in big ways.

Causes of flight delays The reasons flights are delayed, broken down by percentage of all flights reported as delayed in October 2003. Explanation  % of delays  System delay*  39.8 %  Air carrier problem  25.6 %  Late leaving  24.1 %  Cancellation  6.8 %  Extreme weather   2.2 %  Diversions  1.0 %  Security  0.04 %   * includes airport operations, heavy flight volumes, air traffic control, etc.  Source:  Bureau of Transportation Statistics

The Bureau of Transportation Statistics Thursday released its first-ever study of the underlying causes of airline delays; the bureau plans to release monthly surveys.

A surprising finding of Thursday's report for October is that extreme weather (blizzards, hurricanes and the like) was the culprit in just 2.2 percent of all flight delays.

Dave Smallen, a spokesman for the bureau, cautions that drawing fuller conclusions will require additional data. "October was one of the best months ever for weather," he noted. "So we really need to look at long-term data to see trends emerging."

Even so, few things are as infuriating to travellers as delays, so the new statistics provide a helpful look at one of the enduring mysteries of air travel.

The biggest percentage of delayed flights were caused by slowdowns to the entire aviation system -- for example, when extra-heavy traffic slows down operations in one section of the country, producing a domino effect in other areas.

More than half of delays attributable to the system in October had something to do with the weather. Unlike the category "extreme weather," however, system-related slowdowns imply a fix, at least in theory.

"You might, for example, improve things with better equipment or instruments," Smallen said. "Or maybe you could modify flight patterns, or explore some other ways to improve performance."

In other words, when a blizzard hits, travellers just have to adapt. But a little rain might not have to slow you down.

To be sure, the airlines have not been doing so badly this autumn, at least in terms of arrivals and departures.

Worst on-time records Airlines reporting the worst on-time records during the 12 months ending 10/31/03. Airline  On time %  American Eagle  79.7 %  US Airways  80.1 %  America West  81.6 %  Alaska Air  81.5 %  Continental  81.6%     Source:  Bureau of Transportation Statistics

According to the BTS, 86.4 percent of U.S. commercial air flights arrived on-time in October, one of the best monthly performances since the Department of Transportation (DOT) began keeping track in 1995.

In other words, the chances that any flight you take will depart or arrive "on time" -- defined as leaving or arriving at the gate within 15 minutes of its scheduled time -- have been quite good lately.

Of course, the travel industry's doldrums have lightened the traffic load considerably over the past few years, so some bit of improvement might be expected.

The BTS survey provides the following definitions for the causes of flight delays:

Air carrier: The cause of the cancellation or delay was due to circumstances within the airline's control (e.g. maintenance or crew problems, aircraft cleaning, baggage loading, fueling, etc.).

Extreme weather: Significant meteorological conditions (actual or forecast) that, in the judgment of the carrier, delays or prevents the operation of a flight (e.g. tornado, blizzard, hurricane, etc.).

National Aviation System (NAS): Delays and cancellations attributable to the national aviation system that refer to a broad set of conditions -- non-extreme weather conditions, airport operations, heavy traffic volume, air traffic control, etc.

Best on-time records The airlines reporting the best on-time records during the 12 months ended 10/31/03. Airline  On time %  Southwest  85.9 %  United  85.0 %  Northwest  83.7 %  American  83.1 %  Delta  82.5 %   Source:  Bureau of Transportation Statistics

Late-arriving aircraft: A previous flight by same aircraft arrived late, causing the present flight to depart late.

Security: Delays or cancellations caused by evacuation of a terminal or concourse, re-boarding of aircraft because of security breach, inoperative screening equipment and/or long lines in excess of 29 minutes at screening areas.