As the summer travel season heats up, vacationers who are traveling by plane naturally want to know if their flights will arrive on time this summer. The last thing most travelers want to do is spend lots of extra time in the airport, rather than on the beach!

Unfortunately, the summer tends to be the worst season for air travel delays. Since demand is higher from June to August than for the rest of the year, airlines schedule more flights. If severe thunderstorms -- or worse, tropical storms -- slow departures and arrivals in some of the major hub cities, there's no slack in the system. The potential result is rippling delays and cancellations nationwide.

If you happen to be flying on a day when severe weather hits the country, brace yourself for some long delays. On the other hand, if the weather cooperates, you may have a decent chance of arriving on time this summer, as airlines have cut a lot of capacity in recent years. However, a lot will depend on where you're coming from and where you're going. The most crowded airports in the country will be as busy as ever. If you're flying to or from the likes of New York, Chicago, or Los Angeles, you should be prepared to be late.

Big is the new small
As I wrote back in April, airlines are increasingly favoring larger planes these days. Larger planes are almost always cheaper to operate on a per-seat basis, so as long as there's enough demand to fill the extra seats, it makes sense to "up-gauge." In many cases, it's more profitable to offer a few flights on large airplanes rather than many flights on small regional jets. This industry shift toward larger planes means that airlines have been cutting a lot of flights at small- and medium-sized airports.

Hubs in smaller markets such as Cincinnati, Cleveland, and Memphis have been particularly hard hit. Delta Air Lines (DAL -2.62%) and United Continental are two carriers that have realized they're better off directing traffic through their larger hub cities, such as Detroit, Chicago, and Atlanta. As a result, airports in medium-size cities all have many fewer flights today than they had before the Great Recession. As a result, these airports are likely to experience fewer delays than was previously the case.

The favored few
On the other hand, there are a handful of airports that continue to have chronic delays, because they're highly desirable to the airlines. The three major New York airports -- LaGuardia, JFK, and Newark -- top that list. The two major Chicago airports -- O'Hare and Midway -- also tend to be very busy, as do a few other airports, including Washington's Reagan National Airport.

A rendering of Delta's hub at JFK Airport (courtesy of Delta)

Many of these airports are slot-controlled, meaning the government has capped the number of flights that airlines are permitted to schedule, To mitigate delays. These slots are highly sought after. For example, JetBlue Airways (JBLU -3.12%) bid $40 million for eight slot-pairs (the rights to operate eight round trips) at Reagan National Airport in late 2011. On peak days, airlines will use every single slot that they own at these popular airports.

Delays ahead?
On busy summer travel days, it's very easy for minor problems (often weather-related) to cause major headaches at these airports. For example, in March of this year -- a lighter travel month -- approximately 80% of flights in the U.S. arrived on time (officially, any flight that arrives within 14 minutes of the scheduled time is considered on time). However, at New York's JFK Airport, that figure was 77%; at LaGuardia, it was 73%, and at Newark, it was just 67%.

If you have to fly through one of the busiest airports in the country this summer, your best bet is to drag yourself out of bed and fly early in the morning. When delays hit the busiest airports, they tend to "cascade," getting worse throughout the day, because there's no spare capacity to catch up once delays creep into the schedule.

For example, in April, flights scheduled to arrive before 1 p.m. at 29 of the busiest airports in the U.S. arrived on time more than 80% of the time. However, flights scheduled to arrive between 8 p.m. and 11 p.m. were on time less than 70% of the time. At Newark Airport -- one of United's largest hubs and often the most-delayed airport in the country -- morning flights were near the national average in terms of timeliness, but nearly half of all flights scheduled to arrive after 4 p.m. were late!

Bring a book (or a movie)
If you're lucky enough to be avoiding the most congested airports this summer, you have a good chance to get where you're going in a timely fashion, as airlines have been continually pruning their schedules for the past several years. This move has reduced delays in many mid-sized cities that have significantly fewer flights today than they did a few years ago.

By contrast, airlines are still cramming as many flights as they can into the busiest airports in the country. On the heaviest travel days, inclement weather or any other disruption can cause massive cascading delays at these airports, leading to nightmarish delays in the evenings. If you can use alternative airports to get where you're going, the extra driving time might save you a lot of aggravation. If not, your best bet is to fly early in the day.

On the other hand, if you're flying through busy airports in big cities such as New York or Chicago during the afternoon or evening this summer, my advice to you is to bring a book!