Patrick T. Fallon | Bloomberg | Getty Images

A ground operations employee loads baggage onto a Southwest Airlines plane at John Wayne Airport in Santa Ana, California.

Southwest's big twice-a-year fare sale is back, with round-trip fares below $100 on dozens of the carrier's shortest routes.

The sale fares also include longer routes, with the price of flights loosely tied to distance. Flights begin at $49 each way on Southwest's shortest routes and increase to $79, $99 or $129 each way for longer flights.

The sale launched Tuesday morning and is good for travel from Aug. 21 through Dec. 12. Flights on Fridays and Sundays are excluded from the sale as are certain dates around the Labor Day and Thanksgiving holidays. Schedules for Southwest's Florida, Nevada and Puerto Rico flights have additional day-of-week restrictions. Albany, N.Y., is excluded from this sale, though some $49 fares were showing up during the sale period.

More fine print: The sale applies specifically to non-stop flights, though many connecting itineraries may also show lower-than-usual fares. Seats sold at the sale prices are capacity controlled, meaning the cheapest seats will likely sell out on individual flights as the sale progresses.

Some of Southwest's international routes are also included in the sale, though those routes come with significant day-of-travel restrictions. One-way fares on those routes range from $59 to about $263, but are good only for flights on Tuesdays and Wednesdays during the sale period.

Regardless of the details, bargain seekers will have to act quickly to snag the fares. The sale ends on Thursday (June 7) at 11:59 p.m. local time in the city of the departing flight. (Full sale details)

A Tuesday morning spotcheck of fares showed fairly broad availability of the sale fares. The $49 fares do appear on most days on the advertised routes, though some routes show greater availability than others. Some city-pairs showed sale fares on nearly every eligible flight; a handful of routes showed the lowest fares only on unpopular early-morning or late-night flights.

Fares on some routes dropped even lower than the advertised fares. Flights between Sacramento and Boise – for example – were selling for as little as $92 round-trip early Tuesday morning. The same was true for a handful of other routes, including some round-trip combinations on routes such as Charlotte-Nashville and Louisville-Chicago Midway.

The broad fare sale has become a staple for Southwest. It has rolled out similar three-day sales each June and October for the past several years. One sale from June 2015 proved so popular that it crashed Southwest's website, prompting the carrier to extend that particular sale by an additional 24 hours. Southwest's website did not appear to be having any such issues during the current sale.

Southwest has used the sales to generate buzz and -- perhaps more importantly -- to sell seats during what are usually some of the slowest travel periods of the year. The summer sale covers a relatively slow period for the industry. It includes flights from the end of summer -- when demand falls off as vacation season ends -- through mid-December. With the exception of Thanksgiving, which is blacked out, the latter part of that calendar window also marks a period of low demand before travel ramps up again for the winter holidays.

This summer Southwest also has reported a year-over-year drop in bookings following an April accident that resulted in the first-ever accident related death of a passenger on one of the carrier's flights. In a filing with the Securities and Exchange Commission on Monday, Southwest suggested the drop was "primarily driven by lower bookings largely due to reduced marketing efforts following the Flight 1380 accident."

In previous iterations of Southwest's big sales from earlier this decade, the airline pegged fares to mileage thresholds. For example, flights of 500 miles priced at $49 each way and increased from there. Flights of 501 to 1,000 miles cost $99 each way and flights of 1,001 to 1,500 miles cost $129 each way. Flights of more than 1,500 miles went for $149 each way.

While Southwest has ended the precise mileage component of its big sales, its latest versions do closely mimic the previous distance-based sales. The carrier's four advertised sale-fare tiers — $49, $79, $99 and $129 each way — are similar to the prices offered during the distance-based sales of years past.

Whatever the details, travelers can snag advertised round-trip fares for less than $100 on short routes.

Among the sub-$100 options with reasonable availability as of Tuesday morning were:

Albuquerque-Los Angeles; Atlanta-Jacksonville; Amarillo-Dallas Love; Baltimore-Boston; Austin-Lubbock; Baltimore-Charlotte; Buffalo-Baltimore; Richmond, Va.-Atlanta; Manchester, N.H.-Baltimore; Boise-Sacramento; Burbank,Calif.-Phoenix; Chicago Midway-Detroit; Chicago Midway-Omaha; Cincinnati-Baltimore; Cleveland-Atlanta; Cleveland-Milwaukee; Kansas City-Chicago Midway; Columbus, Ohio-Washington Reagan National; Corpus Christi, Texas-Houston Hobby; Dallas Love-Houston Hobby; Denver-Long Beach, Calif.; Des Moines-St. Louis; El Paso-Los Angeles; Fort Lauderdale-Tampa; Harlingen, Texas-Austin; Hartford-Baltimore; Indianapolis-Kansas City; Kansas City-Minneapolis/St. Paul; Kansas City-Nashville; Las Vegas-Ontario, Calif.; Los Angeles-Las Vegas; Little Rock-St. Louis; Long Island/Islip-Baltimore; Louisville-Chicago Midway; Memphis-Chicago Midway; Memphis-Houston Hobby; Midland/Odessa, Texas-Dallas Love; Minneapolis/St. Paul-Chicago Midway; Nashville-Charlotte; New Orleans-Houston Hobby; Norfolk-Baltimore; Oakland-Burbank; Oakland-Portland, Ore.; Ontario, Calif-San Jose, Calif.; Orange County, Calif.-San Francisco; Panama City, Fla.-Nashville; Phoenix-Las Vegas; Portland, Ore.-San Jose, Calif.; Providence-Washington Reagan National; Raleigh/Durham-Atlanta; Salt Lake City-San Diego; San Antonio-Dallas Love; San Francisco-Los Angeles; Tucson-San Diego and Wichita-St. Louis.