The International Air Transport Association released airline passenger demand metrics for the first month of the summer travel season late last week, and the results proved to be strong.However, the airline industry has fluctuated so far this summer. So let’s take a look at some of the biggest airline stocks as the vital summer travel season nears its end.Traffic DataIATA reported global passenger traffic data and demand for June on August 3. The firm announced that global passenger demand soared by 7.8% year-over-year based on passenger traffic data. According to previous reports, demand rose 7.7% in May.On top of June gains, the industry reached a 12-year high in traffic growth at 7.9% for the first six months of the year. The industry’s first half 80.7% load factor, a full flight measurement, reached an all-time high.Overall June capacity, measured in available seat kilometers, rose by 6.5%, while load factor gained 1% to 81.9%.“A brighter economic picture and lower airfares are keeping demand for travel strong,” IATA’s CEO Alexandre de Juniac said in a statement. “But as costs rise, this stimulus of lower fares is likely to fade. And uncertainties such as Brexit need to be watched carefully. Nonetheless, we still expect 2017 to see above-trend growth.”June airline demand climbed 4.4% in North America and capacity rose 4.1%, while load factor grew marginally to 84.5%. Travel to North American was up, but travel to and in international destinations rose far more. European airlines traffic jumped 8.8% in June, and the region’s load factor rose 1.8% to 85.9%, which is the highest overall. Asia-Pacific, Latin American, and African airlines’ traffic all skyrocketed over 9% in June, with Africa posting the highest jump at 9.9%.With all of this increased traffic airline companies must be doing well, right?AirlinesDelta DAL, Air France-KLM AFLYY, Lufthansa DLAKY, and Ryanair RYAAY are all currently Zacks Rank #1 (Strong Buy) stocks, and the airline transportation industry currently ranks in the top 7% of the 256 different industries that Zacks tracks.U.K. based Ryanair rests only slightly below its 52-week and all-time high. Lufthansa currently sits just $1 below its 52-week high water mark. However, the airline industry as a whole has suffered since mid-July.Some of the biggest U.S. airlines, Southwest LUV, United UAL, and American Airlines AAL, all fell less than 1% on Tuesday as part of a growing downward trend. Smaller carriers Spirit SAVE, JetBlue JBLU, and Hawaiian HA also dipped slightly.The US Global Jets ETF JETS, which tracks the airline industry as a whole, fell as well. Boeing BA stock was also dragged down slightly along with the airline companies.July and August flight metrics won’t be reported for a while. But June flight demand, traffic, and load increases have not seemed to help airlines at the moment. So for now, investors might have to see if airline companies can turn heightened demand into soaring stock prices.Looking for Stocks with Skyrocketing Upside?Zacks has just released a Special Report on the booming investment opportunities of legal marijuana.Ignited by new referendums and legislation, this industry is expected to blast from an already robust $6.7 billion to $20.2 billion in 2021. Early investors stand to make a killing, but you have to be ready to act and know just where to look. See the pot trades we're targeting>>Want the latest recommendations from Zacks Investment Research? Today, you can download 7 Best Stocks for the Next 30 Days. Click to get this free report Southwest Airlines Company (LUV): Free Stock Analysis Report Ryanair Holdings PLC (RYAAY): Free Stock Analysis Report JetBlue Airways Corporation (JBLU): Free Stock Analysis Report Delta Air Lines, Inc. (DAL): Free Stock Analysis Report United Continental Holdings, Inc. (UAL): Free Stock Analysis Report Spirit Airlines, Inc. (SAVE): Free Stock Analysis Report Hawaiian Holdings, Inc. (HA): Free Stock Analysis Report Deutsche Lufthansa AG (DLAKY): Free Stock Analysis Report American Airlines Group, Inc. (AAL): Free Stock Analysis Report Air France-KLM SA (AFLYY): Free Stock Analysis Report Boeing Company (The) (BA): Free Stock Analysis Report US GLOBAL JETS (JETS): ETF Research Reports To read this article on Zacks.com click here. Zacks Investment Research