“Always refreshingly cool inside,” reads a board outside the movie theater. The photo, taken around 1940, is a clue to how the summer blockbuster got started.

In the 1920s, many theaters would close during the hottest months because the dark, windowless rooms were boiling hot and customers found them stifling.

While Balaban and Katz opened the world’s first mechanically cooled movie theatre in Chicago in 1917, it wasn’t until 1925 that Willis Carrier, the inventor of air conditioning, installed a more sophisticated system in the Rivoli Theatre in Times Square.

According to Rotten Tomatoes , Carrier persuaded Paramount Pictures to install the air con while it was still under construction.

The Rivoli Theatre opened on Memorial Day weekend in 1925, and initially the air con didn’t work. As the moviegoers filed in, they got out their fans to try and cool themselves. But as the unit finally kicked in, people stopped fanning themselves, and they were able to focus on the movie far better.

Adolph Zukor, who was head of Paramount Pictures, witnessed this and realized immediately what a powerful technology this was for his business. Now, instead of summer being a dead period for the nascent movie industry, the theaters could sell coolness itself by providing a place to escape from the heat and watch a film.

The Blend reports that one of the first summer blockbusters was a 1943 war drama called Bombardier . It was advertised as the “blockbuster of all action-thrill service shows.”

It wasn’t until the 1970s that Jaws and Star Wars became the type of quintessential summer blockbuster movie that has become a hallmark of the Hollywood calendar. By that time, air con was becoming more and more common in American homes.

Nevertheless, it is possible to draw a straight line from these early photos of theaters advertising their air conditioning to the summer blockbuster phenomenon that still exists today .