Travelers to Europe may have anticipated the crowded sites, full hotels and workers’ strikes as industry experts pointed to a returning tourism demand that is outpacing even prepandemic levels. But the latest extreme weather has flagged another danger for visitors: summer heat waves, which climate researchers say will become longer, more frequent and more intense in Europe.

The soaring temperatures in recent days have coincided with a popular time to visit Europe, the most popular overseas destination for American travelers this year, industry watchers say. And while extreme weather has caused travel disruptions in the past, particularly in countries not set up to withstand it, the heat can also be deadly: More than 61,000 people died in last summer’s heat waves in Europe, according to a recent study.

In the latest heat wave, European officials have taken measures to try to shield visitors from the extreme weather. After a tourist collapsed in front of the Acropolis in Athens, the authorities closed the site briefly on Friday and Saturday, citing the heat. And volunteers from the Hellenic Red Cross have handed out free bottles of water to visitors to help them stay hydrated.

On Monday, Britain’s Foreign Office updated its travel advice for Greece, Italy and Spain to note the extreme temperature and the risk of wildfires, cautioning British tourists to stay abreast of local weather updates.

In Rome, which was expecting especially baking temperatures on Tuesday and Wednesday, officials mobilized a task force of workers and volunteers to patrol sites like the Colosseum and outdoor markets to hand out water and spot people suffering from heat stress. Officials in many European countries have warned people to stay indoors during the day’s hottest hours, and measures like cooling stations have been introduced in many cities that are popular with tourists.

Even with large numbers of visitors to Europe this summer, travel advisers say there are signs of changing travel patterns that are likely to become the norm if extreme heat persists. July and August are typically peak travel months, but the season has started stretching into April, May, September and October.

Some travelers are also opting to visit European countries further north, while others are planning their itineraries around the cooler hours of the day.