As heatwaves become increasingly frequent, lasting longer and hitting harder, travelers are opting for cooler vacation spots. The U.S. Environmental Protection Agency reports that heatwaves are having a growing impact on travel choices, especially during the warmer summer months.

"Traditionally, when we think of vacations, beaches and tropical getaways come to mind," an Expedia spokesperson told CNBC Travel. "However, vacations in cooler destinations allow you to pack a light sweater instead of a sunhat, as temperatures are milder."

From January to June 2024, flight searches to Lapland, Finland, increased by 705%, according to Expedia. The online travel agency also noted rising interest in Canada, with searches for Banff up 65% and Lake Louise up 55%.

Home rental site Vrbo identified several destinations among the top 40 most booked for this year, including Breckenridge, Colorado, the White Mountains of New Hampshire, and the Pocono Mountains in Pennsylvania.

Stephanie Grayscher from California shared that she planned a "family trip to Whistler for the summer."

"We live in the San Fernando Valley in Los Angeles, where it's extremely hot for most of the summer, with temperatures well above 38°C," she said. "We want to be comfortable when we travel and fully experience a city without having to escape the extreme heat."

European cities like Rome and Madrid are usually top summer destinations, but rising temperatures across Southern Europe are taking a toll on the summer travel season. This summer, Italian officials issued warnings for 12 cities expected to reach dangerously high temperatures.

"For the entire summer, from June to August, temperatures in Southern Italy were, on average, about 3°C higher than usual," Jason Nicholls, Chief International Forecaster for AccuWeather, wrote in an email to CNBC. "There were multiple heatwaves across Italy and the Balkans between July and August."

"Northern Europe as a whole has truly seen a boom in visitors over the past two years," says Delphine Combes, product executive at travel agency Scott Dunn. "Norway has emerged as one of the top five European destinations, climbing eight spots since 2022."

Combes added that the agency is expanding its offerings for summer travel in Finland, Sweden, and Switzerland to meet the growing demand.

"These are destinations where we previously only offered winter and holiday packages," Combes noted.

According to Visit Finland, foreign visitors made more than 4.8 million trips to the Nordic country over the past 12 months. Foreign overnight stays from January to June 2024 were up 9% year-on-year, with most visitors coming from Germany, France, the UK, Sweden, the U.S., and the Netherlands.