More than 300 historic heat records have been broken in the US this summer. See where it was the hottest.

Between June 1 and September 12, a subset of 356 all-time heat records from California to Maine were tied or broken, according to a CNN analysis of National Oceanic and Atmospheric Administration data. CNN analyzed the highest maximum and minimum temperatures collected by weather stations across the country with at least 30 years of data.

While large parts of Texas and California endured several days in a row of triple-digit temperatures, the state that tied or broke the most heat records this summer was Oregon. The Beaver State has recorded 66 different times this summer, nearly 60 of them in July alone.

As stifling as things could feel during the day, the heat after sunset was even more unusual. In the lower 48 states, this summer’s overnight lows were the warmest on record. More than 70% of all heat records tied or broken were the lowest temperatures reached during the day.

In addition to several high-pressure systems in the west and northeast, the higher number of nighttime records than during the day may be partly explained by wet conditions in parts of the country, according to NOAA National Center Climatologist Ahira Sanchez-Lugo. For Environmental Information.

“Clouds help to cool the daytime temperature a bit by blocking the sun’s rays, while trapping the Earth’s heat at night,” said Sanchez-Lugo.

A North American monsoon was active in the Southwest this summer and rain clouds have raised high daytime temperatures and low nighttime temperatures, Sanchez-Lugo said. In New Mexico, for example, all 15 heat records tied or broken in July and August were overnight lows.

Warming nighttime temperatures pose a particular threat to communities without air conditioning, such as metro areas in California and Oregon. More than half of residents in the San Francisco metro area do not have air conditioning, according to the 2019 American Housing Survey. In San Antonio and Boston, which are in wetter climates, the same is true for less than 10% of homes.

However, the heat records broken in California and Oregon occurred in sparsely populated inland mountain and desert regions. While temperatures in the 90s and even 100s and 110s can occur in parts of metro Los Angeles, San Francisco and Portland, the biggest reason those cities have fewer air-conditioned homes is because the ocean temperatures are much cooler than on the Gulf and East Coasts.

“The sea breeze almost always blows cooler air over the ocean to keep coastal areas relatively mild,” said Ryan Littrell, a meteorologist with the National Weather Service in Los Angeles. “Consequently, air conditioning is not necessary for the vast majority of the year.”

Although areas closer to the coast are still warming, the cooler waters of the Pacific Ocean have helped moderate the heat spikes experienced further inland. In downtown Los Angeles, the average high temperature has risen 0.7 degrees over the past 30 years. During that time period, San Antonio’s average high has risen 2.8 degrees, according to NOAA.

“It’s warmer here than the first half of the 20th century, but we still have the cool Pacific Ocean to help,” Littrell said.

CNN’s Judson Jones contributed to this report.