If you think this summer has been extremely hot, you’re not wrong. According to the National Oceanic and Atmospheric Administration (NOAA), the U.S. is being blanketed in swelteringly high temperatures—again. CNBC reports that a heat dome is affecting much of the country, raising temperatures 15 degrees higher than normal.

In simple terms, a heat dome occurs when warm oceanic air becomes trapped in Earth’s atmosphere by high pressure. In years when La Niña warms the western Pacific, hot air begins to build over the ocean and pushes east across land. A high pressure “dome” traps the hot air, and it sinks toward the ground. This dome blocks the heat from escaping, thus making everything hotter.

Although heat domes are common, some are more severe and can cause widespread problems.

The Pacific Northwest has been dealing with the high heat for weeks. As The New York Times reports, both Washington and Oregon have already set triple-digit temperature records. The heat overwhelmed hospitals with weather-related emergencies and resulted the deaths of more than 60 people.

Heat domes can even cause wildfires and disrupt marine life. The heat dome in western Canada last month killed over 1 billion marine animals along the Pacific coast, according to the Guardian.

A recent study in the journal Nature Climate Change [PDF] predicts that climate change will amplify the frequency and intensity of extreme heat. Think about investing in a great (energy-efficient) air conditioner, because there may be more heat domes in the coming years.