The summer solstice arrives on Wednesday, June 21. It marks the longest day of the year and the first day of summer. But people north of the equator are already living with record heat one day before the start of summer. In the southwestern United States, airlines canceled flights and doctors told people to stay indoors during the warmest parts of the day. Cities like Phoenix and Las Vegas are known for their heat. But even residents of those cities are worried about temperatures as high as 49 degrees Celsius. It has not been that hot for 20 years.