Summer is the season for warm weather. So, when temperatures reach 5°F to 10°F above average, it can be excessively hot. When this type of weather lasts for multiple days, it is usually the result of a phenomenon known as a “heat dome”.
Although not an official meteorological term, it does help paint a picture of what is happening. To start, an area of high pressure develops under a ridge in the jet stream. Acting like a lid in the upper atmosphere, it forces warm air that would normally rise to sink back toward the surface. As it sinks, it compresses and warms even further. Unable to escape, the hot air is remains in place until the ridge breaks down or moves.