Source: Heat Wave | Wikimedia
Heat waves are extended periods of unusually hot weather relative to the normal temperature of an area for a given season. They commonly occur during the summer when weather patterns are slower to shift. Heat waves can worsen drought conditions, put added stress on crops, and have health impacts for humans and livestock.
Most often heat waves happen when a ridge of high pressure sits over an area for an extended period of time. Dry air descends, compresses and warms, which quickly heats the ground. This raises the air temperature, and since high pressures are typically cloud free, the increased sunlight also contributes to the warming air.