Heatwaves are caused by a system of higher atmospheric pressure, whereby air from upper levels of the atmosphere descends and rotates out. As it descends, it compresses, increasing the temperature. The outward flow, meanwhile, makes it difficult for other systems to enter the area, and the large size and slow speed of the hot air causes the heatwave to remain for days or even weeks. The lack of clouds means that an affected area is struck with strong sunlight.