The planetary boundary layer is the lowest layer of the troposphere where wind is influenced by friction. The thickness (depth) of the PBL is not constant. At night and in the cool season the PBL tends to be lower in thickness while during the day and in the warm season it tends to have a higher thickness. The two reasons for this are the wind speed and thickness of the air as a function of temperature. Strong wind speeds allow for more convective mixing. This convective mixing will cause the PBL to expand. At night, the PBL contracts due to a reduction of rising thermals from the surface. Cold air is denser than warm air, therefore the PBL will tend to be shallower in the cool season.