When the atmosphere is well-mixed through its depth, air temperature tends to fall as pressure reduces with height. However in some circumstances, the opposite is true. For example, at night, the lowest layers of the atmosphere can cool rapidly as heat is radiated from the ground into space. During, clear calm conditions, this often means that low-lying areas become colder than surrounding higher ground. This dense, cold air can linger through some winter days, especially when fog forms and prevents the sun from warming the ground.