Sea breezes, land breezes, and coastal fronts are caused by variations in heating or cooling at the Earth's surface. The direction of surface air flow is from colder air toward warmer air. The leading edge of a sea breeze, land breeze, or coastal front is an air mass boundary with a strong wind shift, called a sea breeze front, land breeze front, or coastal front respectively. In addition to causing air temperatures to be colder than they would otherwise be, the sea breeze causes an enhancement of precipitation along the front and a suppression of convection behind the front. Land breezes tend to be much weaker than sea breezes. Coastal fronts can be quite strong and last for days. They are caused by the heating of air over water, and are often reinforced by the trapping of cold air along the coastline by the mountains. Coastal fronts can lead to freezing rain within the cold air.