One of the driving forces of a hurricane is heat energy in oceanic surface waters. Warm water evaporates more quickly, and warm air rises. If it moves onto land it loses that warm water source, and so dies down. The single most important factor in a hurricane losing energy is friction.
Similarly, why do hurricanes weaken as they move over land?
In the forward right quadrant of hurricanes, the storm's forward motion adds to the wind speeds, because they are both in the same direction. Second, as a minor weakening effect, when the storm moves on land, surface wind speeds are decreased by friction.
What happens to a hurricane when it crosses land?
As a hurricane crosses over land, it begins to dissipate, or break apart and reduce in strength. At this point, a hurricane can still cause a lot of damage because of high winds, rain, and flooding, but unless it makes its way back over the open ocean, it is downgraded from a hurricane back to a tropical storm.
What can cause a hurricane to weaken?
Sometimes, even in the tropical oceans, colder water churned up from beneath the sea surface by the hurricane can cause the hurricane to weaken (see Interaction between a Hurricane and the Ocean). Hurricane decay can also be caused by strong vertical wind shear, a change in wind direction or speed with height.