The combination of wind setup and the inverted barometer effect associated with storms can create a pronounced increase in the sea level. This is often called a storm surge. An additional process in the form of a long surface wave travelling with the storm depression can further exaggerate this sea level increase. A negative surge is the opposite effect, generally associated with high pressure systems and offshore winds, and can create unusually shallow water. This effect is of great importance to very large vessels which may be navigating with small under-keel clearances.