Most persistent shifts are characterized initially by a transient TWS. The new wind, typically colder than the wind it replaces, appears at one edge of the course, undermines and reduces the strength of the old wind. When its surface flow gradually increases in velocity it spreads across the course. As the old wind is displaced, the shift (abrupt or gradual) is toward the new wind and, if on a beat, an affected boat must be headed toward the new wind. In most parts of the world, new winds that develop just before or during a daytime race are sea or lake breezes (or late-in-the-day gradient winds returning to replace sea or lake breezes). The same Category II condition can also be created by the appearance of an upslope or downslope wind, by the appearance of a new gradient wind, by the down flow from a thunderstorm, by the separation of the over-water flow from the overland segment of a parallel-to-shore gradient wind, and by the separation of the surface elements of a sea breeze coming at water level around a point from its overland elements coming off the point.