N8 N7 N6 N5
Figure 14.10 Prevailing surface winds at Brisbane. The numbers indicate the mean speed (km/h). Daytime winds from the north-east, east and south-east are sea breezes, which are fostered at Brisbane in summer by easterly gradient winds resulting from the shift southwards of the highs across Australia (Section 13.6).
period October-March) are sufficiently strong to distort trees growing 16 km inland. The sea breeze on the desert west coast of South Africa typically reaches 10 m/s.
A sea breeze brings welcome relief in hot weather in summer, especially if the front arrives before mid-afternoon, thus preventing temperatures reaching their normal maximum. That happens only within a few tens of kilometres of the beach, depending on the time of the breeze starting and the speed of the front's advance. This limit to the area benefiting from sea breezes affects real-estate values.
There are other effects too. A sea breeze spoils the waves for surf-riding by creating a bumpy surface called 'chop'. Separate sea breezes on opposite sides of Cape York Peninsula in the north of Queensland approach each other and collide, creating a Morning Glory (Note 8.L). A similar confluence of sea
breezes from opposite sides of tropical islands such as Viti Levu (Fiji) or Bathurst Island (north of Darwin) leads to ascent inland and then rainfalls there in the afternoon. Sometimes the arrival of a sea-breeze front may trigger deep convection if the air ahead is unstable, so that there are thunderstorms, e.g. in moist low-latitude environments, such as those of Brisbane in summer and Port Moresby (9°S).
The opposite to a sea breeze happens at night, when the land has cooled below the local SST. On shore, such 'land breezes' are slower than 1 m/s and less deep than the daytime sea breezes, because the stability of the atmosphere at night confines the flow to a shallow layer close to the ground's friction.
Land breezes are usually strongest offshore, and may propagate as a density current more than 100 km out to sea, possibly triggering nocturnal thunderstorms over the relatively warm waters. Nocturnal storms of this kind are common off the east coast of Australia in winter. Thunderstorms at night are also provoked by the convergence of land breezes from the shores of Lake Victoria in Africa, or within large bays (e.g. the Gulf of Carpentaria to the north of Australia) or between the islands of the Indonesian archipelago. Likewise, there is convergence offshore from an ocean coast on nights when the land breeze meets an onshore gradient wind which is weak (and moist), again creating uplift and the possibility of rain. In fact, most thunderstorms in the tropics are initiated by sea breezes, land breezes and mountain winds, unlike the situation in midlatitudes, where cold fronts are usually responsible.
The daily alternating between a sea breeze and a land breeze produces a circular hodograph in Sydney, for instance (Figure 14.6).
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