Sea Breeze Front

The leading edge of the sea breeze is called the 'sea-breeze front', which propagates inland

Figure 14.8 The growth of a sea-breeze cell during the day.

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6am noon 6pm midnight 6am

Figure 14.8 The growth of a sea-breeze cell during the day.

6am noon 6pm midnight 6am

Figure 14.9 The diurnal variation of the winds at Jakarta (6°S) at different elevations. Wind speed is shown in units of km/h. The diagram shows onshore surface winds (solid lines) from about 9 a.m. till 9 p.m., though the strongest winds occur around 4 p.m. at about 200 m. The return flow (dashed lines) is fastest at 2 km around 5 p.m. The sea-breeze flow deepens during the daytime until well after 4 p.m.

(Figure 14.8) because of the difference between air densities ahead and behind. In other words, the sea breeze is an example of a density current (Note 14.D), studied in the 1970s by John Simpson in England. The front is a zone of convergence of marine air and the air there already, leading to ascent and therefore, sometimes, a line of cumulus cloud.

The front moves inland as continued heating of the land enlarges the cell, advancing at a fraction of the sea-breeze speed and generally accelerating during the day, from below 3 m/s to over 6 m/s, for instance. The result is that a sea breeze may be felt well inland. An extreme example is shown in Figure 14.11. Inland penetration of sea breezes is especially notable in Australia, because of its aridity, high temperatures and flatness. A sea breeze which starts at the coast near Perth in Western Australia might reach 400 km inland by the evening and last there for 2-3 hours, having already died away at the coast. Similarly, a sea breeze from the Gulf of Carpentaria in the north of Australia has been observed from 2 a.m. to 9 a.m. at Tennant Creek (20°S), which is more than 500 km inland.

The arrival of the front is indicated by a switch of the wind to the onshore direction, a drop of air temperature, a rise of surface pressure by a hectopascal or so, and an abrupt decrease of the air's wet-bulb depression (Section 6.3). There is often a considerable increase of wind speed and of turbulence (Figure 14.12), especially where a dry surface allows rapid heating, and a cold ocean current along the coast accentuates the land-sea difference of temperature. The arid coast of northern Chile has sea breezes which approach gale force, and those on the coast of Western Australia at Geraldton (at 29°S, where the total rainfall is only about 30 mm during the

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