![]() However, such breezes generally extend only a mile or two offshore and die as soon as the sun warms the land. Most are weak, but where cool, dense air is able to flow smoothly down a valley, it should be sufficiently strong for early morning sailing. ![]() However, at night, if the temperature of the land drops below that of the sea, it is possible for the sea breeze effect to be reversed, causing a land breeze that blows off the shore. In Worbarrow Bay, Dorset, high cliffs overlooked by steep hillsides can generate katabatic gusts on clear nightsĪs sea breezes depend upon the sun’s warmth, they inevitably die in the evening, or earlier if clouds move across the sky. Another explanation is that in late summer, the atmosphere is generally warmer than in early spring, so we should expect fine-weather thermals to be less vigorous, irrespective of sea temperature. The sea breeze effect is stronger in spring than in autumn, and this is usually attributed to the greater temperature difference between the warm land and the cool sea during the spring months. If such a wind creates intimidating conditions, perhaps when a boat is approaching a tricky harbour entrance, the problem can probably be resolved by standing offshore for a couple of hours until the breeze loses some of its strength. The good news is that the strongest winds will only persist for a short time, fading before the waves have become fully developed. The sea breeze effect should never be underestimated and is sometimes capable of producing a moderate degree of violence. On one occasion, the day started with a wind of Force 3, but this later increased until sustained anemometer readings of 32 knots (on a boat sailing downwind) suggested that the wind was at least Force 7, and the French coastguard service issued a gale warning. Perfect offwind sailing with the Biscay sea breeze providing plenty of horsepower. When planning a cruise around a large island, such as Britain or Ireland, sea breezes will normally be more favourable during an anti-clockwise circumnavigation (in the northern hemisphere). On the eastern side, the late afternoon wind is coming off the land, but it is nevertheless a sea breeze, blowing over from the opposite coast. In this example, the fully developed breeze cuts across the peninsula. ![]() When sea breeze fronts move well inland, the sea breezes from different coastlines will coalesce, as shown in the diagram above. If the breeze is channeled up a valley, constrained by the valley sides, it will resist the normal tendency to veer during the day. A shallow layer of breeze will flow readily on to a low coastal plain but may not be able to rise over high cliffs. Geographical features modify and deflect sea breezes. Thus, the sea breeze may begin by blowing directly on to the land but end the day blowing parallel with the shore. While the breeze is strengthening, the Coriolis effect changes its direction and it veers so that the low pressure is on its left. Local sea breezes form over peninsulas and coastal islands but later merge into a general airflow towards the larger landmass that lies beyond. When a sea breeze develops during a fine day, an accumulation of warmed, less-dense air causes a progressive drop in atmospheric pressure over larger areas of the land. In the northern hemisphere, the change of direction is a veer to the right, so that the wind vector has low pressure on its left. What if a wind is already blowing onshore?Ī developing sea breeze, penetrating inland and veering during the day: late afternoonĪs soon as any wind starts to blow, from high pressure towards low pressure, the Earth’s rotation compels it to change direction. It may be necessary to make a firm decision: either to steer inshore, hoping for a more consistent sea breeze near the land, or to head offshore and stay in the background wind. The position and extent of this zone will vary during the day and present a puzzle for anyone who is on a sailing craft, hoping for a fast passage. When a sea breeze blows against the background wind, the latter re-establishes its influence further offshore, leaving a zone of calm between the two winds. High-level plumes remain in the background wind while the sea breeze changes the alignment of the lower plumes. The leading edge of a sea breeze is usually quite shallow and we may be able to see this directly if it crosses an industrial site, where smoke stacks or steam vents emit plumes at various levels. To anticipate the onset of this kind of breeze, watch for a dark line to seaward as the band of new ripples moves inshore. In its early stages, a sea breeze may flow inland as a very shallow layer of air
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