Sea breezes

Most afternoons in the summer half of the year it is windy at the beach. Why? Because, if there is no wind caused by the current atmospheric pressure pattern, then a sea breeze is likely to develop. A sea breeze is a moderate or fresh wind blowing from the sea to the land, and is usually noticeably cooler than conditions prevailing before it started.

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It is caused by the different responses of land and water to heating by the sun’s radiation. When sunlight warms the sea surface, some of the heat is con­ducted away to water below, and also some mixing of the water occurs. Consequently, the sea surface temperature does not rise much during the day. The land, however, heats up comparatively quickly—especially asphalt and concrete. Daytime temperatures can often reach 10°C higher than the overnight minimum tempera­ture.

Once the land heats up, it warms the air immediately above it by conduction. This air then rises and mixes with the air above. As the air in the lowest 1000 metres gradually warms up, it expands. This has the effect of lifting the centre of gravity of the column of air vertically above the land in comparison with the column of air above the sea.

So, for example, the height at which the atmospheric pressure is 900 hPa (which is usually about 1000 metres above mean sea level) will now be a little higher over the land than over the sea.

This means that along a horizon­tal surface at 1000 metres altitude, the pressure above the land has become a little higher than it is over the sea. Consequently, air at this altitude begins to flow away from the land and towards the sea.

But this means that there is now more air in total in the column over the sea than in the column over the land. So, down at mean sea level, the atmospheric pressure is now a little higher over the sea than over the land, and air begins to move from the sea over the land.

This circular air movement—up over land, out to sea aloft, down over the sea, then back in over the land at sea level—is called the sea breeze circulation.

The sea breeze usually starts up late morning and drops off around dusk. Its strength depends on the temperature contrast between land and sea. So it is often stronger in late spring and early summer than in late summer or autumn, because the sea temperatures are warmer in autumn than in spring.

If there is a pre-existing wind from land to sea at 1000 metres. this generally helps the sea breeze circulation and brings the sea breeze in earlier and more strongly than would otherwise be the case. This can happen at Nelson in a southwest airstream when the sea breeze can come in abruptly at 25 knots (45 km/hr). However, if the pre­existing wind is too strong, it will prevent the sea breeze devel­oping.

If the pre-existing wind at 1000 metres is blowing from sea to land. this can suppress the sea breeze, either weakening it or preventing its development. Also, this wind will often bring a layer of cloud in over the land, thus reducing the sunlight reaching the land surface.

As the sea breeze develops, it reaches further out to sea and penetrates further inland. As its length or “fetch” increases, it begins to be influenced by the Earth’s rotation. This causes its direction to change slowly in an anticlockwise direction in the southern hemisphere. That is. a westerly sea breeze blowing on to a west coast will gradually tend southwest during the afternoon, while an easterly sea breeze on an east coast tends northeast.

Over an enclosed piece of water such as the Manukau harbour, the sea breeze begins blowing every­where perpendicular to the beach. However, after a while, one direc­tion usually prevails in response to the predominate distribution of land and sea. In the case of the Manukau, this is the southwest.

Knowledge of these sorts of changes in a developing sea breeze may have played a vital role in the history of western civilisation. The Greek fleet under the Athenian admiral Themosticles is thought to have used sea breeze fluctuations to their advantage when they defeated a much larger Persian fleet in the battle of Salamis in 479 BC.

Sometimes two sea breezes moving inland from opposite coasts of a peninsula will give rise to a line of thunderstorms where they meet in the middle. This happens over the Florida peninsula, and also over Auckland and Northland.

An example of this occurred on Friday February 26, this year. By midday, sea breezes of about 10 knots (20 km/hr) were blowing towards the centre of the Auckland peninsula from each coast. A line of cumulus clouds developed along the length of the peninsula with isolated thunder­storms, as can be seen from the very strong echoes on the radar scan (left). Once developed, these thunderstorms moved slowly northeast, under the influence of the southwest winds blowing in the middle and upper atmosphere. Places in the path of the thunder­storms experienced heavy rain, but those on either side had none.

Sometimes showers that form where sea breezes come together remain stationary. When this happens, a relatively small area can receive more than 50 mm of rain in an hour or two, while streets a few kilometres away remain dry.

Sometimes whirlwinds form in the cool sea air as it moves inland. This is most likely to occur when the temperature difference between the hot land and the air from the sea is greatest, such as over coastal deserts in the tropics. In Libya, the temperature of the air just next to the ground can reach 9°C higher than the air about one metre up. The whirlwinds there usually last just a few minutes, and are be­tween 50 m and 100 m tall.

Whirlwinds become visible when they pick up dust or sand. Then they are called “dust devils… Occasionally, they pick up water. and are called “water devils.” Usually, whirlwinds that pick up water form over land, then move over water. However, there have been accounts of whirlwinds forming over inland lakes. This is most likely after a long hot sum­mer, when the surface water of the lake has become very warm.

A particularly vivid description of a whirlwind lifting, then dropping, a column of water over a Scottish lake in the 18th century has given rise to speculation that many sightings of the so-called Loch Ness Monster have, in fact, been water devils, seen from a distance.

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