Royal Meteorological Society Weather and Climate

Local Winds

Local winds occur on a limited spatial scale, their horizontal dimensions typically several tens to a few hundreds of kilometres. They also tend to be short-lived, their time scales typically several hours to a day or so. There are many such winds around the world, some of them cold, some warm, some wet, some dry. The hazards associated with the winds are many and various.

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Sea Breezes and Land Breezes

These winds result from differential heating of land and sea. Sea breezes occur by day, when the land becomes warmer than the sea. Land breezes occur at night and in the early morning, when the land is cooler than the sea.

Air that is heated near coasts cannot expand seaward because air over the sea is cooler and therefore more dense; and it cannot expand along the coast or inland because the air there is also expanding. It therefore expands upward. On account of the ascent, barometric pressure falls over the heated land, so a thermal low develops, as shown on the diagram below, in which P₁, P₂, P₃ and P₄ denote air pressure.

sea breeze diagram
Above - Sea Breeze

Pressure does not fall over the sea, so a pressure gradient is created, and this gradient is responsible for the onshore flow of air which characterizes a sea breeze. The circulation is completed by air moving seaward at a height of 500 to 1,500 metres and subsiding over inshore waters. Where there is ascent of air, cumulus clouds tend to form.

In contrast, air contracts when it is cooled, so air descends over land on a clear night. Pressure thereby becomes higher over land than over the sea; and this pressure gradient causes the offshore flow of air which characterizes a land breeze. As the diagram below shows, the circulation is completed by air moving landward at a height of 100 to 300 metres and descending over the coast. Again, cumulus clouds tend to form where ascent of air occurs.

sea breeze
Above - Land Breeze

The air in a sea breeze is cool and moist compared with the air it encounters over land; and the boundary between the two types of air takes the form of a convergence zone, a so-called “sea breeze front”, which is typically 100 to 250 metres wide. In temperate latitudes, these fronts may penetrate 70 km or more inland by 21:00 hours Local Time. In the tropics and subtropics, they may penetrate as much as 150 km inland by that time. Where sea breezes from differently aligned coasts intersect, up-currents tend to be particularly strong (5 to 10 m/s). Indeed, such intersections are often marked by cumulonimbus clouds.

Sea breeze circulations are usually noticeable several kilometres out to sea, particularly in the tropics, where they can be detected as much as 20 km off the coast. Land breeze fronts generally do not progress more than 10 to 15 km offshore.

Wind speeds in sea breezes are typically 4 to 8 m/s but may be stronger. Wind speeds in land breezes are typically only two or three metres per second.

Sea breezes initially (around noon) blow directly from sea to land. In middle latitudes, however, Coriolis force causes the breezes to be deflected so that, by early evening, they blow almost parallel to the coast. Land breezes are also deflected but generally by no more than 20 to 30 degrees.

Rarely do sea or land breezes constitute a hazard. However, fog or drizzly weather may occur in coastal areas where damp air is drawn shoreward by a sea breeze. The names “haar” and “sea fret” are used along the east coasts of Scotland to describe this occurrence, which is most likely to occur in spring and early summer, times of year when the sea is still comparatively cool and the sun strong enough to create sea breezes.

Coastal fog or low cloud formed in this way may be called a holiday hazard though the occurrence is, in reality, no more than a disappointment, particularly for those who discover that the weather inland has been warm and sunny! For the operators of airfields in coastal areas, though, the matter can be more serious.

Fog and low cloud occur frequently over the coasts of California and Oregon in summer, when sea breezes are drawn across the cool waters of the California Current almost every day. Poor visibility constitutes a hazard on the coastal highway.

Anabatic and katabatic winds

Above - Anabatic Wind

Like sea and land breezes, anabatic and katabatic winds are induced thermally. Anabatic (upslope) winds occur over slopes which are heated by the sun. Katabatic (downslope) winds occur over slopes which are cooled.

Air which is in contact with slopes that are warmed expands upward and subsequently sinks over neighbouring valleys. In the diagram below, P₁, P₂, P₃ and P₄ indicate pressure surfaces, showing that pressure is lower close to the warmed hill-side than at the same heights over the valley.

Katabatic winds occur where air in contact with sloping ground is colder than air at the same level away from the hillside over the valley (see diagram below). Surface cooling generally results from a net loss of radiation at night, particularly where skies are clear of cloud. Accordingly, katabatic winds are nocturnal phenomena in most parts of the world, their speeds typically not exceeding 3 or 4 m/s. Where ground is covered with snow or ice, however, katabatic winds can occur at any time of day or night, their speeds often reaching 10 m/s, or even more if funnelling through narrow valleys occurs.

Above Katabatic Wind

Anabatic winds are usually very light (generally only 1 or 2 m/s) and seldom of much significance, except near coasts, where they tend to augment sea breezes. In contrast, katabatic drainage may lead to the formation of frost, mist and fog in valleys. Where strong katabatic winds occur frequently, as on the coasts of Greenland and Antarctica, observed wind speeds and directions are likely to be unrepresentative of the pattern of barometric pressure existing over the area as a whole.

The Bora and Mistral

The bora is a strong, cold and gusty north-easterly wind which descends to the Adriatic Sea from the Dinaric Alps, the mountains behind the Dalmatian coast (the coast of Croatia). It is a winter phenomenon that develops when a slow-moving depression is centred over the Plain of Hungary and western Balkans so that winds are blowing from the east towards the Dinaric Alps. These mountains form a barrier which trap the cold air to the east of them whilst the Adriatic coast remains comparatively mild. Gradually, though, the depth of the cold air increases until the air flows over passes and through valleys to reach the Adriatic Sea (see diagram below).

The bora begins suddenly and without warning and the cold air typically descends to the coast so rapidly that it has little time to warm up. The bora can reach speeds of more than 100 km/h and has been known to overturn vehicles and blow people off their feet.

bora
Above Bora

The mistral is also a strong and often violent wind. It blows from the north or north-west down the Rhône Valley of southern France and across the Rhône Delta to the Golfe du Lion and sometimes beyond. Though strongest and most frequent in winter, it may blow at any time of year and develops when stable air is forced through the Rhône Valley. It occurs when a depression is centred over north-west Italy and the Ligurian Sea and a ridge of high pressure extends north-eastward across the Bay of Biscay. It may blow continuously for a day or two and attain speeds of 100 km/h, causing considerable damage to crops and making driving conditions difficult in the Rhône Valley.

Shooting Flows

So-called “shooting flows” can reinforce katabatic winds over gentle slopes in high latitudes; and they usually begin suddenly and without warning. In their formation, an essential factor is a marked temperature inversion at an altitude of 100 to 300 metres.

Given an upstream wind of suitable speed and direction, air moves rapidly (shoots) through the duct which exists between the land surface and the inversion (with the inversion forming an impenetrable lid). The sudden commencement of a shooting flow is associated with a phenomenon of fluid flow known as a hydraulic jump. The flow ceases abruptly when the speed and direction of the wind upstream are no longer suitable for its maintenance.

Shooting flows of 30 to 40 m/s often occur over Antarctic coasts. Indeed, at the foot of an ice slope near Commonwealth Bay (67°S 144°E), winds of 30 m/s blow for about nine months of the year. Moreover, apart from occasional short-lived lulls, they blow remarkably steadily. Strong shooting flows also occur frequently at various coastal locations in the Arctic. At Franklin Bay (69°45’N 126°00’W) on the Beaufort Sea coast of the Canadian mainland, for example, wind speeds sometimes reach 35 m/s. At the same time only 10 to 15 km away, however, winds can be calm or very light. At Resolute Bay (74°40’N 95°00’W), on Cornwallis Island, one of the Queen Elizabeth Islands, shooting flows are sometimes so strong that ships cannot be loaded or unloaded.

The föhn (or foehn)

The föhn is a warm, dry, gusty wind which occurs over the lower slopes on the lee side of a mountain barrier. It results from the forcing of stable air over a mountain barrier and its onset is generally sudden. For example, the temperature may rise more than 10°C in five minutes and the wind strength increase from almost calm to gale force just as quickly. Föhn winds occur quite often in the Alps (where the name föhn originated) and in the Rockies (where the name chinook is used). They also occur in the Moray Firth and over eastern parts of New Zealand’s South Island. In addition, they occur over eastern Sri Lanka during the south-west monsoon.

The danger of a föhn where there are steep snow-covered slopes is that avalanches may result from the sudden warming and blustery conditions. In föhn conditions, relative humidity may fall to less than 30%, causing vegetation and wooden buildings to dry out. This is a long-standing problem in Switzerland, where so many fire disasters have occurred during föhn conditions that fire-watching is obligatory when a föhn is blowing. The sudden dryness brought by a föhn also has an aggravating effect on the human constitution, affecting blood circulation and the nervous system. An increased suicide tendency is a consequence.

The föhn explanation which is found in most textbooks cannot be wholly correct. The essence of this explanation is as follows.
When precipitation occurs over windward slopes, the temperature inside the rain clouds falls with height at the saturated adiabatic lapse rate. Moisture is lost from the air when precipitation falls on the windward side of the mountain barrier, so the cloud base is higher on the lee side than the windward side. Level for level, the air is warmer on the lee side because the air which is descending warms at the dry adiabatic lapse rate.

This explanation does not account for the sudden onset of most föhn occurrences or the magnitude of the temperature increases that occur on many föhn occasions. Moreover, it does not explain why the föhn can occur when skies are cloudless over windward slopes!

The occurrence of a föhn depends upon the presence of a temperature inversion over the mountain barrier, which can be the case in an anticyclone or when there is a warm front, occluded front or warm sector over the barrier. The inversion becomes distorted over the lee slopes, its altitude there decreasing abruptly when the vertical profile of the wind speed upstream becomes suitable. As with a shooting flow, a phenomenon called a hydraulic jump takes place. The level of the inversion drops so much that air from above the inversion reaches the atmospheric layer next to the ground. This air is warm and very dry. Wherever stable air crosses a mountain barrier, föhn development may occur.

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