Wind flow over land is not simple Actual wind accelerates down valleys, bends around headlands, bumps about abutments, is contorted by the coastline, dips and dives over hills and dales, eddies behind mountains, and gallops through gaps. There is a natural reluctance for surface wind to move over land in the cool of the night or near dawn, and it may be drawn onshore during a hot day sea breeze , or offshore at night land breeze. Mountain air cooled at night sinks down valleys and flowsout to sea katabatic wind. These effects do not show up in the isobars on the weather map but can halve or double the local wind speed, and distort its direction always towards low pressure.
One terrain effect does show up on the weather map: chains of mountains distort isobars crossing them. In the example to the upper right, the Southern Alps buckle the isobars coming in from the Tasman Sea, with a build-up of pressure on the windward side and a counter-balancing drop of pressure down-wind from the mountains.
Because of this distortion, avoid trying to use isobars over mountains to work out wind direction. Also, in general, avoid using isobars to work out windflow over high ground. Also shown in the example to the upper right, in red, are observations of the surface wind at MetService stations for the same time as the isobars.
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The "long" part of the wind arrow shows the direction. As can be seen, some winds line up nicely with the isobars and others are almost at right angles to them. When isobars enclose an area of high pressure this is called a High or anticyclone and its centre is labelled on a weather map by an 'H'. The term 'anticyclone' is a bit of meteorological jargon.
The central pressure of a weak High is about hPa,while a strong or intense High has a central pressureabove about hPa. An intensifying High has a rising central pressure, while a weakening High has a falling central pressure.
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Near a High's centre are light winds and sometimes areas of low cloud called anticyclonic gloom. Round the edge of a High, the winds are sometimes strong. Intense Highs tend to squeeze the isobars together creating areas of strong winds. Winter Highs often bring frost; summer Highs may bring thunderstorms and hail. The bigger Highs are, the slower they tend to move, sometimes 'blocking' the fronts that are trying to follow them. Isobars make shapes and patterns.
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When they enclose an area of low pressure this is called a 'Low' or 'depression' and its centre is labelled on a weather map with an 'L'. The term depression is a bit of meteorological jargon. A low pressure system is like a giant funnel of wind spiralling inwards and upwards forcing warmish air in the centre to rise.
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As air rises it cools and clouds form. The central pressure of a shallow Low is above hPa, of a moderate Low hPa, and of a deep or intense Low below hPa. If there are two or more centres the Low is said to be complex. If the central pressure is rising the Low is said to be filling or weakening. If the central pressure is falling the Low is said to be intensifying or deepening.
The satellite picture to the upper right from the satellite GMS-5, courtesy of the Japan Meteorological Agency shows cloud patterns fairly typical of those associated with Highs and Lows. An air flow originating from a prescribed location warm, cold,moist or dry is called an air-mass. Air-masses are named according to where they have come from and each has its own characteristic temperature and humidity.
A front marks the boundary between two air-masses, and appears on the weather map as a line with triangles or semicircles attached.
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In this type a strong jet runs over the Occlusion point. The low forms usually between the Occlusion point and the midpoint of the occluded Front. Nearly all Secondary Lows form in systems over the sea during the winter season. This follows from the fact that the basic westerly flow is stronger during the winter, and the deepening of a low is easier over the relatively warm water. Although the secondary lows are associated with strong jets, the processes in the left exit region are involved in less than half of the cases.
But if the left exit region is favourably situated in the vicinity of the occluded front, it can enhance the development, as in the case on 07 February In the process of the secondary low development there is temporarily a back bent part in the occluded Front. The situation, however, is different from the conceptual model Back - Bent Occlusion see Back - Bent Occlusion - Meteorological physical background. Three stages can be distinguished in the development of a Secondary Low within an occluded front: Initial stage A Secondary Low begins to deepen within the occluded front in the area of a local vorticity maximum.
The part of the front around the low has a Wave - like structure for a short time compare Wave. This Wave does not amplify, however, but turns into an Occlusion spiral. Developing stage Whilst the original low fills up, the secondary deepens. At this stage the occluded front with the Secondary Low still moves forward.
Mature stage The occluded front splits into two parts that spiral around their respective lows. The movement of the Secondary Low slows and it begins to fill up. Below, initial stage at