Highs and Lows and Winds

Wind is defined as the horizontal motion of air across the surface of the Earth, described by convention, as the direction from which the wind is blowing. Consequently winds blowing from the north are called northerly winds. Wind direction is usually indicated by a wind vane and is usually shown on a wind rose in steps of 10 degrees. A westerly wind comes from west, which is 270° and shown as 27 on a wind rose or display. However, most winds also have a vertical component, w hich is normally much smaller, but will gain some importance under certain conditions.

Wind speed is commonly measured in knots (kn), kilometres or miles per hour (kph or mph). The international SI unit for wind speed is given in metres per second. Wind speed is measured by an instrument called anemometer. Also wind speed or wind force can be described in terms of its effects on sea or land. Sir Francis Beaufort who introduced the famous Beaufort scale running from force 0 to force 12.

Winds arise from differences in air pressures, known as pressure gradient. High pressure at one place sets up a force pushing air towards areas of low pressure. The greater the difference in pressure, the stronger the force. The smaller the distance between the two pressure areas, the faster the moving air is accelerated. The steeper the 'slope' between two pressure systems, the larger the 'pressure gradient force' will be. The unit for pressure is hectopascals (hPa), they are indicated by the littl e numbers on a pressure map. For example, 990 hPa is very typical for a low, 1025 would be very typical for a high.

Look at the map above. The pressure difference between the low south of Island and the ridge north of Scotland is 25 hPa (1020hPa - 995hPa) and the distance is relatively small resulting in strong winds flowing clockwise out of the high and counterclockwise into the low. On the other hand the pressure difference between the blocking high over Russia (1025 hPa) and the cut-off low over northern Germany (1010 hPa) is comparatively small (15 hPa) and the distance relatively long. The resulting wind force is relatively moderate.

High and low pressure are relative. There's no set number that divides high and low pressure. Atmospheric pressure differences normally originate from a temperature (and thus density) difference between different regions. In the planetary boundary layer changes in wind speed and direction are related to the Earth's rotation (via the Coriolis effect), to surface friction and, at higher levels, to the thermal wind.

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