loading
City:

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.


Add to social bookmarking:  | more |
A is for Air
Accessory clouds
Advection
Air masses and their sources
Air-mass Thunderstorm
Alpine Glow
Altocumulus
Anticyclone
Atmosphere - Diagram
Aurorae - Northern Lights
Average rainfall over England and Wales
Azores High
Banner Cloud - the peak's flag
Beaufort Scale
Blizzard
British Weather Terms
Brocken Spectre
Bubble High
Burning Time
CAPE - Convective Available Potential Energy
Cap cloud
Cc floccus
Cc lacunosus
Cc stratiformis
Cc undulatus
Central England Temperature
Centres of action
Ci fibratus
Ci radiatus
Ci spissatus
Ci uncinus
Ci vertebratus
Cirrocumulus
Cirrostratus
Cirrus
Clocks go Back from BST to GMT
Cloud classification
Cloud seeding
Cloud species
Cloud streets
Cloud types (genera)
Cloud variety
Clouds - sentry of the sky
Cold low
Comma Cloud
Comma Cloud
Convection
Coriolis effect
Corona
Crepuscular rays
Cut-off low
Dew Point
Dew
Discovery of the Jet Stream
Doppler radar
Drifting snow - blowing snow
Drought
Earth's Atmosphere
Easterly wave - the Hurricane's cradle
El Nino
Föhn (foehn) wind
Föhn wall
Flash Flood
Fog and Mist
Fogbow
Forecasting weather
Frost hollow
Fujita Scale Statistics
Fujita Tornado Scale
Funnel cloud
Genoa Low
Geostationary Satellites
Geostrophic Wind
Glaze and Black Ice
Glory
Grass Minimum Temperature
Gustnado
Hail
Hailstorms in Britain
Highs and Lows and Winds
History of Hurricane Names
Hoar Frost
Humidity
Inversions
Isobars on surface maps
Jack Frost
Jet stream cirrus from space
Katabatic winds
Key to our weather symbols
Lake-effect snow
Latent Heat
Levanter cloud
Millibar and hectopascal
Mirages
Mizzle
NOAA satellites
Nimbostratus
North Atlantic Drift (Gulf Stream)
Polar Orbiting Satellites
Polar low - the arctic hurricane
Precipitation Map
Radiosonde
Rain gauge
Rime
Roll cloud
Rotor cloud
Saffir-Simpson scales
Sc duplicatus
Sc perlucidus
Sc undulatus
Shelf Cloud
Sometimes a bit fishy
Sounding
Southern Oscillation
St. Swithun's Day
Standard Reference Period
Stevenson Screen
Stratocumulus
Stratosphere
Sun pillar
Supercooled clouds
Surface wind
Swell
TORRO
Thermocline
Thunderstorm Probability
Thunderstorms
Tornado Alley
Troposphere
Troposphere - Diagram
UV Index
Ultraviolet radiation
Virga or Fallstreak
What Makes Northern Lights Happen?
What does it mean?
Why Skies are Blue
Why Thunder Rumbles
World Meteorological Organisation (WMO)