![]() 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. it is like water flowing over a rocky brook, eddying around corners and rushing between boulders. Isobars are only smoothed-out approximations and only tell about the general wind flow, not the details. Where the flow curves strongly around the Low, the brown arrow is longer than the blue arrow, meaning the wind is weaker than the isobar spacing suggestĥ.Where the flow curves strongly around the High, the blue arrow is longer than the brown arrow, meaning the wind is stronger than the isobar spacing suggests.The longer the arrow, the stronger the wind. The blue arrows show the wind as it is likely to be the red arrows show how the wind would be if it flowed parallel to the isobars.As much as 20-40% lighter than the isobar spacing would suggest as the air turns around (and into) a Low.Up to 20% higher than the isobar spacing would suggest as the air turns around (and out of) a High.Because of the 'spin-out' effect when turning corners, the wind speed can be: This time, the red arrows show the wind directions as they are more likely to be - gently flowing across the isobars towards lower pressure.Ĥ. Surface wind 'leaks' across the isobars towards lowpressure, by about 15 to 20 degrees over the open sea, but by as much as 30 to 90 degrees over and around land. on a weather map with isobars 4 hectoPascals apart, a spacing of about two degrees latitude (with straight isobars) means fresh winds about Auckland but a gale over Fiji.ģ. The closer the isobars, the stronger the winds. In the picture to the right, the wind direction is given by the red arrows.Ģ. In the Northern Hemisphere the flow is the other way around. In the Southern Hemisphere the flow is CLOCKWISE around LOWS and COUNTER-CLOCKWISE around HIGHS. This is just another way of giving Buys-Ballot's law. Winds blow almost directly (but not quite) along the isobars. So from the isobars you can estimate the winds, but it is not quite as easy as Buys-Ballot's law. ![]() If you LOOK into the wind, the LOW pressure is on your LEFT. ![]() ![]() In the Southern Hemisphere, his rule is as easy to remember as three L's: Christopher Buys-Ballot (1818-90), who was a Dutch meteorologist, made the vital link between isobars and wind in 1857. Some have numbers on them showing this value in hectoPascals. They join together places with the same mean sea level air pressure (weight per square area of air above). Those plain lines that curve across the map are called isobars (iso = equal, bar = pressure). They do NOT show what is happening at higher levels, where the wind flow may be doing something entirely different. They show what is happening at a set time where most of us need it - at the Earth's surface. Weather maps as they appear on TV, in a newspaper or here are called 'surface charts' or, more correctly, 'Mean Sea Level' (MSL) charts. ![]()
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