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Old 08-31-1997
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Wind

We obtain wind information in many different ways; through onboard instruments, weather forecasts and charts, and our own "eye ball" estimates. Often we depend on the skills of a professional meteorologists to make wind determinations for us, but we can also learn the basics of predicting winds from weather charts, and in doing so refine and tailor forecasts to our needs.

Gusts, Lulls and Squalls

To determine wind speed and direction we first need to understand a few wind concepts and terminology. Wind is the horizontal movement of air, with its speed usually measured in knots (Note: in some areas of the world speed is measured in meters per second or kilometers in lieu of knots), and direction always in degrees true. Wind speed and direction is shown using arrows with tail feathers that denote speed in five knot increments, with the arrow pointing in the direction of motion. A given speed and direction is actually a mean value measured-or expected-over a ten minute period. A mean value is used since winds oscillate, and an instantaneous reading could be misleading.

Increases in speed are called gusts and decreases lulls. Prolonged gusts, where speed increases by at least 15 knots, are called squalls. Squalls are most often associated with rapidly developing and moving weather systems and not always depicted on weather charts.

Energy within our atmosphere is continually being moved around-both daily and seasonally-by convection, advection, and radiation, and in this ongoing process masses of air are warmed and cooled, causing them to rise, sink and move horizontally.

It is this daily, seasonal and yearly warming and cooling that brings about wind. And winds, therefore are a combination of many ingredients: large scale weather systems-such as high and low pressure areas, and local conditions-such as sea breezes, funneling, and convergence.

Generally speaking, official weather forecasts and charts provide wind information representative of large scale (synoptic) weather conditions. Most weather models-computer simulations of forthcoming weather based upon data gathered from ships, balloons, buoys, and satellites-are highly accurate in describing overall weather events, but are often not capable of describing specific localized weather, and wind, in great detail. We must determine and introduce local effects, those caused by topography, currents and temperature differences after examining the "big" picture.

Isobar Lines and Geostrophic Wind Diagram

We can calculate wind speed and direction at our location, or any desired point, by measuring the spacing and shape of isobar lines drawn on weather charts. Isobar lines connect points of equal (iso) barometric (bar) pressure, and surface winds generally blow parallel to isobar lines, bending inwards around low pressure and outwards around high pressure systems. Closely spaced, or tight, isobar lines indicate strong winds and isobar lines with wide separation portend light winds.

Wind speed is calculated using isobar lines by first measuring the perpendicular distance between lines and then using that value to enter a geostrophic (earth driven) wind diagram. A geostrophic wind diagram allows you to calculate winds based on two factors: difference in pressure (gradient) between isobar lines, and effect of earth's rotation on air movement, known as Coriolis effect.

A geostrophic wind scale does not take into account friction from land and sea surface, increases or decreases due to water and air temperature differences, or interaction of weather systems within close proximity to each other, so these factors need to be factored in after geostrophic wind is calculated.

To find geostrophic wind we first we measure the perpendicular distance, or spacing, of isobar lines at our location. Isobar lines are normally placed at 4 mb intervals, but spacing should be verified, as occasionally 3 mb and 5 mb intervals are used. The only effect isobar spacing has on wind determination is in how the geostrophic wind diagram is entered.

It may be helpful in identifying isobar lines to remember that normal sea-level pressure is 1013 mb and in the labeling of isobar lines often the left digits of the pressure are not printed: for example an isobar line representing 1016 mb would be labeled "16", or a pressure of 993 mb would be written as "93".

Actual wind speed is less than that derived from a Geostrophic wind scale due to friction. We use 60% of our deduced wind speed since friction reduces wind over water by 40%. (Over land, where friction is greater, geostrophic wind is reduced by 60%).

Another aspect to wind prediction is that wind speed is calculated for a standardized height of 10 meters (33.3 feet) above sea-level. Below this height increased surface friction reduces wind speed, so under 33.3 feet winds are normally less than predicted and above 33.3 feet winds are greater than expected.

Boats equipped with wind sensors at mastheads will often record speeds significantly higher than those felt in the cockpit, and when a weather report-or geostrophic calculation-predicts winds of 10 knots, for example, you may actually feel calm winds near the surface, and winds greater than 10 knots at a masthead above 33.3 ft.

Stability

Another adjustment often made to geostrophic wind speed is for difference of water and air temperature. When cold air moves over warm water an unstable situation occurs since cold air tends to sink and warm air sitting directly over the warm water rises, bringing about convective, or vertical motion. This convective motion produces gusty winds that are often 50% greater than calculated geostrophic winds. An air-water temperature difference of 10 degrees C (or approximately 20 degrees F) and greater is considered an unstable condition.

When warm air sits over cooler water a stable situation exists since cold air will remain beneath the rising warm air, and a minimal amount of convection will occur. A stable condition exists when the air-water temperature difference is, once again, 10 degrees C (20F) or greater-with air warmer than the water.

When winds associated with a developed low and high pressure system are stable they tend to blow more directly in towards the center of lows and out from the center of highs than unstable winds, which tend to parallel isobar lines.

Unstable winds are found most often on the backside of cold fronts, where cold, dry arctic or polar air sweeps south over warm Atlantic and Pacific waters. Dense, cold, air can produce formidable seas.

Race Track Effect

Lastly in our determination of surface winds we need to examine the effect that bends in isobar lines have on wind speed. As winds circles around low pressure areas they spiral inwards, but as these winds follow curves in isobar lines a force called centrifugal (outward) force counteracts the winds inward tendency.

This situation is analogous to race cars going around a track. On the straight-aways cars move with forces in balance, but as a curve is entered centrifugal force moves cars outward, while drivers try to keep their cars on the track and moving through the curve. Thus the reason for banked turns on race tracks, to counteract the centrifugal-outward-force.

Therefore around a low pressure system winds tend to be diminished in areas where isobar lines make sharp or distinctive bends or turns, outward and inward forces are acting against each other.

Just the opposite occurs around high pressure areas, where winds are blowing outward and centrifugal force accentuates this outward flow, increasing wind strength at turns or bends in isobar lines. Geostrophic wind speeds can be adjusted 20% up (highs) or down (lows) to account for bends in isobar lines, most noticeable in the vicinity of warm and cold fronts.

So study those isobar lines and keep a geostrophic wind scale close at hand to assist in determining wind speed and to properly evaluate and forecast your local winds.

Winds are determined using a Geostrophic wind scale which requires measuring the perpendicular distance between isobar lines and latitude. Here a Pacific Gale is producing northwest winds of 30 knots. These winds produce unstable conditions since cold air is moving over warm water which leads to strong convection as cold air sinks and warm air rises. This convection brings about gusty winds which are often 50% greater than predicted winds, producing large seas as shown on the accompanying Wind/Wave Forecast.

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