Wind Air has a tendency to move, for seeking equilibrium between areas of more air molecules (higher pressure) and those with less (lower pressure). If we consider a container, which is air-vacuumed, so it has less air (lower pressure) than outside world (higher pressure). If we open the container mouth, then we will see, the air rushes into the container, from outside. This air rushing from outside to the container, is an air in motion. This air in motion in the atmosphere is called wind. So the wind is the movement of air molecules from high pressure area to low pressure. Winds are named for the direction from which they begin, not the direction they flow. The forces which drives the air, from high area to low area, is called the pressure gradient force. The earth s surface is made of various land and water forms, so it absorbs the sun s radiation unevenly. Again due to sun s declination, some parts of the earth receive direct rays from the sun all the year and are always remaining warm, similarly some parts remain colder. Accordingly the air on earth surface also becomes warmer or colder. Since the warm air has less weight than cold air, so the warm air goes up and makes the area as low pressure and the opposite phenomena makes an area of high pressure. This way, when a difference in air pressure exists between two adjacent areas, then the air tends to flow from the region of high pressure to the region of low pressure, due to pressure gradient. The pressure gradient force acts at right angles to isobars. The speed of the wind is controlled by the strength of the pressure gradient, the stronger the pressure gradient the higher the wind speed. The strength of the pressure gradient can be decided from the spacing of isobars, on a weather map. Below shows two different pressure gradients. The distances between points A & B and C & D are the same but the amount of pressure change is quite different. So the pressure gradient is greater between A & B, because the spacing of the isobars is much closer than between C & D. As a result, the wind blows much faster between A & B. The pressure gradient force, however, is not the only force, which acts on a moving parcel of air. The other forces are surface friction, coriolis force and centrifugal force. 1
Coriolis force is a force, which created due to Earth's rotation. It is negligible at the equator and increases with latitude, becoming highest at the poles. It always acts at right angles, at which to the direction, in which the wind is blowing. Once the wind motion becomes established, the coriolis effect causes continuous deflection further to the right of the pressure gradient, until the coriolis force and gradient force become equal. The result is that the wind blows parallel to the isobars. The deflection caused by coriolis force, is to the right in the northern hemisphere and to the left in the southern hemisphere. Geostrophic wind is the theoretical wind, which is caused from an exact balance between the coriolis force and the pressure gradient force. This condition is called geostrophic balance. The geostrophic wind is directed parallel to isobars.this balance seldom holds exactly in nature. The true wind almost always differs from the geostrophic wind, due to other forces such as friction, centripetal force etc. Thus, the actual wind would equal the geostrophic wind only, if there were no friction and the isobars were perfectly straight. Geostrophic wind scale is provided, on weather map, drawn as per the scale of map. Here first needs to measure the perpendicular distance between isobars, spaced at 4 millibar intervals, at the required position. Then the distance is to transfer to the geostrophic wind scale (as given below), in such way that the starting point remains on the vertical line, at required latitude. Whilst the finishing point is to be at a position, on or between the wind speed curves, by keeping parallel to the base line. From that finishing position, by following the curve down to the base line, the wind speed can be read off either directly or by interpolation. 2
The below type of table can also be used to get Geostrophic wind speed. 3
Surface friction is an important surface force, which slows the wind and also changes wind direction. It also reduces the influence of the Coriolis force. Since the gradient force remains the same, the air is generally shifted toward low-pressure regions. How much it is shifted depends on the nature of the surface. In rough areas, the impact of friction is greater. Above land surface, the wind could be diverted by about 30, but above sea surface, it often ranges from 0 to15. At heights of more than 1000 m (3000 feet) above the ground, this effect dies out and the winds blow parallel to the isobars. Below diagram, is indicating wind flow direction at land surface and also at a height of 1000m, where the friction is negligible. Centripetal force occurs, when the air flow curves in. Any motion in a curved path represents accelerated motion and requires a force directed toward the center of curvature of the path. This force is called the centripetal force, in other words "center seeking" force. It is the external force required to make a body follow a curved path. Centripetal force should not be confused with centrifugal force. 4
Since centripetal force is added to the gradient force, the wind blows a bit more strongly around high-pressure centers. This force is weaker than the Coriolis and pressure gradient forces. 5
World s wind circulation at January 6
World s wind circulation at July 7
Global Winds Tropical Easterlies (it is also called Trade wind) is moving from high pressure zone of about 30º latitudes to the equator. Coriolis effect creates its wind direction. It veers (a clockwise change of direction, from which the wind is blowing is called veering, similarly anticlockwise chance of direction, is called backing) to the west because of the rotation of the earth, so it is easterly wind. It dominates the weather of tropics. Prevailing Westerlies flow from west to east, during air movements towards the poles. It occurs between 30 and 60 degrees latitudes. This wind moves towards the poles and appears to curve to the east. These winds are responsible for many of the weather movements across the US and Canada. Polar Easterlies occur at 60 degrees latitude in both hemispheres. The Polar Easterlies are formed, when the air over the poles cools and sinks, flowing away from the earth's poles toward the west. Wind circulation at Bay of Bengal (July) 8
Courtesy : Bay of Bengal pilot (NP21) 9
Wind circulation at Bay of Bengal (January) Courtesy: Bay of Bengal pilot (NP21) Beaufort scale is a measure for describing wind speed, based mainly on observed sea and land conditions. Its full name is the Beaufort wind force scale. 10
The scale was created in 1805 by Sir Francis Beaufort, an Irish-born British admiral and hydrographer. The scale was made for ship's log entries on Royal Navy vessels in the late 1830s and then started to use on non-naval vessel from the 1850s. While judging the wind speed at sea; the swell, tide, rain, distance from land etc should be counted. Force Wind (Knots) Descriptive Appearance of Wind Effects term On the Sea On Land 0 Less than 1 Calm 1 1-3 Light Air 2 4-6 3 7-10 4 11-16 5 17-21 6 22-27 Light Breeze Gentle Breeze Moderate Breeze Fresh Breeze Strong Breeze 7 28-33 Near Gale 8 34-40 Gale 9 41-47 Strong Gale Sea surface smooth and mirror-like Scaly ripples, no foam crests Small wavelets, crests glassy, no breaking Large wavelets, crests begin to break, scattered whitecaps Small waves 1-4 ft. becoming longer, numerous whitecaps Moderate waves 4-8 ft taking longer form, many whitecaps, some spray Larger waves 8-13 ft, whitecaps common, more spray Sea heaps up, waves 13-20 ft, white foam streaks off breakers Moderately high (13-20 ft) waves of greater length, edges of crests begin to break into spindrift, foam blown in streaks High waves (20 ft), sea begins to roll, dense streaks of foam, spray may reduce visibility Calm, smoke rises vertically Smoke drift indicates wind direction, still wind vanes Wind felt on face, leaves rustle, vanes begin to move Leaves and small twigs constantly moving, light flags extended Dust, leaves, and loose paper lifted, small tree branches move Small trees in leaf begin to sway Larger tree branches moving, whistling in wires Whole trees moving, resistance felt walking against wind Whole trees in motion, resistance felt walking against wind Slight structural damage occurs, slate blows off roofs 10 48-55 Storm Very high waves (20-30 Seldom experienced on land, 11
11 56-63 Violent Storm 12 64+ Hurricane ft) with overhanging crests, sea white with densely blown foam, heavy rolling, lowered visibility Exceptionally high (30-45 ft) waves, foam patches cover sea, visibility more reduced Air filled with foam, waves over 45 ft, sea completely white with driving spray, visibility greatly reduced causes considerable structural damage. Beaufort weather notation is the symbolic Beaufort letters, used on bridge log book and weather log book. Below it is given, at various groups, As per cloud, b bc c o Total cloud amount: 0 to 2 oktas (eighths of sky covered) Total cloud amount: 3 to 5 oktas Total cloud amount: 6 to 8 oktas* Uniform thick layer of cloud, completely covering the sky As per thunder storms and its related phenomena, l t t l Distant lightning (storm too far away for sound to reach observer) Thunder heard; no lightning seen Thunderstorm As per rain, snow, hail etc, d Drizzle, freezing drizzle dr Drizzle and rain mixed h Hail, small hail, snow pellets, diamond dust, ice pellets hr Hail and rain mixed hs Hail and snow mixed r Rain, freezing rain rs Rain and snow mixed, or partially melted snow ('sleet') s Snowflakes sh Snow grains( known in some texts as 'granular snow' ) 12
Visibility related items, f Fog, ice fog (visibility < 1000 m) fe Wet fog (visibility < 1000 m) fg/fs Shallow land/sea fog(visibility above the fog >= 1 km) F Fog, ice fog (visibility < 200 m) ks Drifting or blowing snow kz Dust or sandstorm (visibility < 1000 m) m Mist (visibility >= 1 km) z Haze Others e g G i kq p u w x y Wet air, without rain, snow etc. falling Gale (mean speed 34-47 knots over a 10 minute period) Storm (mean speed 48 knots or more over a 10 minute period) Intermittent (used with precipitation & fog) Line Squall Shower Ugly, threatening sky Dew Hoar frost Dry air -- less than 60 % relative humidity Sea Breeze Land surface heats up faster than water surface. So during the day-time, the land surface becomes extremely hot. So a low air pressure develops on the land, as due to heat the air molecules expand and rise. In contrast, due to large difference in diurnal range of atmosphere temperature, over the ocean, the temperature is much cooler and consequently the air pressure is relatively higher. At costal area, where this low and high pressure areas remain very near and isobars run roughly parallel to the cost. Since the distance between these two areas are very small and the pressure gradient is high, so wind will move from sea to land, which is called sea breeze. It sets in by about 1000 hrs local time, goes to maximum force of 3 to 4 by about 1400 hrs local time and dies down at about sunset. 13
Land Breeze Land surface cools down faster than water surface. So during the cloud less night-time, the land gets cold. So a high air pressure develops on the land, as due to cold the air molecules compressed and descend towards the land surface. In contrast, over the ocean the temperature remains warmer and consequently the air pressure is relatively lower. At costal area, where this low and high pressure areas remain very near and isobars run roughly parallel to the cost. Since the distance between these two areas are very small and the pressure gradient is high, so wind will move from land to sea, which is called land breeze. It sets in a couple of hours after sunset and dies down at about half an hour after sunrise. Monsoon is a seasonal prevailing wind for a particular area (Bay of Bengal & some parts of Indian ocean), which lasts for several months and caused by the larger amplitude of the seasonal cycle of land temperature, compared to that of nearby oceans. The English word monsoon came from Arabic word mawsim (means "season"), 14
North East Monsoon takes place from December to early March, with wind force 3/4 at Arabian sea and 5/7 at China sea. Due to northern hemisphere winter, the temperature over central Asia is less than 25 C, which is creating a zone of high pressure of 1036 mb. Where as the equatorial low of 1012 mb remains unaffected by the change of season. As a result of this pressure distribution, a NE wind flows from the interior of India, towards the Indian Ocean, which is known as the winter monsoon circulation (or NE monsoon). A stream of this circulation flows down towards Bangladesh and then to the Bay of Bengal. NE monsoons cause the cold wind to sweep down from the Himalayas, towards the Indian Ocean. While traveling towards the Indian Ocean, the dry cold wind picks up some moisture from the Bay of Bengal and pours it over water surrounding area of India. This North-East Monsoon is not able to bring as much rain, as the South-West Monsoon. South West Monsoon occur from June to September, with wind force 7/8 force at Arabian sea and 6/7 at Bay of Bengal. The Great Indian Desert (Thar Desert) and adjoining areas of the northern and central Indian subcontinent heats up considerably during the hot summers. This causes a low pressure area of about 994 mb, over the northern and central Indian subcontinent; this is considerably lower than the equatorial low of 1012 mb. Due to this pressure gradient, the moisture laden winds from the Indian Ocean rush in to the subcontinent and causes SW monsoon. The Himalayas act like a high wall and do not allow this winds to pass into central Asia, forcing 15
them to rise. With the gain in altitude of the clouds, the temperature drops and precipitation occurs. Some areas of the subcontinent receive up to 10,000 mm of rain. Monsoon creates a great break in the general circulation of the atmosphere. This area of monsoon is basically an area of trade wind, which is blowing from high pressure zone of about 30º latitudes to the equator and caused by Coriolis Effect. Trade winds blow NE at north of the equator; SE at south of the equator. But due to monsoon, we can see variation of wind direction, as well as wind force, at these monsoon areas. For this reason, some areas have monsoon, where as others in similar latitudes have trade wind. Buys Ballot's law With the true wind to one's back, a low-pressure center (L) will be to one's left, and high pressure (H) to one's right (in the Northern Hemisphere). Simply we can tell, if we face the true wind at northern hemisphere, then low pressure area will be at our right hand side and high pressure area will be at our left hand side. Cautions: 1. It should not be used near equator, because due to negligible coriolis force at the equator, winds blow directly from high pressure to low pressure areas. 16
2. It should not be used near land, because the land surface diverts wind direction. This law helps masters of vessels, to assist them in steering the vessels, to keep themselves away from hurricanes or any other TRS at sea. Before introducing modern weather forecast instruments, previously it was the only method for a ship s master, to forecast the weather, on observation of meteorological conditions (visible cloud formations, wind direction, atmospheric pressure etc) at his location. Anabatic wind is a wind, which blows up a steep slope or mountain side, driven by heating of the slope through insolation. It is also known as an upslope flow. These winds normally occur during the daytime in calm sunny weather. A hill or mountain top will be radioactively warmed by the sun, which in turn heats the air just above it. Air at a similar altitude over an adjacent valley or plain does not get warmed, so much because of the greater distance from the below ground. Katabatic winds are down-slope winds, frequently produced at night by the opposite effect, the air near to the ground losing heat to it faster than air, at a similar altitude over adjacent low-lying land. They are generally much stronger than mountain breezes, with winds speed more than 100 kts. An optimal condition is snow-covered elevated plateau. They are frequently observed in the Adriatic Sea and some other areas with high hinterland. 17
True-wind is the wind, which we feel, when ship is not moving, for example at anchor. Apparent wind is the wind, which we feel, when ship is moving. It is the resultant of true wind and ship s reversed movement. To solve the problem, related to true wind and apparent wind, we may draw the below tangle OAT, where, AT is the course and speed of the ship, OT is the direction and speed of the true wind, OA is the direction and speed of apparent wind. If we know any two, then we can find out the third one, by a simple geometry. N.B. Here NS line is representing north-south direction. At sea, the direction of true wind can also be judged by the direction of waves. The direction of true wind is at right angles to the line of waves. But this reading can be wrong, if both sea (the waves, which is created by wind, which is presently blowing) and swell (the waves, which is created by wind, which is already stopped or is blowing some other places) are from different directions. Mistral is a cold and dry, northerly or northwesterly katabatic wind, is blowing over the Gulf of Lion and adjoining coastal areas, particularly the Rhone Valley, from the southern coast of France, with wind force more than 40 kts and gusts sometimes to 100 kts. Its effects are often felt past Sicily into the eastern portion of the Mediterranean basin. Wave heights can be 4 / 6 m, with maximum height of about 9 m. 18
The mistral is most common in winter and in spring. Duration of effects with winds exceeding 30 kts for over 65 hours, have been reported at some areas. It usually occurs with high pressure to the NW over France and low to the SE over the Tyrrhenian Sea and strengthened by the katabatic flow of air from the mountains and also by the canalizing of the air down the narrow, deep Rhone Valley. For this reason, strong wind blows over the Gulf of Lion (Golfe du Lion) Some local winds Abroholos (squall frequent wind, occurs from May to August, between Cabo de Sao Tome and Cabo Frio, on the coast of Brazil) Alize (northeasterly across central Africa and the Caribbean) Amihan (NEly wind across the Philippines) Bayamo (violent wind on Cuba's southern coast) Bora (northeasterly from eastern Europe to northeastern Italy) Cape Doctor (dry south-easterly wind that blows on the South African coast in summer) 19
Fremantle Doctor (afternoon sea breeze from the Indian Ocean, which cools Perth, Western Australia during summer) Habagat (southwesterly wind across the Philippines) Harmattan (dry northerly wind across central Africa) Khamsin (SEly from north Africa to the eastern Mediterranean) Kona (southeast wind in Hawaii, replacing trade winds, bringing high humidity and often rain) Nor'easter (strong storm with winds from the northeast in the eastern United States, especially New England) Nor'wester (wind that brings rain to the West Coast, and warm dry winds to the East Coast of New Zealand's South Island, caused by the moist winds) Rasha-ba (strong wind in Iraq Kurdistan area, particularly in Sulaimaniya. The word is Kurdish means Black-wind) Southerly Buster (rapidly arriving low pressure cell that dramatically cools Sydney, Australia during summer) Zonda wind (on the eastern slope of the Andes in Argentina) 20