Environment of Earth

September 16, 2009


Filed under: Environment — gargpk @ 3:05 pm

Wind  is  simply defined as air in motion. Local  winds are produced on a local scale by processes of heating and cooling  of lower air. Following two categories of local winds may be recog­nized.

(i) Katabatic winds: The  first category includes local winds in hilly or  moun­tainous regions, where on clear and clam nights, heat is rapidly lost  by ground radiation. This produces a layer of  cold,  dense air close to ground. A component of the force of gravity, acting in the downslope direction, causes this cold air to move down the mountain  sides, pouring like a liquid into ravines  and  thence down  the grade of the larger valley floors. Mountain breezes of this origin are of a variety termed katabatic winds. Particular­ly strong, persistent katabatic winds are felt on the great ice caps  of Greenland and Antarctica where the lower air layer  becomes intensely chilled. Certain occurrences of severe  blizzards in these regions are katabatic winds.

(ii) Convection winds: In  the second category are included land and  sea  breezes, which affect only a coastal belt a few km in width. Heated during the  day by ground radiation, the air over land  becomes lighter and  rises  to higher elevations. Somewhat cooler air over the adjoining water then flows land-ward to replace the rising warmer air  creating a pleasant sea breeze. At night, rapid  cooling  of the land results in cooler, denser air which descends and spreads seaward to create a land breeze. These daily alternations of  air flow are parts of simple convection systems in which flow of  air takes a circular pattern in vertical cross section. Land and sea breezes are limited to periods of generally warm, clear weather when regional wind flows is weak, but they form an important element of the summer climate along coasts.

Irrespective of whether there are pressure centers or belts, a  pressure gradient always exists, running from higher to lower pressure.  If isobars are closely placed, it indicates  that  the pressure  gradient is strong and pressure changes occur rapidly within a short horizontal distance. Widely placed isobars indicate a weak pressure gradient. Most of the widespread and per­sistent  winds of the earth are air movements set up in response to pressure differences. The pressure gradient force acts in  the direction  of pressure gradient and tends to start the air flow from  higher  to lower air pressure.  Strong pressur gradients cause strong winds and vice versa. Calm exists in the centers  of high pressures.

Coriolis force and geostrophic winds

If  the  earth  did not rotate upon its  axis,  winds  would follow the direction of pressure gradient. However, the rotation of earth upon its axis produces another force, the Coriolis force which  tends to turn the flow of air. The direction of action  of Coriolis  force is stated in the Ferrels’s Law‚ which states  that any  object  or fluid moving horizontally in the  Northern  hemi­sphere tends to be deflected to the right of its path of  motion, regardless of the compass direction of the path. In the  Southern hemisphere,  similar  deflection occurs towards the left  of  the path  of motion. The Coriolis force is absent at the equator  but increases  progressively poleward. It should be noted  especially that the compass direction is not of any consequence. If we  face down the direction of motion, turning will always be towards  the right hand in Northern hemisphere. Since the deflective force  is very  weak, it is normally apparent only in freely moving  fluids such  as air or water. Ocean currents patterns are, to some ex­tent, governed by it, and streams occasionally will show a  tend­ency  to undercut their right-hand banks in hemisphere. Driftwood floating in rivers at high latitudes in Northern  hemi­sphere, concentrates along the right-hand edge of the stream.

Applying these principles to the relation of winds to pressure,  the gradient force (acting in the direction  of  the pressure gradient) and the Coriolis force (acting to the right of the  path of flow) reach a balance or equilibrium only  when the wind has been turned to the point that it flows in the direction at  right angles to the pressure gradient i.e. parallel with the isobars. The ideal wind in this state of balance with respect to the  forces, is termed the geostrophic wind for cases in which the isobars are straight. In general, air flow at high altitudes parallels the isobars. The rule for the relation of wind to air pressure  in the Northern hemisphere states that:  Standing with back  to the wind, the low pressure will be found on  the left-hand side and high pressure on the right-hand side.

Between  the  ground level and altitude of  about  2000-3000 ft., still  another force modifies the direction of wind. This force is the friction of air with ground surface. This force acts in  such a way as to counteract, in part, the Coriolis force  and to  prevent  the wind from being deflected until  parallel  with isobars.  Instead, the wind blows obliquely across  the  isobars, the angle being from 20 to 45 degrees.


The  wind  systems present on the earth’s  surfaces  may  be categorized as following:

(1) Doldrums: In  the  equatorial trough of low pressure, intense solar heating causes the moist air to break into great  convection columns, so that there is a general rise of air. This  region, lying  roughly between 5 degrees N and 5 degrees S latitudes  was long known as the equatorial belt of variable winds and calms  or the doldrums. There are no prevailing surface winds here, but  a fair distribution of directions around the compass. Calms prevail as much as a third of the time. Violent thunderstorms with strong squall winds are common. Since this zone is located on a belt  of low  pressure, it has no strong pressure  gradients  to  induce persistent flow of wind.

(2) Trade wind belts: In  the north and south of the doldrums are the trade wind belts. These roughly cover the two zones lying between  latitudes 5 degrees and 30 degrees N and S. These winds are the result of a pressure  gradient from the subtropical belt of high pressure to the equatorial  trough of low pressure. In  the  Northern  hemi­sphere,  air moving towards equator is deflected by  the  earth’s rotation to flow southwestward. Thus the prevailing wind is from the northeast and the winds are termed northeast trade winds. In the Southern hemisphere, deflection of moving air towards left causes  the southeast trades. Trade winds have a high degree of steadiness and directional persistence. Most winds come from  one quarter of the compass.

The  systems of doldrums and trades shifts seasonally  north and  south,  through several degrees of latitudes  alongwith  the pressure  belts that cause them. Because of the large land areas of northern hemisphere, there is a tendency for these belts to be shifted  farther  north in summer (July) than  they  are  shifted south  in  winter (January). The trades are best  developed over Atlantic  and Pacific oceans, but are upset in the Indian Ocean region due to proximity of the great Asian land mass.

(3) Winds of horse latitudes: Regions  between latitudes 30 and 40 degrees in  both  hemi­spheres  have long been called the subtropical belts of  variable winds  and  clams or the horse latitudes.  These  coincide with subtropical high-pressure belts. However, these are not continu­ous belts and high-pressure areas are concentrated into  distinct centers  or cells located over the oceans. The apparent outward spiraling movement of air is directed equatorward into the east­erly  trade  wind system; poleward into the westerly  trade  wind system. The cells of high pressure are most strongly developed in the summer (January in Southern and July in Northern hemisphere). There is also a latitudinal shifting following the sun’s declina­tion. This amounts to less than 5 degrees in Southern hemisphere, but it is about 8 degrees for the strong Hawaiian high located in the north eastern Pacific.

Winds in these regions are distributed around a considerable range of compass directions. Calms prevail upto quarter  of  the time. The  cells  of high pressure have generally fair, clear weather,  with a strong tendency to dryness. Most of the  world’s great  deserts  lie in this zone and in the  adjacent  trade-wind belt.  An explanation of the dry, clear weather lies in the  fact that  the  high  pressure cells are centers  of  descending  air, settling  from higher levels of the atmosphere and spreading  out near the earth’s surface and the descending air becomes  increas­ingly dry.

(4) Westerlies: Between  the latitudes 35 and 60 degrees, both N and  S,  is the belt of westerlies or the prevailing westerly winds. Moving from  the subtropical high-pressure centers towards the  subpolar lows,  these surface winds blow from a southwesterly quarter in the Northern  hemisphere and from a  northwesterly  quarter in Southern  hemisphere. This generalization is somewhat  misleading because winds from polar direction are frequent and strong.  More accurately,  winds within the westerly wind belts blow from any direction  of the compass but the westerly components are defi­nitely  predominant. In these belts, storm winds are common cloudy days with continued precipitation are frequent. Weather is highly changeable.

In  Northern  hemisphere, land masses cause considerable disruption of the westerly wind belt but in Southern  hemisphere, there  is an almost unbroken belt of ocean between the  latitudes 40 and 60 degrees S. Therefore, in Southern hemisphere the  west­erlies gain great strength and persistence.

(5) Polar easterlies: The characteristic wind systems of the arctic and  antarctic latitudes  is  described as polar easterlies. In the Antarctic, where an ice-capped mass rests squarely upon the south pole and is surrounded by a vast oceanic expanse, polar easterlies show an outward  spiraling flow. Deflected to the left in Southern  hemi­sphere, the radial winds would spiral counterclockwise, producing a system of southeasterly winds.

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