Wind is defined as air in motion and it has speed and direction. In other words, wind is the horizontal flow or movement of air masses on the earth’s surface.
Wind ranges from simple, light and comfort breeze to chaotic and destructive hurricane.
FACTORS AFFECTING WIND
- Rotation of the earth: Owing to rotation of the earth, winds are deflected to the right in the northern hemisphere and to the left, in the southern hemisphere. This is often referred to as the Ferrel’s Law of Deflection.
- Temperature: It is known that high temperature creates low pressure in areas where such temperature occurs. When the air above the ground surface gets heated, it expands and rises since it has become light. The surrounding air, which is denser than the heated air, then moves in to take the place of the ascending air.
- Pressure: Owing to differences in the rate of temperature in various areas which either create high pressure belt (low temperature) or low pressure belt (high temperature), it is known that winds; therefore, tend to blow from the high pressure belts to the low pressure belts. Winds blow from the sub-tropical high pressure belt in the northern hemisphere towards the equatorial low pressure belt to become North East Trade Winds while those blowing from the southern hemisphere become the South East Trade Winds.
TYPES OF LOCAL WINDS
Local winds are winds that blow over limited area for a short period of time. Popular examples of local winds are:
- Fohn Wind or Chinook Wind
- Both winds are dry winds associated with leeward side of mountains.
- Fohn wind is experienced in the valleys of Northern Alps in Switzerland in spring.
- Chinook winds are experienced on the eastern slopes of the Rockies in U.S.A and Canada in winter.
- Fohn wind is known to increase the temperature by 8 degrees Celsius to 17 degrees Celsius within an hour. It melts snow and causes Avalanches.
- Chinook is known to raise temperature by 19 degrees Celsius within 15 minutes and favours crops and pasture growth.
- Sirocco is warm, dry and dusty wind.
- It is associated with the Mediterranean regions of North Africa, especially in summer season.
- It originates from the Sahara desert and blows towards the Mediterranean regions.
- It only becomes moist after crossing the Mediterranean Sea and brings little rain.
- Mistral is a cold wind from the north and blows from the Rhine valley towards the Mediterranean region.
- It is also associated with the Mediterranean region of North Africa in winter season.
- The speed ranges from 64 to 130 km/hr.
- Temperature of the wind may be below freezing point through the sky may be clear and cloudless.
- Typhoons occur mainly in regions between 6 degrees and 20 degrees North and South of the equator and are very frequent from July to October e.g. in the tropics such as in China.
- They are small with a diameter of 80-300km.
- They are associated with torrential downpour accompanied by thunder and lightning.
- It can cause lots of damage to crops and properties.
- Hurricanes are found in the West Indian Islands in the Caribbean.
- They have calm and rainless centres where the pressure is lowest.
- They have great speed of about 120km/hr.
- Dense dark clouds gathers and violent stormy weather lasts for several hours. Hurricanes are destructive to life and property.
- Tornadoes are found in Guinea lands of West Africa and the South U.S.A, where they are called whirlwind and in North West Australia where they are called willy-willies.
- They are small, but very violent tropical and subtropical cyclones with speed of 800km/hr.
- They are mostly frequent in spring but can also occur at almost any time.
- They have a dark funnel cloud 80-400 metres in diameter and are very destructive.
- They are also called depressions and are confined to temperate latitudes.
- They vary from 240-3200km in diameter with lowest pressure at the centre and therefore, blow inwards.
- They are associated with precipitation either in form of rain, snow or sheet.
- These are the opposite of cyclones with high pressure in the centre and blow outwards.
- The pressure gradient is gentle and the winds are light.
- They usually herald in fine weather.
- The skies are clear, the air is calm and temperatures are high in summer, but cold in winter.
- Anticyclones may last for days or weeks and then, fade out.
- Harmattan winds
- They are strong, dry, cold and dusty winds.
- They occur mainly in West Africa.
- The winds blow from the north east across the Sahara desert.
- They create a hazy atmosphere which produces poor visibility.
- They are active during the dry season and blow from November to March.
- Land and Sea Breezes
- These are local winds which blow alternately between land and sea on a daily basis.
- Breeze exchange between the land and the sea is attributed to differential heating of land and sea.
- These winds are common in areas very close to the sea.
- Sea breeze: This breeze blows from the sea to the land. During the day, the land is heated much faster than the sea and it is cooler than the sea at night. As the land is heated during the day, it causes the air to rise, creating a low pressure situation. Air, therefore blows from the sea to replace the heated and rising air on the land, thereby resulting in lower temperature and relatively cool air on the land. Hence, it results in sea breeze.
- Land Breeze: This breeze blows from the land to the sea. At night, the land becomes cooler than the sea, having lost much of its temperature at a much faster rate than the sea. Thus, there is a high pressure situation on the land and relatively lower pressure situation on the sea; hence, land breeze blows from the land to sea.
DISTRIBUTION OF PLANETARY WINDS
It is an established fact that wind blows from areas of high pressure to areas of low pressure and this forms the basis of wind movement.
- The North East Trade and South East Trade winds blow from the sub-tropical high pressure belts to the equatorial low pressure belts.
- Converging on the temperate low pressure belts are, the south westerlies from the horse latitudes of the north to the temperate low in the north.
- There is also the meeting with the polar easterlies which originate from the polar high.
- Similar pattern also occurs in the southern hemisphere where the south-east trade winds and the polar easterlies also meet.
MEASUREMENT OF WIND
Wind has direction and speed. The direction of wind is measured by an instrument called Wind Vane while the speed of the wind is measured by a different instrument called Anemometer.
- Wind Vane: Wind Vane is made up of two parts – one part is the arrow or vane on the top, which is free to move with the prevailing wind. The other part is stationary and consists of a frame and four compass points. The Vane or arrow points to the direction from which the wind is blowing so that wind is named from the direction it blows e.g. South West wind blows from South West direction.
- Anemometer: This consists of three or four semi-circular cups attached to the ends of horizontal spokes mounted on a high vertical spindle. As the wind blows, the cups rotate. The higher the speed of the wind, the greater the speed of the cups and vice versa.