Atmospheric Circulations And Weather Systems Class 11 Notes Geography Chapter 9 - CBSE

Chapter : 9

What Are Atmospheric Circulations And Weather Systems ?

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Atmospheric pressure: The weight of a column of air contained in a unit area from the mean sea level to the top of the atmosphere is called atmospheric pressure. The atmospheric pressure is expressed in units of a millibar.
Wind: The air in motion is called wind.
Pressure gradient force: The differences in atmospheric pressure produce a force. The pressure gradient is the rate of change of pressure with respect to distance.
Frictional force: It affects the speed of the wind. It is greatest at the surface, and its influence generally extends up to an elevation of 1-3 km. Over the sea surface, the friction is minimal.
Coriolis force: The rotation of the earth about its axis affects the direction of the wind. This force is called the
Coriolis force, after the French physicist who described it in 1844.
Geostrophic wind: When isobars are straight and there is no friction, the pressure gradient force is balanced
by the Coriolis force, and the resultant wind blows parallel to the isobar. This wind is known as the geostrophic
wind.
General circulation of the atmosphere: The pattern of the movement of the planetary winds is called the general circulation of the atmosphere.
Cell: The easterlies from either side of the equator converge in the Inter Tropical Convergence Zone (ITCZ).
Such circulations from the surface upwards and vice-versa are called cells.
Hadley cells: A cell in the tropics is called the Hadley cell.
Ferrel cells: In the middle latitudes, the circulation is that of sinking cold air that comes from the poles and the rising warm air that blows from the subtropical high. At the surface, these winds are called westerlies, known as the Ferrel cell.
Polar cell: At polar latitudes, the cold, dense air subsides near the poles and blows towards middle latitudes as the polar easterlies. This cell is called the polar cell.
Valley breeze: In mountainous regions, during the day the slopes get heated up and air moves upslope and to fill the resulting gap, the air from the valley blows up the valley. This wind is known as the valley breeze.
Polar high: Near the poles, the pressure is high known as the polar high.
El Nino: The warm water of the central Pacific Ocean slowly drifts towards the South American coast and replaces the cool Peruvian current. Such appearance of warm water off the coast of Peru is known as the El Nino.
Southern oscillation: The change in pressure condition over the Pacific is known as the southern oscillation.
Katabatic wind: During the night, the slopes cool, and the dense air descends into the valley as the mountain wind. The cool air of the high plateaus and ice fields draining into the valley is called katabatic wind.
Air mass: The air with distinctive characteristics in terms of temperature and humidity is called an air mass.
It is defined as a large body of air having little horizontal variation in temperature and moisture.
Source regions: The homogeneous surfaces which are formed over air masses are called the source regions.
Fronts: When two different air masses meet the boundary zone between them is called a front.
Cold front: When the cold air moves towards the warm air mass, its contact zone is called the cold front.
Warm front: If the warm air mass moves towards the cold air mass, the contact zone is called the warm front.
Occluded front: If an air mass is fully lifted above the land surface, it is called the occluded front.
Extratropical cyclone: The systems developing in the mid and high latitudes, beyond the tropics, are called the middle latitude or extratropical cyclones.
A mature tropical cyclone is characterized by the strong, spirally circulating wind around the centre, called the eye. The diameter of the circulating system can vary between 150 and 250 km. The eye is a region of calm with subsiding air.
Tropical cyclones: Tropical cyclones are violent storms that originate over oceans in tropical areas and shift to the coastal areas bringing about large-scale destruction caused by violent winds, very heavy rainfall, and storm surges. This is one of the most devastating natural calamities.
Tornado: From severe thunderstorms, sometimes spiralling wind descends like an elephant’s trunk with great force, with very low pressure at the centre, causing massive destruction on its way. Such a phenomenon is called a tornado.
Water spouts: The tornado over the sea is called water spouts.