• weight of a column of air contained in a unit area from mean sea level to top of atmosphere is known as atmospheric pressure.
• atmospheric pressure is expressed in units of a millibar. At sea level, average atmospheric pressure is 1,013.2 millibar.
Vertical Variation of Pressure
• In lower atmosphere, pressure decreases rapidly with height.
• vertical pressure gradient force is much larger than that of horizontal pressure gradient.
Horizontal Distribution of Pressure
• Small pressure differences are highly significant in terms of wind direction and velocity.
• horizontal distribution of pressure is studied by drawing isobars at constant levels.
World Distribution of Sea Level Pressure
• Along 30° N and 30° S are found high-pressure areas called subtropical highs.
• Near poles pressure is high and it is called polar high.
Forces Affecting Velocity and Direction of Wind Pressure Gradient Force
• air in motion is known as wind. wind blows from high pressure area to low pressure area.
• force exerted by rotation of earth is called Coriolis force.
• horizontal winds near earth’s surface respond to combined effect of three forces – pressure gradient force, frictional force and Coriolis force.
• differences in atmospheric pressure produce a force. rate of change of pressure concerning distance is pressure gradient.
• It affects speed of wind. This is greatest at surface and its influence usually extends up to an elevation of 1 – 3 km.
• rotation of earth on its axis affects direction of wind. This force is known as Coriolis force after name of French physicist who described it in 1844.
• Coriolis force acts perpendicular to pressure gradient force. pressure gradient force is perpendicular to an isobar.
Pressure and Wind
• velocity and direction of wind are net results of wind generating forces.
• When isobars are straight and when there is no friction, pressure gradient force is balanced by the
Coriolis force and resultant wind blows parallel to isobar.
• wind circulation around a low pressure area is known as cyclonic circulation. Around a high pressure area, it is known as anticyclonic circulation.
Circulation of air
• pattern of movement of planetary winds is known as general circulation of atmosphere.
• air at Inter-Tropical Convergence Zone [ITCZ] rises because of convection caused by high insolation and low pressure is thus created.
• easterlies from either side of equator converge in Inter-Tropical Convergence Zone [ITCZ].
• pattern of wind circulation is modified in different seasons due to shifting of regions of maximum heating, pressure & wind belts.
• Differences in heating and cooling of earth surfaces and cycles that develop daily or annually can create several common, local or regional winds.
Land and Sea Breezes
• During day time, air over land heats up faster creating low pressure comparing to sea, thus pressure gradient from sea to land is created and wind blows from sea to land as sea breeze.
• During night reversal of condition takes place. land loses heat faster and is cooler than sea.
• During night slopes get cooled and dense air descends into valley as mountain wind.
• pressure gradient is from land to sea resulting land breeze.
• When air remains over a homogenous area for sufficiently a long time, it acquires characteristics of area.
• air masses are classified according to source regions. There are five major source regions. They are [i] Warm tropical and subtropical oceans; [ii] subtropical hot deserts; [iii] relatively cold high latitude oceans; [iv] very cold snow-covered continents in high latitudes; [v] Permanently icecovered continents in Arctic and Antarctica.
• When two different air masses meet, boundary zone between them is known as a front. process of formation of fronts is called frontogenesis.
• If an air mass is fully lifted above land surface, it is known as occluded front.
• Tropical cyclones are violent storms that originate over oceans in tropical areas and move over to coastal areas bringing about large-scale destruction caused by violent winds, very heavy rainfall and storm surges.
• Tropical cyclones originate and intensify over warm tropical oceans.
• energy that intensifies storm, comes from condensation process in towering cumulonimbus clouds, surrounding centre of storm.
Thunderstorms and Tornadoes
• A thunderstorm is a well-grown cumulonimbus cloud producing thunder and lightning.
• A thunderstorm is characterised by an intense updraft of rising warm air, which causes clouds to grow bigger and rise to a greater height.
• Tornadoes usually occur in middle latitudes. tornado over sea is known as water spouts.
• systems developing in mid and high latitudes, beyond tropics are known as middle latitude or extratropical cyclones.
• Extratropical cyclones form along polar front. Initially, front is stationary. In northern hemisphere, warm air blows from south and cold air from north of front.
• processes of wind circulation, both at surface and aloft are closely interlinked. extratropical cyclone differs from tropical cyclone in several ways.