winds ,stroms and cyclones

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y Atmospheric pressure is the force perunit area exerted against a surface by theweight of air above that surface in theEarth's atmosphere. In mostcircumstances atmospheric pressure is

closely approximated by the hydrostaticpressure caused by the weight ofairabove the measurement point. Lowpressure areas have less atmosphericmass above their location, whereas highpressure areas have more atmosphericmass above their location. Similarly, as

elevation increases there is less overlyingatmospheric mass, so that pressuredecreases with increasing elevation. Acolumn of air one square inch in cross-section, measured from sea level to thetop of the atmosphere, would weigh justover a stone (and a column one square

centimetre in cross-section would weighjust over a kilogram).PRESSURE

ATMOSPHERIC PRESSURE


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WHENAIR IS HEATED,THE PARTICLES OFHOT AIR BECOME FURTHERAPART FROM EACHOTHER .THEYMOVE FASTER THAN THE PARICLES

OF COLD AIR.SOWE SAYTHAT AIREXPANDS ONHEATING

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Because its density is lower.

At constant pressure, a given volume of hot air thusweighs less than the same volume containing colder air.

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WIND CURRENTSy Although wind currents are

circular patterns of moving area,they really start at the equatorwith the help of the sun.

However, important factors suchas low- and high-pressure zonesas well as the tilt of the earthalso affect how wind blows. Themost common global winds aretropical easterlies, prevailingwesterlies and polar easterlies.

But wind belts exist on smallerscales, too, such as NorthAmerica's Chinook and theMistral in France.

y

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MONSOONWINDSy Monsoon is traditionally defined as a seasonal

reversing wind accompanied by correspondingchanges in precipitation,[1] but is now used todescribe seasonal changes in atmosphericcirculation and precipitation associated with

the asymmetric heating of land and sea.[2] [3]Usually, the term monsoon is used to refer tothe rainy phase of a seasonally-changingpattern, although technically there is also a dryphase.

y The major monsoon systems of the worldconsist of the West African and Asia-Australian monsoons. The inclusion of theNorth and South American monsoons withincomplete wind reversal may be debated.

y The term was first used in English in BritishIndia (now India, Bangladesh and Pakistan)and neighbouring countries to refer to the bigseasonal winds blowing from the Bay ofBengal and Arabian Sea in the southwest

bringing heavyrainfall to the area.[4][5]

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NATURAL DISASTERSy Anatural disaster is the effect of a

natural hazard (e.g., flood, people,tornado, hurricane, volcanic eruption,earthquake, or landslide). It leads to

financial, environmental or humanlosses. The resulting loss depends on thevulnerability of the affected populationto resist the hazard, also called theirresilience.[1] This understanding isconcentrated in the formulation:"disasters occur when hazards meet

vulnerability."[2] A natural hazard willhence never result in a natural disaster inareas without vulnerability, e.g. strongearthquakes in uninhabited areas. Theterm naturalhas consequently beendisputed because the events simply arenot hazards or disasters without human

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THUNDER STROMSy Athunderstorm, also known as an

electrical storm, a lightning storm,thundershower or simply a storm is aform of weather characterized by the

presence oflightning and its acousticeffect on the Earth's atmosphere knownas thunder.[1] The meteorologically-assigned cloud type associated with thethunderstorm is the cumulonimbus.Thunderstorms are usually accompaniedbystrong winds, heavy rain and

sometimes snow, sleet, hail, or noprecipitation at all. Those which causehail to fall are known as hailstorms.Thunderstorms mayline up in a series orrainband, known as a squall line. Strongor severe thunderstorms may rotate,known as supercells

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CYCLONEy In meteorology, a cyclone is an area of closed,

circular f luid motion rotating in the samedirection as the Earth.[1][2] This is usuallycharacterized by inward spiraling winds thatrotate counter clockwise in the Northern

Hemisphere and clockwise in the SouthernHemisphere of the Earth. Most large-scalecyclonic circulations are centered on areas oflow atmospheric pressure.[3][4] The largest low-pressure systems are cold-core polar cyclonesand extratropical cyclones which lie on thesynoptic scale. Warm-core cyclones such astropical cyclones, mesocyclones, and polar

lows lie within the smaller mesoscale.Subtropical cyclones are of intermediatesize.[5][6] Upper level cyclones can exist withoutthe presence of a surface low, and can pinchoff from the base of the Tropical UpperTropospheric Trough during the summermonths in the Northern Hemisphere.Cyclones have also been seen on other planetsoutside of the Earth, such as Mars andNeptune.[7][8]

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ANEMOMETERy An anemometer is a device for

measuring wind speed, and is acommon weather stationinstrument. The term is derivedfrom the Greek word anemos,meaning wind. The first knowndescription of an anemometer wasgiven byLeon Battista Albertiaround 1450[1].

y Anemometers can be divided intotwo classes: those that measure thewind's speed, and those that

measure the wind's pressure; but asthere is a close connection betweenthe pressure and the speed, ananemometer designed for one willgive information about both.

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BEAUFORT SCALEy The Beaufort Scale (pronounced /bofrt/) is an

empirical measure for describing wind speed based

mainly on observed sea conditions. Its full name is theBeaufortWind Force Scale

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TORNADOy Atornado (often referred to as a

twister or, erroneously, a cyclone) is aviolent, dangerous, rotating column ofair that is in contact with both thesurface of the earth and a cumulonimbus

cloud or, in rare cases, the base of acumulus cloud. Tornadoes come in manyshapes and sizes, but are typically in theform of a visible condensation funnel,whose narrow end touches the earth andis often encircled by a cloud ofdebrisand dust. Most tornadoes have windspeeds less than 110 miles per hour (177km/h), are approximately250 feet (80

m) across, and travel a few miles (severalkilometers) before dissipating. The mostextreme can attain wind speeds of morethan 300 mph (480 km/h), stretch morethan two miles (3 km) across, and stayon the ground for dozens of miles (morethan 100 km).[1][2][

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D .N.ARAVINDH RAJ VII D 20

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winds ,stroms and cyclones

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