Airmass :
Meaning and Characteristics “An air mass may be defined as a large body of air whose physical properties, especially temperature, moisture content, and lapse rate, are more or less uniform horizontally for hundreds of kilometres.”

According to A N Strahler and A. H. Strahler: “a body of air in which the upward gradients of temperature and moisture are fairly uniform over a large area is known as an air mass.”

* An air mass may be so extensive that it may cover a large portion of a continent and it may be so thick in vertical dimension that it may vertically extend through the troposphere. An air mass is designated as cold air mass when its temperature is lower than the underlying surface while an air mass is termed warm air mass when its temperature is higher than the underlying surface.

* The boundary between two different air masses is called front.

Source Regions

* The extensive areas over which air masses originate or form are called surface regions whose nature and properties largely determine the temperature and moisture characteristics of air masses. * An ideal source region of air mass must possess the following essential conditions: * There must be extensive and homogenous earth's surface so that it may possess uniform temperature and moisture conditions * There should not be convergence of air; rather there should be divergence of air flow so that the air may attain the physical properties of the region. * Atmospheric conditions should be stable for considerably long period of time so that the air may attain the characteristics of the surface. * * * Regions of Air Masses * There are six major source regions of air masses on the earth's surface: 1. Polar oceanic areas (North Atlantic Ocean between Eurasia and North America, and Arctic region during winter season) 2. Tropical oceanic areas (anticyclonic areasthroughout the year) 3. Tropical continental areas (North AfricaSahara, Asia, Mississippi Valley zone of the USA most developed in summers) 4. Equatorial regions (zone located between trade windsactive throughout the year) 5. Monsoon lands of SE Asia

Classification of air masses

There are two approaches to the classification of air masses, e. g. Geographical Classification
Thermodynamic classification.

Geographical Classification 1. The geographical classification of air masses is based on the characteristic features of the source regions. 2. Trewartha has classified air masses on the basis of their geographical locations into two broad categories, viz. 3. Polar air mass (P), which originate in the polar areas. Arctic air masses are also included in this category Tropical air mass (T), which originate in tropical areas. Equatorial air masses are also included in this category. 4. These two air masses have been further divided into two types on the basis of the nature of the surface of the source regions: 5. Continental air masses (indicated by small letter V) maritime air masses ( ‘m’ ) 6. Thermodynamic Modifications and Classification of Air Masses Thermodynamic modifications of an air mass involves its heating from below while passing through different surfaces away from the source region. 7. The modification of air masses depend on 4 factors: 8. Initial characteristics of air mass in terms of temperature and moisture content: 9. Nature of land or water surface over which a particular air mass moves 10. Path followed by the air mass from the source region to the affected area Time taken by the air mass to reach a particular destination. 11. A warm air mass (w) is that whose temperature is greater than the surface temperature of the region visited while if the air mass is colder than the surface temperature it is called cold air mass (k). 12. Such mechanical modifications are introduced due to cyclonic and anticyclonic conditions, Based on the thermodynamic and mechanical (dynamic) modifications air masses are divided into: Cold air mass and warm air mass.

Geography air stability

Stability: When the dry adiabatic lapse rate of ascending dry air is higher than the normal lapse rate and if it is not saturated and does not attain dew point, it becomes colder than surrounding air at certain height with the result it becomes heavier and descends. This process causes stabilityof atmospheric circulation due to which vertical circulation of air is resisted.

When the ascending parcel of air reaches such height that its temperature equals temperature of surrounding air, its further upward movement is stopped. Such air is said to be in the state of neutral equilibrium.

Mechanical Instability: It is a case of abnormal conditions when the normal lapse rate is exceptionally very high (15 degree C to 35 degree C per. lOOm.). The upper layers are cold and denser than the underlying layers, therefore, cold and denser upper layers automatically descend. Such situation is called mechanical instability and helps in the formation of tornado.

Conditional Instability: When a parcel of air is forced to move upward, it cool at dry adiabatic lapse rate (10 degree C per 1000 m. Or 5.5 degree F per 1000 feet), normal lapse rate is 6.5° C per 1000 m. The air is initially forced to move upward but rises automatically due to its own properties after condensation point is reached.

Dew
The earth receives radiation fromihe sun during day and reflects in the night. * When the earth reflects the heat the surface becomes cooleLand the air around it also becomes cooler. * Then the water vapour in the air condenses and then is called ‘dew’ * But there are two preconditions: There must be vapour in the air; and the surface must be cool enough to condense that water vapour. * That's why after rainy season, the water vapour content in the air increases. * After the rainy season in the winter, the air becomes cool and therefore adequate dews are formed in Oct. Nov. Fog * It is a special type of thin cloud consisting of microscopically small water droplets which are kept in suspension in the air near the ground surface arid reduces horizontal visibility. * Fog is generally associated with inversion of temperature and occurs in the morning hours but sometimes also continue till noon.
When there is a mixture of smoke and fog, it becomes Smog.

Radiation Fog

Radiation fog is formed when warm and moist air lies surface. Due to this situation overlying warm and moist aircools and the dew point is reached, with the condensation of water vapour around hygroscopic nuclei (dust particles and smokes) forms numerous tiny water droplet and thus fog is originated.
When fog is combined with sulphur dioxide it becomes poisonous and causes human deaths. Such fog is called urban smog.

Advectional Radiation Fog

The fog formed due to mixing of warm moist air and cold air due to arrival of warm and moist air over cold ground surface is called advectional radiation.

The fogs occurring over sea surfaces are called sea fogs, which are generally formed, near the coastal areas frequented by cold ocean currents.
Steam Fogs

Steam fogs are in fact advectional fogs, which are formed when cold air moves from land over oceanic surface and there is evaporation of large quantity of moisture from water surface to saturate the overlying cold air. They are also called evaporation fogs.

Upslope or Hill Fogs

It originate when continental warm and moist air rises upslope along the hill slopes because the rising air is saturated due to cooling and condensation of moisture around hygroscopic nuclei and forms fogs which cover the lower segments of hill slopes.

Frontal Fogs

Fronts are formed when two contrasting air masses (warm and cold air masses) converge along a line. Warm air is pushed upward by cold air and hence overlying warm air is cooled from below due to underlying warm air is cooled from below due to underlying cold air and fogs originate after condensation.

Frost

When the temperature falls below freezing point, it forms a frost. Temperature either 0° C or less, than the water droplets take the form of ice cubes.

Rime

It is a deposit of white opaque ice crystals formed by the freezing of super cooled water droplets on the surface below 0° C.

Geography anti cyclone winds

Temperate Cyclone

It is also called ‘Depressions’ It has low pressure at the centre and increasing pressure outward. It has varying shapes such as near circular, elliptical or wedge, therefore, it is also called Low or Troughs or Mid Latitude Depressions.

Secondly, they are formed in the regions extending between 35 degree to 65 degree latitudes in both the hemispheres.

Thirdly, Tropical Cyclone is confined strictly over sea, whereas Temperate Cyclone form over both land and sea.

Fourthly, Tropical cyclone is produced in summer and autumn and Temperate cyclone largely in winter.

Fifthly, Temperate cyclone has low pressure gradient, whereas tropical cyclone has steep pressure gradient; and

finally, rainfall in temperate cyclones is slow and continuous, whereas in tropical cyclone the rainfall is violent and torrential.

Anticyclone

A system of atmospheric pressure in which the isobars on a synoptic chart indicate a relatively, highpressure in the centre and decreasingly low pressures outwards to the periphery of the system. The isobars are generally widely spaced, indicating light winds which may be absent near the centre. Air movement is clockwise in the Northern hemisphere and anticlockwise in the Southern hemisphere. The term anticyclone was coined by Galton in 1861 Anticyclones do notexperience any precipitation and tend to be dry. It is characterized by larger, slow moving and more persistent with a high pressure at the centre but with a weaker pressure gradient and light variable winds diverging from the centre.

* They are usually circular in shape but sometimes also assume V shape. * They are much larger in size and area than temperate cyclones as their diameter is 75% larger than that of the latter. * Anticyclones do not have fronts. Winds descend from above at the centre and thus weather becomes clear and rainless.

Fohn Winds

(Latin: Meaning growth) A warm and relatively dry wind which descends on the leeward side of a mountain range.

Fohn winds are:

Chinook-Andes and Rockies
Yamo-Japan
Tramontane-C. Europe
Samun-Iran
Norwester-New Zealand
Berg-S. Africa
Santa Ana-California
Zonda-Argentina

Depression Winds

A moving wind involves air masses originating both on its poleward and equatorward side, therefore, both warm and cold wind result. Depression winds are:

Warm Winds

* Sirocco-Italy * Leveche-Australia * Khamsim-Egypt * Gibli-Tunisia * Brickfielders-Victoria (Australia)

Cold Winds

* Southerly Burster-New South Wales * Pampero-Argentina * Friagem or Surazo-Brazil * Papagayo-Mexico * Mistral-Rhone Valley (France) * Levanter-West Mediterranean * Etesian-East Mediterranean * Bora-Adriatic Coast * Convectional Wind: They are basically desert winds with dusty and gusty surface winds * Karaburan-Tarim Basin * Haboob-Sudan Harmattah-West Africa

Tornadoes:

A rapidly rotating column of air developed around a very intenselow pressure centre. It is associated with a dark funnel shaped cloud and with extremely violent wind blowing in a counterclockwise spiral; but accompanied by violent down draughts. Common in USA.

Zones

There are four principal climate regions: * Tropical (hot) * Subtropical (warm) * Temperate (cool) * Polar Regions (cold)

geography atmosphere composition

1. Nitrogen (N2) 78.08% 2. Oxygen (O2) -20.9% 3. Argon (Ar) -0.9% 4. Carbon Dioxide (C02) -0.033%. These 4 constitute 99.997%.
Water vapour (H20) Trace Constituents: Neon (Ne) Helium (He) Krypton (Kr) Xenon (Xe) Hydrogen (H2) ethane (CH4) Nitrous Oxide (N2O) Radon (Rn) Highly variable constituents: Water vapour Ozone (03) Sulphur dioxide (S02) Nitrogen dioxide (N02) Carbon Monoxide (CO) Particles (dust, salt)

Characteristics

Nitrogen: When the weathering of igneous rocks takes place, it adds nitrogen in the atmosphere. It is found between 50 − 100 km. But dominates the lower 50 km.

Oxygen: It occurs up to 120 km. But up to 6 km. As 02, while above it occurs in dissociated form or O.

Carbon dioxide: Absorbs heat radiation from the earth in the atmosphere. It is: Transparent and keeps the earth temperature at high level. The rocks gradually remove away the C02 from the atmosphere. It dissolves in water to form carbonic acid, a compound i.e.. C02 + H20 = H2C03. The ocean contains 60 times more C02 than the atmosphere.

Argon, Neon, Krypton, Xenon: Chemically inactive; present in tiny proportion; known as noble gases.

Water Vapour: Most variable in proportion and largely concentrated in the lowest Kms. recycles in evaporation-condensation. It is mainly found in lowest region: 6 km of atmosphere. Therefore it becomes less with height. Neon: Bright Red. Used in Neon sign, tubelights and advertisement boards.

Helium: Chemically inert. It is added in the atmosphere by the oil fields. The amount of helium has increased 10 times. Since it is chemically inert hence it can only be lost by escape in the space. Hydrogen: It is negligible in low atmosphere but present above 1500 km. Protons and electrons are found in hydrogen.

Ozone (03) -Absorbs ultraviolet and infrared radiation and therefore increases the. Temperature above stratosphere. Maximum production of ozone occurs at 30 − 40 km above the earth's surface but its maximum concentration occurs at 20 − 30 km above the earth. Ozone hole was first sighted above Antarctica. Ozone immediately reacts with chlorine.

Variations in Atmospheric Composition Variation with height

* Water Vapour comprises up to 4% of the atmosphere by volume near surface but non existent above 10 km. Of the atmosphere * Ozone is mainly concentrated between 15 − 35 km. * 100 − 200 km. Is the nitrogen layer * 200 − 1, 100 km. Is the oxygen layer * 1, 100 − 3, 500 km. Is helium layer * Above 3, 500 km. Is the oxygen layer again.

Variations with latitude and seasons

* Above 30° latitude north, C02 is least * Ozone content is low over the equator and high over 50 degree north latitude, particularly in spring.

Atmosphere can be divided into following layers: Troposphere; Stratosphere; Mesosphere; Ionosphere; Thermosphere; Exosphere; Magnetosphere

Troposphere:

* Troposphere It is the most important zone for weather phenomenon, because of: Gradual decrease of temperature with height i.e.. 6.5 degree C per km. Temperature decreases except at winter pole; lowest part of troposphere up to 1.5 − 2 km. Is called friction layer, where topography greatly influences wind speed and circulation * It contains all the major atmosphere pollutants. This is also called Connective layer where the clouds are formed * it roughly extends to a height of 8 kms near the poles and about 18 kms. At the equator the thickness at the equator is greatest it contains dust particles and over 90% of the earth's water vapour * aviators of jet aeroplanes often avoid this layer due to presence of bumpy air pockets.
The upper limit of the troposphere is called Tropo-pause, literally means zone or region of mixing. Its height is 17 km during January and July over the equator and the temperature of this height is 700 C

Stratosphere

* From Tropopause to about 50 Km * It is an Isothermal region and extremely dry free with clouds, water vapour and dust; here air is at rest and movement is almost horizontal * Some clouds found are called Mother of Pearls or Nacreous. * Contains much of Ozone (03); therefore called Ozono-sphere, especially between 15 kms to 35 km from the sea level. The combining of atmosphere oxygen 02 with individual oxygen results in the creation of ozone. * In the lower stratosphere (up to 25 km.) temperature remains constant, temperature increase gradually with height up to 50 Kms; and at 50 kms becomes 0° C or 32° F. * The upper limit of the Stratosphere is called Stratopause. * Winds decrease with height in the lower stratosphere and then increase with height in the upper stratosphere. * Feable winds and Cirrus Clouds are found in the lower stratosphere

Chemosphere

* Chemosphere extends from troposphere to an altitude of 50 kms. Overlapping both homosphere and heterosphere. * In this air glove occurs at night especially green and red. It is a part of Stratosphere. In this air glow occurs at night, especially green and red.

Mesosphere

* Height from 50 Km to 80 Km. * The temperature decreases fairly with the height with the minimum temperature of about-90 degree. * Mesopause (the top of the layer); above Mesopause temperature increases with increasing height * The presence is because of meteoric dust particles.

Thermosphere

* The part of the atmosphere beyond Mesopause is known as thermosphere wherein temperature increases rapidly with increasing height. * it is above 200 km. And N02 and 02 are found. * its lower portion is composed mainly of nitrogen and oxygen in molecular and atomic forms * rapid temperature increase approaching 1700 degree C, at 350 kms. * thermosphere is divided into two layers: (1) Ionosphere and (2) Exosphere Ionosphere: 80 kms. To 640 kms. And above * Radio waves found; it is a region of electrically charged or ionized air lying next to Mesosphere * High electron density * 150 kms. 380 kms. Known as Appleton * Absorbs deadly X-rays * The northern lights or aurora borealis are found. * This layer is called Kennelly Heaviside Layer (99 − 130 kms) here interaction takes place between solar-ultra-violet photons with nitrogen * Sporadic Layer is associated with high velocity winds. The bulk of the atmosphere consists of electrically neutral atoms and molecules. At high altitudes, however, a significant fraction of the atmosphere is electrically charged. This region is generally called the Ionosphere. * It extends throughout the mesosphere and thermosphere but is most important and distinct at altitudes above about 80 kilometres. * Most of the ionization in the ionosphere is effected by pho-toionization. Photons of short wavelength (i.e.. high energy) are absorbed by atmospheric gases. A portion of the energy is used to eject an electron, converting a neutral atom or molecule to a pair of charged species: An electron, which is negatively charged, and a com-panion positive ion. Ionization in the Fl region is produced mainly by ejection of electrons from 02, 0, and N2. The threshold for ionization of 02-corresponds to a wavelength of 102.7 nanometres. Thresholds for 02 and N2 are at 91.1 and 79.6 nanometres, respectively.

Exosphere:

640 kms and above:

The atoms of oxygen, hydrogen and helium form the tenuous atmosphere

The density becomes extremely low and the atmosphere resembles a nebula because it is highly rarefied. Aurora Australis and Aurora Borealis are produced-magnetic storms on the sun discharge electrified particles in the space. The earth's magnetic poles attract these particles. Aurora Australis (the southern dawn). Aurora Borealis (the northern dawn).

Chemical Composition

It is basically divided into two parts: Homosphere

Heterosphere

Homosphere

It represents the lower portion of the atmosphere and extends upto the height of 90 km from the sea-level.

The main constituent gases are Oxygen (20.946%), Nitrogen (78.084%). Others are Argon, Carbon-dioxide, Neon, Helium, Krypton, Xenon, Hydrogen, etc.

The proportion of different gases is uniform at different levels in this zone.

Heterosphere

This zone extends from 90 km to 10, 000 km.

There are four parts of it:

Molecular nitrogen layer-it is dominated by molecular nitrogen and extends upward up to the height of 200 km (90 to 200 km)

Atomic oxygen layer: Extends from 200 to 100 km

Further upward there is helium layer which extends up to the height of 3500m
Atomic hydrogen layer-it is the top most layer of the atmosphere and extends up to the outer most limit of the atmosphere.

Geography climatology humidity

The process of transformation of liquid (water) into gaseous from is called evaporation. The process of conversion of vapour into (water) and solid form (ice, snow, frost) is condensation.

Latent Heat

Energy in the form of heat is required for the conversion of water into gaseous torm (water vapor). Heat energy is generally measured in the unit of calorie.

The potential energy of water is more than ice and that of vapour is more than water. This hidden amount of heating water is the latent heat.

Humidity Capacity

The moisture content (humidity) of the air is measured in grain per cubic foot or in gram per cubic centimetre. Evaporation is the main mechanism through which wa-sher is converted into humidity. Temperature and evaporation are directly positively related and hence humidity and temperature are also directly positively related.

The moisture retaining capacity or humidity capacity refers to the capacity of an air of certain temperature to retain maximum amount of moisture content.

Humidity capacity is directly positively related with temperature

Higher the temperature, higher the humidity capacity and vice-versa.
The ratio of increase of humidity capacity also increases with the increasing temperature.

Absolute Humidity The total weight of moisture contained (water vapour) per volume of air at definite temperature is called absolute humidity.

Evaporation is the main mechanism through which water is converted into humidity or vapour. Temperature and evaporation are directly and positively related. The absolute humidity decreases from equator towards thepoles and from ocean to the continents.

The possibility of preeipitation largely depends on absolute humidity.

The air having moisture content equal to its humidity capacity is called ‘saturated aiR'Generally absolute humidity does not change with increase or decrease of temperature. Specific Humidity-It is defined’ as the mass of water vapour in grams contained in a kg Qf air and it represents the actual quantity of moisture present in a definite air

It is seldom affected by change in the air temperature measured in the units of weight It is directly proportional to vapour pressure, which is the partial pressure exerted by water vapour in the air and is independent of other gaseS'and is inversely proportional to air pressure.

It decreases from equator to pote ward.

In real sense, specific humidity is a geographer's yardstick of a basic natural resource water to be applied from equatorial to Polar Regions. It is a measure of quantity of water that can be extracted from the atmosphere as precipitation.

In Arctic, it is 0.2 gm/kg. while in equatorial region, it is 18gm/kg.

Relative Humidity (RH)

Relative humidity is defined as a ratio of the amount of water vapour actually present in the air having definite volume and temperature (i.e.. absolute humidity) to the maximum amount the air can hold (i.e.. humidity capacity).

(Relative Humidity = Absolute Humidity/Humidity Capacity) There is inverse relationship between air temperature and relative humidity, i.e.. relative humidity decreases with increasing temperature while it increases with decreasing tem-perature.

When the humidity capacity and absolute humidity of the air are the same, the air is said to be saturated and relative humidity becomes lOO percent. Relative humidity (RH) can be changed in two ways: First, if the absolute humidity increases due to additional evaporation and secondly, change takes place temperature. Importance of RH: The possibility of precipitation depends on it. High and low relative humidity is indicative of the possibility of wet and dry conditions respectively. Distribution of RH: Equatorial regions are characterized by highest relative humidity.

It gradually decreases towards subtropical high pressure belts where it becomes minimum (between 25° 35° latitude) -Latitudes largely control seasonal distribution of relative humidity.

Maximum R H is found during summer season between 30° Nand30° S latitudes. Condensation

The transformation of gaseous form of water (i.e.. water vapour) into solid form (ice) and liquid form (water) is called condensation. Condensation is opposite to evaporation.

The temperature at which an air becomes saturated is called Dew point temperature. Condensation depends on: the percentage of relative humidity of the air the degree of cooling of air.

Cooling of Air and Adiabatic Change

Temperature decreases with increasing height at the rate of 6.5° C per 1000m or 3.6° F per 1000 feet. This rate of decrease of temperature with increasing height is called normal lapse rate.

A definite ascending air with given volume and temperature expands due to decrease in pressure and thus cools. It is apparent that there is a change in temperature of air due to ascent or descent but without addition or subtraction of heat. Such type of change of temperature of air due to contraction or expansion of air is called adiabatic change of temperature.

Adiabatic change of temperature is of two types, viz. Dry adiabatic change and moist adiabatic change. * The temperature of unsaturated ascending air decreases with increasing height at the rate of 5.5 F per feet or 10° C per 1000 m This type of change of temperature of unsaturated ascending or descending air is called dry adiabatic rate. * The rate of decrease of temperature of an ascending air beyond condensation level is lowered due to addition of latent heat of condensation of the air. This is called moist adiabatic rate * In this case temperature of an ascending air beyond condensation level decreases (and hence the air cools) at the rate 3° F per 1000 feet. This is also called retarded adiabatic rate.

Stability and Instability of the Atmosphere

Different types of precipitation (dew, rainfall, frost, snowfall, hailstorm, etc.) depend on stability and instability of the atmosphere.

The air without vertical movement is called stable air while unstable air undergoes vertical movement.
An air mass ascends and becomes unstable when it becomes warmer than the surrounding air mass while descending air mass becomes stable. The stability and instability depends on the relationship between normal lapse rate and adiabatic change in temperature.

geography climatology pressure winds

Temperature is affected by:

Latitude: At high latitude there is low temperature and at low latitude, there is high temperature due to longer and shorter distances. Thus equator will be affected due to isolation or sunstroke and tropics will be most affected between 6° north-6° South. Because the vertical motion is relatively rapid during its passage over the equator, but its rate slows down as it reaches the tropic

Altitude: Places near the earth's surface are warmer, thus the temperature decreases with the increasing height above the sea level because of the lapse rate i.e.. Every 1 km decreases by 6.5 degree c.

Continentality: Continental Climate: Summer-70 degree F; winter-28 degree F; range-42 degree F. Maritime Climate: Summer-62 degree F; winter-48 degree F; range-14 degree F.

Oceanic Currents and Winds

Pressure And Planetary Winds

1. 0' -5 degree North South:

A. Called Equatorial Low Pressure Belt B. Intense heating, with expanding air and ascending convection currents C. It is the closest point to the sun, therefore, the air is relatively more hot due to which, the air becomes less denser, lighter and moves upward. D. It is called DOLDRUMS or calm E. It is a Zone of Wind Convergence

2. 10° 15° North and South: A. Due to high pressure belt around this area, there is subtropical high pressure belt where the air is comparatively dry, light and calm. B. This is very beneficial in maritime trade, hence, is called maritime trade. Since the air becomes hotter at the equator, it raises upward and around 30 degree north and south starts coming down. Due to this, a high-pressure belt is created. Hence, horse latitude i.e.. 25° 35° north and south, no wind blows.

3. 30° 45° North-South: A. Subtropical High Pressure Belt B. Air is comparatively dry and winds are calm and light. C. It is a region of descending air currents of wind divergence with cyclonic activity D. Referred as HORSE LATITUDE.

4. 30° 60° North and South:

A. It is the area of temperate low pressure belt or the antitrade wind area. There is rainfall all round the year and cyclones and anti-cyclones are developed. B. Comparatively, anti-trade winds are faster in southern hemisphere than in the northern C. Due to Coriolis force, they become South Westerlies in the north and North Westerlies in the south. In the southern hemisphere, due to oceans between 40 de-gree-60 degree South Westerlies blow with much greater force with regularity throughout the year. Here three types of winds are found: Roaring 40S, Furious 50S, and Shrieking or Storming 50S.

5. 60° North-South:

A. Two Temperate Low Pressure Belts which are also zones of convergence with cyclonic activity
The sub-polar low pressure areas are best developed over oceans.

6. 90° North-South

A. Temperatures are permanently low, are the Polar High Pressure Belt. HORSE LATITUDE-The dynamically induced subtropical high pressure belt extends between 30° 35° (25° 35° ); latitudes in both the hemispheres. B. This belt separates two wind systems, viz. Trade winds and westerlies. C. This zone 30° 35° is characterized by weak and variable winds and calm. D. It is known as horse latitude because of the fact that in ancient times had be sailed through the calm conditions of these latitudes.

Doldrum

* A belt of low pressure, popularly known as equatorial trough of low pressure, extends along the equator within a zone of 50 degree N and 50 degree S latitudes. This is the belt of calm or doldrums because of light and variable winds. This belt is subjected to seasonal and spatial variations due to northward and southward movement of the overhead sun (summer and winter solstices). Polar Belt: * Temperature is permanently low, so this region is the high pressure belt. In the northern hemisphere, they blow north east and in southern hemisphere, south east. The polar easterlies blows towards the temperate low pressure belt. They are extremely cold as they come from Tundra and Icecap region. They are more regular in the south than the north. Planetary Winds: Winds tend to blow from the high pressure belts to the low pressure belts, are the planetary winds. Coriolis Force or Ferrel's Law of Deflection: * Instead of blowing directly from one pressure belt to another, however the effect of the rotation of the earth (Coriolis force) tends to deflect the direction of the winds. In the northern hemisphere, winds are deflected to their right and in the southern hemisphere to their left. * This is known as Ferrel's Law of Deflection. * The Coriolis Force is about along the equator but increases progressively towards the Poles.

Trade Winds

* These winds blow out from the Sub-Tropical High Pressure Belt in the northern hemisphere towards the Equatorial low become North East Trade Winds and those in the southern hemisphere become the South East Trade Winds. These trade winds are the most regular of all the planetary winds. * They blow with great force and in constant direction. * Therefore, helpful to traders to sail. Trade winds bring heavy rainfall. * They sometimes contain intense depressions. * The word ‘trade’ comes from the Saxon word ‘tredan’ which means to tread or follow a regular path. * They blow from north-east towards the equator, in the northern hemisphere and from south-east towards the equator, in the southern hemisphere.

Permanent Winds

They blow throughout the equator.

Westerlies

* From the Subtropical High Pressure Belts, winds blow towards the Temperature Low Pressure Belts. * Under the effect of Coriolis Force, they become the South-Westerlies in the northern hemisphere and North-Westerlies in the southern hemisphere. * This warming effect and other local pressure differences have resulted in a very variable climate in the temperature zones, dominated by the movement of cyclones and anti-cyclones. * In the southern hemisphere, where there is a large expanse of ocean, from 40 degree south to 60 degree south; westerlies blow with much greater force and regularity throughout the year. * There is much variation in the weather conditions in their poleward parts where there is convergence of cold and denser polar winds and warms and lighter westerlies. * Their velocity increases south ward and they become. Stormy. They are also associated with boisterous gales. The velocity of the westerlies be-cpme so great that they are called: * Roaring forties between the latitudes 40 − 50 degree S Furious fifties at 50 degree S latitude * Shrieking sixties at 60 degree S'latitude. Polar Easterlies: * It blows from the Polar Easterlies towards the Temperate Low Pressure Belts. * It is extremely cold winds as it comes from Tundra and Ice-Cap regions. y It is more regular in the south than in the north. * It is defected to the right to become the N. E. Polar Winds in the Northern Hemisphere and to the left to become the S. E. Poter Winds in the Southern Hemisphere. * These polar cold winds converge with warm westerlies near 60 − 65 latitudes and form polar front or mid-latitude front or mid-latitude front, which becomes the centre for the origin of temperate cyclones.

Fohn and Chinook

Fohn is a warm, dry and local wind-Northern Alps-Switzerland in spring; and called climate oasis.

* Chinook is a warm, dry and local wind-Eastern slopes in Rockies in USA and Canada in winters. * It increases temperature 35 degree F within 15 minutes. * It causes Avalanches. * In North America, it is called Chinook, meaning ‘the snow eater’ * Chinook winds are more common during winter and early spring along the eastern slopes (leeward side) of the Rocky Mountains from Colorado (USA) in the south to British Columbia (Canada).

Sirocco

Sirocco is a warm, dry and dusty local wind, which blows in northerly direction from Sahara desert and after crossing over the Mediterranean Sea, reaches Italy and Spain.

* Becomes extremely warm and dry while descending through the northern slopes of the Atlas Mountain. * It is known as Khamsin in Egypt; Gibli in Libya; Chili in Tunisia; Simoom in Arabian Desert; Blood Rain in South Italy; Leveche in Spain; Gharbi in Adriatic and Aegean Sea.

Mistral

* It is a cold wind which blows in Spain and France from North-east direction; especially in winter * The average velocity of mistral is 56 − 64 km/h to 128 kmph Bora: * Bora is an extremely cold and dry north-easterly wind in Adriatic Sea, with a velocity of 128 kmph to 196 kmph * It is also called Tramontana and Gregale. Harmattan: It is warm and dry winds blowing from north-east and east to west in the eastern part of Sahara desert. * Called as Doctor in Guinea coastal of Western Africa * Called Brickfielder in Victoria in Australia; Blackroller in the Great plains of USA; Shamal in Mesopotamia; Norwester in New Zealand.

Blizzard

* It is a violent stormy cold and powdery polar wind laden with dry snow and is prevalent in North and South polar regions, Siberia, Canada and the USA. * Northers in the Southern USA and Burran in Siberia. Tropical Cyclones: Typhoons: It occurs mainly in the region 6 degree and 20 degree North and South of the equator and are most frequent from July to October. It's velocity is 100 m. p. h. Torrential downpour is accompanied by Thunder and Lightening. Hurricanes: Same feature, but only differs in intensity, duration and locality. It has calm, rainless centres, where pressure is lowest.

Tornadoes

* Its velocity is 500 m. p. h. * It appears as a dark funnel cloud. * 250 − 1400 ft. In diameter. * It is most frequent in spring.

cyclones:

Cyclone is a system of low atmospheric pressure in which the barometric gradient is steep. Winds circulate, blowing inwards in an anticlockwise direction in the Northern hemisphere and clockwise direction in the Southern hemisphere. Cyclones are classified into two parts:

Tropical Cyclone

A system of low pressure occurring in tropical latitudes, characterized by its very strong winds; found mainly in Indian Ocean, Indonesia, and Australia. Tropical Disturbances are classified into three parts:

1. A Tropical Depression is a system with low pressure enclosed within few isobars and the wind speed is. 33 knot or 61 tens and it lacks a marked circulation. 2. A Tropical Storm is a system with several closed isobars and a wind circulation of 115 Kms. 3. A Tropical Cyclones is a warm core vortex circulation of tropical origin with a small diameter often of an approximately circular shape; they occur only in oceanic areas where the sea temperatures exceed 27 degree C.

Different Names of Tropical Cyclones

* Hurricanes-N. America & Caribbean * Typhoons-Western North Pacific * Willywillies-Australia * Bagulo-Philippine Islands * Taifu-Japan * Cyclones-Indian Ocean

Structure of the Tropical Cyclones

Eye the innermost or central portion of the mature cyclone is the ‘eye’ It is about 10 to 30 km in diameter, depending upon the size of the. Storm and is a more. Or less calm region with little or no clouds and some subsidence. The eye or the calm centre can be described variously as the: Pressure eye (where mean sea level pressure is lowest), Wind eye (light or calm wind conditions), Radar eye (the eye seen in radar echoes) and the Satellite eye (clear or dark spot seen in the cloud mass in satellite imagery).

Eye Wall or Inner Ring Surrounding the eye is a tight ‘inner ring’ of hurricane winds. This core of maximum winds is at the centre of a solid thick wall of towering Cumulonimbus clouds and is called the ‘eye wall’

Outer Ring An outer ring of cyclonic circulation lies beyond the eye wall, where the speed decreases steeply and clouds and rain diminish rapidly outwards.

1. It is essentially radially symmetrical 2. It has six regions A. The eye is the centre of the storm which is characterised by more or less circular with comparatively clear skies, lowest pressure, the highest temperature and highest relative humidities B. The eye is surrounded by a wall of cumulonimbus known as eyeball. Strongest wind is found C. Spiral bands or Rainbands or Feeder bands contain many individual thunderstorms which produce heavy rainfall D. Annular zone is characterized by cloudiness and high temperatures and low humidities E. Outer Convective band F. Main cloudmass. Horizontal structure of tropical cyclone

Copyright © www.www.examrace.com ir has weight and therefore it exerts air pressure or atmospheric pressure. Pressure is felt maximum at the surface and it decreases with height. It is about 2.7 kg per sq. Cm i.e.. 1013.2 millibar, water vapour decreases the pressure, the movement of the earth also affects pressure. At the equator earth rotates with a speed of 1600 km/hour and completes a distance of 40, 000 km in 24 hour. Speed decreases as it goes up and down: At 45 degree latitude speed is 1100 km/hour and becomes 0 degree at the poles, therefore after 60 degree latitude air pressure is constant. Types Of Atmospheric Pressure Vertical distribution of pressure-the pressure is highest at surface and at every 300 m above the earth the pressure decreases by 34 millibar. At 5, 500 m. Pressure reduces to half and 1/4th at 11000m. Horizontal distribution of pressure-In January, sun is tilted towards south and therefore in the southern hemisphere there is low-pressure belt. Therefore in Eurasia and North America high pressure is developed with low temperature. In July, at Atlantic and Pacific ocean low pressure is developed especially Icelandic and Aleutianic. On the other hand, when the sun is tilted towards north low pressure is shifted to Asia and Africa. However in the lower Asia, Africa, South Pacific Ocean and South Atlantic Ocean high pressure is developed. 7 pressure belts Equatorial Low Pressure Belt North of Capricorn-High Pressure Belt South Of Cancer-High Pressure Belt 66 degree North Low Pressure Belt (Northern sub-Polar region) 6 degree South low Pressure Belt (Southern sub-Polar region) North Pole High Pressure Belt South Pole High Pressure Belt Relation between Pressure and Wind The temperature increases when wind expands and density decreases and when the temperature Jails, winds contracts and density increases. Therefore, where temperature is increased, density is lowered and any reduction in temperature produces high pressure. The earth rotates west to east and therefore, it produces centrifugal force and due to this force, there is change in the direction of the wind. There are three laws related to it Ferrel's Law: In the northern hemisphere, wind deflects towards right and in the southern hemisphere it is vice-versa. This means that in the north-em hemisphere, the wind deflects clockwise and in the southern anti-clockwise. This is called deflection of wind and because of high and low pressure the wind tends to be deflected instead of being straightened. Buisballot's Law: In north hemisphere at the right side of your backside, there would be high pressure and at the left, low pressure. In the south, it is vice-versa. This would help in determination of appropriate direction. Hadley's Law: In the northern hemisphere the wind from north to south deviates right and in the southern hemi-sphere, from south to north deviates to the left. The left from east to west does not deflect according to this law. Clouds: Clouds are defined as aggregates of innumerable tiny water droplets, ice-particles of mixture of both in the air, generally above the ground surface Acid Rain Acidity measured by pH. PH scale runs from 1 to 14 measures the balance of Hydrogen ions (H + ) & Hydroxide ions (OH) in a 14 point scale Positive and negative balance is PH7. If there are more hydrogen ions than hydroxide ion, the pH is less than the solution is termed acidic, the lower the pH, the higher the acid content. Rainfall-somewhat acidic pH: 5.5 to 60. Condensation The process of change of water vapour into liquid form is called condensation. Therefore, condensation is opposite to vaporization Surface Tension: Water has the highest surface tension of any liquid except mercury. Surface tension is the attraction of molecules of each other. Humidity Humidity: Water in gaseous form or water vapour in the atmosphere is referred to as humidity. It can also be defined as the number of molecules of water/unit volume. Vapour Pressure: Vapour pressure is simply the pressure, exerted by the water vapour content. Its value increases as the temperature increases. Specific Humidity: Specific humidity is the ratio of the weight of moist vapour (gms) to the weight of moist air. Represents the actual quantity of moisture in a definite air. It decreases fmm equator to poles. In Arctic it is 0.2 gm./kg. while in equatorial region, it is 18 gm./kg. It is used in Climatology Absolute Humidity: Absolute humidity of the air is the mass or weight of vapour per unit volume of air Relative Humidity: It is defined as the ratio of the amount of water vapour actually present in the air having definite volume and temperature (Absolute humidity) to the maximum amount the air can hold. Forms of Condensation Mist: Forms on wet surface, lakes or rivers where the hu-midity is high and condensation in evening was led to wisps of mist over the fields and water especially in sheltered spots. Rime: It is a deposit of white opaque ice crystals formed by the freezing of super cooled water droplets on the surface. Below 0 degree C. Smog: Smoke and Fog. Normal lapse rate: 6.5 degree C/thousand m. Or 3.5 degree F/thousand feet. Dew Point The temperature at which air becomes saturated is called Dew Point. The vapour is limited. If the volume of vapour increases but there may be a point at which the vapour cannot be incorporated, this is called saturation point. Dew points are the temperaing winter than summer. Regional Distribution According to the ancient Greek thinkers the globe is divided into three temperature zones on the basis of latitudes: Tropical Zone-Extends between the tropics of Cancer (23.50N) and Capricorn (23.50S). The sun is more or less vertical on the equator throughout the year. There is no winter around the equator because of high temperature prevailing throughout the year. Temperate Zone Extends between 23.5° and 66.5° latitudes in both the hemispheres. There is a marked seasonal contrast with the northward and southward migration of the overhead sun The range of temperature is exceptionally high.

Frigid Zone

* Extends between 66.5° latitudes and the poles in both the hemispheres. * More oblique sun's rays throughout the year resulting into exceptionally very low temperature characterize it. * The length of day and night is more than 24 hrs. Isanomalous Temperature * The difference ot observed temperature of a place and the mean temperature of the latitude passing through that places called thermal anomaly. * For example, if the average temperature of 30° N latitude is 20° C and the temperature of “S” place located on the latitude is 30° C, then the thermal anomaly is of 10° * If the observed temperature of a particular place is more than the mean temperature of the latitude of that place, * the thermal anomaly is called positive thermal anomaly, but if the observed temperature of a given place is less than that of the latitude of that place then it becomes negative thermal anomaly. * The equal thermal anomaly of several places is called isanomalous temperature and the lines drawn on the world map joining places of equal thermal anomaly are called isanomals.

Types of rainfall :

Conventional Rainfall * Occur due to thermal convention currents caused due to insolational heating of ground surface. * Prevalent especially in equatorial areas * Warm air rises up and expands, then reaches at a cooler layer and saturates and then condenses mainly in the form of cumulus or cumulonimbus clouds and normally precipitation takes place in the second half of the noon * Also rains in the tropical, subtropical and little in temperate regions * But there must be two pre conditions: * Abundant supply of moisture through evaporation to air so that relative humidity becomes high * Intense heating of ground surface through incoming shortwave electromagnetic solar radiations.

Features of Conventional rainfall

1. It occurs daily in the afternoon in the equatorial region. 2. It is for very short duration but occurs in the form of heavy showers. 3. They make Cumulonimbus clouds. 4. In hot deserts it is not regular, but is irregular and sudden.

Cyclonic or frontal rainfall

* Occur due to upward movement of air caused by convergence of extensive air masses * It happens due to the convergence of two different air masses with different temperature. The worm air rises over cold air and cyclonic rain occurs * Cold air pushes up warm air and the sky is clear again.

Orographic Rainfall

Orographic Rainfall occurs due to ascent of air forced by mountain barrier. The preconditions are:

* there should be mountain barrier across the wind direction, so that the moist air is forced on obstruction to move upward e. g. Aravali in Rajasthan is parallel to Arabian Sea and thus forms rain shadow area * there should be sufficient moisture in the air * the height of the mountain also affects rainfall if the height is more but more distance from sea, lesser rainfall; * if the height is less but nearer to sea, more rainfall.

Features of Orographic rainfall

* The windward slope. E. g. Mangalore is located in the western windward slope and receives 2, 000 mm of rainfall, whereas Bangalore is in rain shadow area and hardly receives 500 mm. Rainfall. Similarly Coast Ranges of North America receives 2, 000 mm. But eastern slope doesn't. * The maximum rainfall occurs near the mountain slope and decreases away from the foothills. E. g. In Shimla, 1520 mm. Nainital, 2, 000 mm. And Darjeeling receives 3150 mm. Rainfall because Darjeeling is nearest to Himalayan slopes. Patna 1000 mm. Allahabad 1050 and Delhi 650 mm. * If mountain is of moderate height, the maximum rainfall doesn't occur at the top rather it occurs on the other side. * Cumulus clouds while the leeward side by Stratus clouds characterizes the windward slope of mountain at the time of rainfall. * The amount of rainfall increases with increasing height along the windward slope of mountain up to a certain height but the amount of rain decreases with increasing height because of marked decrease in the moisture content of air. This situation is called ‘inversion of rainfall’ * This type of rainfall may occur in any season. Inversion Point: Maximum rainfall line is at 24, 000 feet or 7, 000 m. At the equator whereas in the Himalayas, it is 12, 000 feet or 3600 m. at Alps 21, 000 or 6, 300 m. And at Pyrenees mountain 18, 00012, 000 feet.

Distribution of Rainfall

1. Rainfall is related with air temperature and atmospheric humidity, while humidity is closely related with temperature through the process of evaporation. 2. The regions having high temperature and abundance of water receive higher amount of rainfall e. g. Equatorial regions. 3. SubTropical regions also have the same conditions but the western parts receive least rainfall due to anticyclonic conditions. 4. Mean annual rainfall for the whole globe is 970 mm. But is unevenly distributed. Some places receive less than 100 mm. Of rainfall e. g. Hot deserts like Kalahari, Thar etc. while some receive more than 12, 000 mm. Like Cherrapunji in India. 5. The equatorial regions receive rainfall throughout the year, while the other areas have seasonal rainfall. 6. The Mediterranean region receives most of the annual rainfall during winters.

Other Forms of Precipitation :

Ice:
If the temperature of the entire atmosphere is below 0° C, the condensation will lead to ice formation and snowfall.

Snowfall: The fall of larger snowflakes from the clouds on the ground surface is called snowfall. It occurs when the freezing level is less than 300m from the ground surface. These crystals reach the ground, without being melted in a solid, form of precipitation as snow

Sleet:
In UK it refers to a mixture of snow and rain but in American terminology it means falling of small pellets of transparent and translucent ice having a diameter of 5 mm. Or less.

Hail: It consists of large pellets or spheres of ice. In fact hail is a form of solid precipitation wherein small balls or pieces of ice, Known as. Hail or stones, having a diameter of 550 mm fall downward as hail storms! They are very destructive and dreaded form of solid precipitation because they destroy agricultural crops and claim human and animal life. After condensation, if the temperature is below 0 degree C, than the water drops would take the form of hails.

Drizzle:
The fall of numerous uniform minute droplets of water having diameter of less than 0.5 mm. Is called drizzle. They fall continuously from low stratus clouds but the total amount of water received on the ground surface is significantly.

Concepts of Rainfall:

Origin of Rainfall :

* The presence of warm, moist and unstable air and sufficient number of hygroscopic nuclei are prerequisite condition for rainfall. * The warm and moist air after being lifted upward becomes saturated and clouds are formed after condensation of water vapour around hygroscopic nuclei (salt and dust particles) but still there may not be rainfall unless the air is supersaturated. * The process of condensation begins only when the relative humidity of ascending air becomes 100% and the air is further cooled through dry adiabatic lapse but first condensation occurs around larger hygroscopic nuclei only. Such droplets are called cloud droplets.

Condensation

For condensation there are following pre conditions:

* The air becomes warm and goes vertical and then spreads * To come into contact of warm air with high mountains and then to climb over then and to come close to the top layer of ice and become cool * To become cool by approaching the colder latitudes * To come in contact with colder air or currents.

Theories of Rainfall Cloud Instability

Theory of Bergeron Findeisen:

* This theory was postulated in 1933; also called Icecrystal theory. This theory is based on the fact that relative humidity of air is greater with respect to an icesurface than with respect to water surface. * Air temperature ranges between 5° C to 25° C, then water droplets become supersaturated. * The aggregation of ice crystals is more prevalent when air temperature ranges between 0 degree to 50 degree C * When the ice crystals fall and pass through layer of air with temperature more than 0 degree C, they change into raindrops.

Collision Theory

* The Bergeron process could not explain the mechanism of rainfall in tropical areas. * The Collision Theory involving collision, coalescence and sweeping for the formation and growth of rain drops was postulated by many meteorologists. * According to this theory, the collision may cause splitting and scattering of cloud droplets. Longmuir modified this theory saying that the larger drops fall with greater velocity than smaller drops hence absorbing them.