Air Masses Defined

An air mass by definition is a large dome of air which has similar horizontal temperature and moisture characteristics. As well, air masses often have a rather homogeneous temperature lapse rate above the influence of the surface layer. At any given time, an estimated fifty distinct air masses are scattered across the face of the planet. Some are newly born entities and strongly reflect their birthing ground. Others are old and travel-scarred with only the smallest commonality with their place of origin remaining.

Air Mass Classification

approximately fifty distinct air masses can be identified globally in the lower atmosphere. Most cover thousands of square kilometres of surface and extend several kilometres vertically. Each one bears the mark of the region in which it was formed. Some of the fifty are young and fresh. Others are old and greatly transformed. Some are moving across the planet at speeds covering several hundred kilometres each day, others are nearly stationary.

Air masses acquire their characteristic temperature and moisture (or absolute humidity) signature from the source regions over which they are born. The ideal source region is one with light winds, particularly in the upper atmosphere so that the air mass remains in place long enough to acquire the temperature and moisture properties of the underlying surface throughout the air mass. Therefore, middle latitude regions where the weather systems move quickly across their surface, driven by fast-moving upper level air currents such as the jet stream are not good air mass breeding grounds

At varied intervals, portions of these semi-permanent high pressure cells break away to form vagabond air masses that hitch a ride on strong upper air currents and travel the globe.
Areas dominated by extensive areas of high pressure and light winds are the ideal breeding grounds
for air masses. There are several such regions of extensive, semi-permanent high pressure around the globe, in particular, two latitude belts in each hemisphere: one in the polar regions, the other in the subtropics.









High polar latitudes and the subtropics around 30 degrees latitude are both good source regions, whose relative strength waxes and wanes with the solar seasons. Open ocean expanses, large deserts and extensive continental plains at high or low latitudes are the ideal birthing grounds within these belts. Mountainous areas are too variable in their properties, and mid-latitude continental plains are not conducive to air masses staying in place for long because of the strength of the prevailing westerly global winds at these latitudes. Mid-latitude oceans can be source regions under certain conditions because their surfaces have very uniform characteristics.

From the characteristic properties picked up in their breeding ground, air masses are designated as hot or cold, wet or dry. The terms are to some degree relative. A cold air mass in summer may be as warm as a warm air mass in winter. Each air mass has a characteristic temperature and moisture content and thus we can distinguish four combinations: hot and dry; hot and wet; cold and wet; and cold and dry. Bergeron actually gave us two additional temperature categories by defining "very hot" and "frigid" air masses for those forming over the equator and polar regions, respectively


The first dimension of the Bergeron classification system is the latitude zone of air source region which governs the air mass's temperature characteristics. There are four such zones in the system

Equatorial (E) Arctic or Antarctic (A or AA)
Tropical (T) Polar (P)


The letter in parentheses is that used to label air masses on weather maps. Next, are the two underlying surface characteristics of the source region that affect the resulting air masses:

Maritime or Oceanic Surfaces (m)
which create relatively humid air masses

Continental or Land Surfaces (c)
which create relatively dry air masses



The combination of the above gives us eight air mass types, but because Arctic (Antarctic) and Equatorial air masses only have one moisture character, we are left with the six basic air mass types given below:




Air Mass Type, Temperature Characteristic, Moisture Characteristic
Arctic or Antarctic
(A or AA) Extremely cold, formed over poles. Very dry due to extreme cold.
Polar Continental
(cP) Very cold, having developed over sub-polar regions. Very dry, due to the cold and having developed over land.
Polar Maritime
(mP) Very cool because of the high latitude but not cold, due to moderating influence of the sea and the warm ocean currents at these latitudes. Moderately moist because of the cool temperature, but not as dry as polar continental air because of evaporation from the water surface.
Tropical Continental
(cT) Very warm because of the lower sub-tropical latitude of formation. Dry because it formed over land
Tropical Maritime
(mT) Very warm because of the sub-tropical latitudes at which it forms. Very humid because of the warm tropical waters below.
Equatorial
(E) Hot. Extremely humid. Continental is not differentiated from maritime because much of equatorial land is covered with humid tropical rainforests. On some weather maps, the lowercase letter "k" or "w" may be attached to the two-letter abbreviation describing an air mass. The "k" indicates that the air moving across a region is colder than the land surface temperature. while "w" indicates that the air is warmer than the land surface temperature. Thus, cold continental polar air flowing over warmer land surfaces would be designated as cPk.


Air Masses Classification

Arctic (A):

Extremely cold temperatures and very little moisture typify Arctic air masses. They usually originate north of the Arctic Circle, where winter days of 24-hour darkness allow the air to chill to extremely low temperatures. Such air masses break southward across Canada and the USA during winter, but very are rarely seen at lower latitudes during the summer because the 24-hour sun warms the Arctic region considerably and the polar front and accompanying jet stream generally remains at higher latitudes.



Continental polar (cP): Cold and dry, Continental polar masses are not as cold as Arctic air masses. These usually form further south in the subpolar Canadian North and Alaska and often dominate the weather picture across the continent during winter. Continental polar masses do form during the summer, but mostly influence only Canada and the northern USA. These air masses are usually responsible for bringing clear and pleasant weather during the summer

Maritime polar (mP): Cool and moist conditions characterize Maritime polar air masses. They usually bring cloudy, damp weather. Maritime polar air masses form over the northern Pacific and the northern Atlantic Oceans. These generally influence the Pacific Northwest and the Northeast, respectively. Maritime polar air masses can form any time of the year and are usually not as cold as continental polar air masses in winter because of the moderating influence of the sea surface beneath them.





Maritime tropical (mT):

Warm temperatures with copious moisture typify Maritime tropical air masses. They are most common across the eastern US and southeastern Canada originating over the warm waters of the southern Atlantic Ocean, Caribbean Sea and the Gulf of Mexico. These air masses can form year round, but they are most prevalent during summer. Maritime tropical air masses are responsible for the hot, humid days of summer across much of the eastern half of the continent. Such air masses are often called the Bermuda High because of their birthplace within the subtropical zone around and east of Bermuda.

Continental Tropical (cT):

Hot and very dry, Continental tropical air masses usually form over the Desert Southwest and northern Mexico during summer, often keeping the region scorching above 38o Celsius (100o Fahrenheit) during summer. They can bring record heat to the US Plains and the Mississippi Valley during summer, but they usually do not make it to the eastern and southeast US or into Canada as cT air masses. As they move eastward, moisture evaporates into the air, transforming the air mass to become more like a maritime tropical air mass. Continental tropical air masses very rarely form during winter.


Equatorial (E) air masses, the remaining category, rarely visit the contiguous United States and almost never reach Canada, but these air masses are an important weather factors for the southern nations of North America: southern Mexico, Central America and many of the Atlantic island nations.

WINDS AND ITS TYPES

Introduction

The horizontal movement of air along the earth’s surface is called a Wind. The vertical movement of the air is known as an air current. Winds and air current together comprise a system of circulation in the atmosphere. The movement and the speed of wind are affected by three main factors:

*Pressure gradient
*Rotation of the Earth
*Friction of the Earth.


Pressure Gradient

We know that a wind always blows from a region of high-pressure to a region of low-pressure. Steeper the pressure gradient, higher is the speed or force of the blowing wind. Slower the pressure gradient, slower is the force of the blowing wind.


Rotation of the Earth

If the Earth did not rotate upon its axis, winds would follow the direction of the pressure gradient. But the rotation produces another force other than the pressure force. It is called the ‘Coriolis force’. This tends to turn the flow of air by changing its direction from its original straight path. The wind starts deflecting more and more to its right from its original path in the northern hemisphere. In the southern hemisphere it starts deflecting more and more to its left from its original path. Thus a wind blowing from north becomes north-easterly in the northern hemisphere. A wind blowing from south becomes south-easterly in the southern hemisphere. The effect of the Coriolis force on wind is stated in Ferrel’s law as follows: “Any object or fluid, moving horizontally in the Northern Hemisphere, tends to be deflected to the right of its path of motion regardless of the compass direction of the path. In the Southern’ Hemisphere, a similar deflection is towards the left of the path of motion.” The deflection is the least at the equator and the greatest at the poles





Global circulation of winds





Friction of the Earth

The friction along the Earth’s surface decreases both the angular deflection and velocity of the wind. It is very little over vast free surface of oceans and is considerable over the mountains and the heavily forested areas.

Types of Winds

On the earth’s surface, certain winds blow constantly in a particular direction throughout the year. These are known as the ‘Prevailing Winds’. They are also called the Permanent or the Planetary Winds. Certain winds blow in one direction in one season and in the opposite direction in another. They are known as Periodic Winds. Then, there are Local Winds in different parts of the world.


Planetary or Permanent Winds


The planetary wind system of the world accompanies the presence of the High and Low-Pressure Belts. We know that winds tend to blow from the high-pressure centres to the low-pressure centres. The effect of the earth’s rotation (Coriolis Force) tends to deflect the direction of these winds. The deflection in the direction of these winds take place according to Ferrel’s Law. Two sets of surface winds, the Trades and the Westerlies are the main planetary winds of the world.



Trade Winds


North and South of the Equatorial Belt of Calms, are the Trade Winds covering roughly the zone lying between 5° and 30° North and South. In other words they cover almost the entire area between 30°N and 30°S latitudes on both sides of the equator. The Trade Winds are a result of a pressure gradient from the Sub-Tropical Belt of High Pressure to the Equatorial Belt of Low Pressure. In the Northern Hemisphere, the wind moving equatorward, is deflected by the earth’s rotation to flow south-westward. Thus, the prevailing wind there is from the North-East, and it has been named as the ‘North-East Trades’. In the Southern Hemisphere, deflection of the wind is towards the left, this causes the ‘South-East Trades’.

Trade Winds are noted for their steadiness and persistent direction. But the system of Doldrums and trades shifts seasonally north and south, through several degrees of latitudes, as do the pressure belts that causes them. The trades are best developed over the Pacific and the Atlantic Oceans, but are upset in the Indian Ocean because of nearness of the great Asian landmass. They are named after the Latin word ‘trado’ which means blowing steadily in a constant direction; hence, the name Trade Winds. As these Trade Winds blow from the warmer, sub-tropical latitudes to the hot tropics, they have a great capacity for holding water-vapour or moisture.

When they cross the open oceans, they pick up a lot of moisture. They bring heavy rainfall to the eastern coasts of continents lying within the tropics because they blow on-shore. On the western coasts of continents, these Trade Winds do not bring any rainfall. It is because here there are ‘off-shore’ winds or winds blowing just parallel to the shores, as they blow off-shore. As such, the western areas within the tropics suffer from aridity. The great deserts of the Sahara, Kalahari, Atacama and the Great Australian Deserts all lie on the western mar¬gins of the continents, lying within the tropical latitudes.


Westerlies


The Westerlies or the Prevailing Westerly Winds blow between 35° and 60° North and South latitudes from the Sub-Tropical High-Pressure Belts towards the Sub-Polar Low-Pressure Belts. We know that the high-pressure belt is a zone of divergence for these outgoing winds. In the Northern Hemisphere, the Westerlies generally blow from the south-west to the north-east, and in the Southern Hemisphere from the north-west to the south-east.

These are on-shore winds on the west coasts and off-shore winds on their east coasts. The on-shore winds bring rainfall while the off-shore winds are lacking in it. These winds are not as constant in strength and direction as the Trade Winds. They are rather stormy and variable though the main direction remains from west to east. But as their general direction is from the west, they are called the “Westerlies”. They are also known as “Anti-[B]Trade Winds”, [/B]because their movement is in the opposite direction from that of the Trade Winds.

In the Northern Hemisphere, land-masses cause considerable disruption in the westerly winds. But in the Southern Hemisphere, between 40°S and 60°S, the westerlies gain great strength and persistence because of the vast expanse of oceans in their belt. This made the mariners of old call them the “Roaring Forties”, the “Furious Fifties” and the “Screaming Sixties”.

In olden days, sailing vessels had to face great danger while sailing in the opposite direction in the face of the prevailing westerly winds. It is to be rioted that the westerlies bring warmth and rainfall throughout the year to all the western coasts of the Temperate Zone. But the areas, which lies in the Mediterranean type of region, get rainfall only in winter. At that time, in December, the Mediterranean parts of Europe and California (U.S.A.) come under the influence of the westerlies and receive rainfall. In the Southern Hemisphere, in this month, the Mediterranean regions (Central Chile, Southern Africa, S.W. Australian coast) do not receive any rainfall, as they shift away from the influence of the westerlies.
In June, the Mediterranean parts of the southern continents come under the influence of the westerlies and receive rainfall. At that time, the Mediterranean areas of the Northern Hemisphere do not receive any rainfall from the westerlies, because they shift away from their influence.





TRADE WINDS









Eastlies and Westerlies on earth.





Polar Winds


The winds blowing in the Arctic and the Antarctic latitudes are known as the Polar Winds. They have been termed the ‘Polar Easterlies’, as they blow from the Polar High Pressure Centres towards the Sub-Polar Low-Pressure Belts. In the Northern Hemisphere, they blow in general from the nort h-east, and are called the North-East Polar Winds; and in the Southern Hemisphere, they blow from the south-east and are called the South-East Polar Winds. As these winds blow from the ice-capped landmass, they are extremely cold. They are more regular in the Southern Hemisphere than in the Northern Hemisphere.


Periodic Winds

Land and sea breezes and monsoon winds are winds of a periodic type. Both are caused by the unequal heating of land and sea. Land and sea breezes occur daily, whereas the occurrence of monsoon winds is seasonal.

See Breeze

During the day, the greater heating of the land causes the air to ascend, causing a low pressure over land and the cool heavy air from the sea moves in to take its place. The strength of the sea breeze depends on the topography of the coast and the regions.

Land Breeze

During the night the land cools quickly so that it is colder than the sea. A low pressure area is caused over the sea and the cooler heavier air from the land begins to flow towards the sea. The general effect of the contrast in heating of land and sea is to produce cooler winters and warmer summers in the centres of continents than along coasts.


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Monsoons

Monsoon winds are seasonal winds. For six months they blow from land to sea, and for the other six months from sea to land. Thus, they may be regarded as land and sea breezes on a large-scale in which the period is a year instead of a day. The word ‘monsoon’ has been derived from the Arabic word ‘Mousim’, which means ‘season’. These seasonal winds greatly disturb the belted system of the earth’s permanent winds.




Local Winds

Local winds are the result of a variety of causes. The mountain and valley winds follow a daily alteration of direction in a manner like the land and sea breezes.
During the day, when slopes are intensely heated by the Sun, the air moves from the valleys upward over rising mountain slopes towards the summits. This is known as a Valley Wind. It decreases the cold of the areas on higher summits.
At night, when the same slopes have been cooled by radiation from ground to the air, the air moves valleywards down the ground slopes and reduces the temperature of the valleys. These winds, thus, respond to local pressure gradients, caused by the heating or cooling of the ground.




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Type of local winds flow, under the influence of gravity, from higher to lower regions. In winter, a cold and dense air may accumulate over highland and flow out upon adjacent lowlands as a strong cold wind.

The Bora of the northern France are well known examples of this type.

Foehn and Chinook are still other types of local winds, passing over a mountain range, are forced to descend on the leeward side with the result that the air is heated and dried.

regards sabahat

__________________
GEO to aise k sab tumhara ha,MARO to aisa jaise tmhara kuch bhi nhi.

Maza deti han zindagi ki thokerin unko,jinhen NAAM-E-KHUDA le kar sanbhal janey ki adat ho.

you are welcome . dont worry first of all buy books for geography and then ask question about topics.

for books

physical geo.by das gupta.

physical geo. by deblij.

physical geo. by starhler ( recommended by most of forum members)

physical geo. of graduation level. to clear your concepts.


human geo. by deblij is the best.

economic geo. by fazal karim khan (new edition).

geo. of pakistan by fazal karim khan.

__________________
GEO to aise k sab tumhara ha,MARO to aisa jaise tmhara kuch bhi nhi.

Maza deti han zindagi ki thokerin unko,jinhen NAAM-E-KHUDA le kar sanbhal janey ki adat ho.

SORRY for being late friends because i was well now i am a bit better and pasting the next topic. plz pray for me.


WEATHER FRONTS AND ITS TYPES


Weather Fronts

__________________
GEO to aise k sab tumhara ha,MARO to aisa jaise tmhara kuch bhi nhi.

Maza deti han zindagi ki thokerin unko,jinhen NAAM-E-KHUDA le kar sanbhal janey ki adat ho.

CLOUDS AND ITS TYPES



The cloud classification system






ATMOSPHERIC DISTURBANCES



1:-THUNDERSTORM


Thunderstorms





2:-TORNADOES


Tornadoes





3:-HURRICANES


Hurricanes (also known as Tropical Cyclones)

__________________
GEO to aise k sab tumhara ha,MARO to aisa jaise tmhara kuch bhi nhi.

Maza deti han zindagi ki thokerin unko,jinhen NAAM-E-KHUDA le kar sanbhal janey ki adat ho.

do u have these nots in the doc format or the pdf format ?? if yes ....then can u plz mail these ?? my email is farooq.zia@nu.edu.pk

__________________
*~The only good is knowledge and the only evil is ignorance~*

i am afraid zia i dont have them in pdf form.

__________________
GEO to aise k sab tumhara ha,MARO to aisa jaise tmhara kuch bhi nhi.

Maza deti han zindagi ki thokerin unko,jinhen NAAM-E-KHUDA le kar sanbhal janey ki adat ho.

Sayeeda one of the member has said to convert the files in pdf ..

I think it would be very helpful..

can you please try to do or any member can he please conerert the file posted by Sayeeda in pdf

can anyone refer me a geography teacher!?

__________________
~Yaseen~

PRECIPITATION

The word precipitation in chemistry refers to material falling out of suspension. The same definition can be applied when studying weather.

Precipitation from a meteorological stand point is water in some form, falling out of the air, and settling on the surface of the earth. This allows us to distinguish between forms of condensation in the atmosphere and condensation that occurs at the surface. Dew is condensation at the surface and thus is not a form of precipitation. Rain, snow, hail, sleet, freezing rain are all forms of precipitation


FORMATION OF PRECIPITATION

Formation of precipitaion is a complex process.clouds composed of only water droplets never cause rain as no single water droplet is likely to become large enough to form a rain droplet has a radius of only .01mm and an average rain drop has a radius of about 1mm. in middle and high latitude areas precipitaion occur from the clouds having a mixture of water droplets and ice crystals. tiny clouds droplets doesnot freeze at 0 dgree c,these having temperature below freezing point are known as SUPER COOLED OR UNDER COOLED CLOUD DROPLETS .in some coluds liquid droplets may be present at temp. as low as -40 degree c.



TYPES OF PRECIPITAION


ICE PELLETS OR SLEET

Ice pellets (also referred to as sleet are a form of precipitation consisting of small, translucent balls of ice. Ice pellets usually are smaller than hailstones.They often bounce when they hit the ground, and generally do not freeze into a solid mass unless mixed with freezing rain. The METAR code for ice pellets is PL.


FORMATION


Ice pellets form when a layer of above-freezing air is located between 1500 meters (approximately 5,000 feet) and 3000 meters (approximately 10,000 feet) above the ground, with sub-freezing air both above and below it. This causes the partial or complete melting of any snowflakes falling through the warm layer. As they fall back into the sub-freezing layer closer to the surface, they re-freeze into ice pellets. However, if the sub-freezing layer beneath the warm layer is too small, the precipitation will not have time to re-freeze, and freezing rain will be the result at the surface.


EFFECTS

In most parts of the world, ice pellets only occur for brief periods and never accumulate a significant amount. However, across the eastern United States and southeastern Canada, warm air flowing north from the Gulf of Mexico ahead of a strong synoptic scale storm system can overrun cold, dense air at the surface for many hundreds of miles for an extended period of time. In these areas, ice pellet accumulations of 2-5 cm (.8-2.0 in) are not unheard of. The effects of a significant accumulation of ice pellets are not unlike an accumulation of snow. One significant difference is that for the same volume of snow, an equal volume of ice pellets is significantly heavier and thus more difficult to clear away. Additionally, a volume of ice pellets will take significantly longer to melt compared to an equal volume of fresh snowfall.



Freezing Rain OR Glaze


Ice storms can be the most devastating of winter weather phenomena and are often the cause of automobile accidents, power outages and personal injury.

Ice storms result from the accumulation of freezing rain, which is rain that becomes supercooled and freezes upon impact with cold surfaces. Freezing rain is most commonly found in a narrow band on the cold side of a warm front, where surface temperatures are at or just below freezing.

Formation
.
Freezing rain develops as falling snow encounters a layer of warm air deep enough for the snow to completely melt and become rain. As the rain continues to fall, it passes through a thin layer of cold air just above the surface and cools to a temperature below freezing. However, the drops themselves do not freeze, a phenomena called supercooling (or forming "supercooled drops"). When the supercooled drops strike the frozen ground (power lines, or tree branches), they instantly freeze, forming a thin film of ice, hence freezing rain.

HAIL


Hail is a form of solid precipitation which consists of balls or irregular lumps of ice, that are individually called hail stones. Hail stones on Earth consist mostly of water ice and measure between 5 millimetres (0.20 in) and 200 millimetres (7.9 in) in diameter, with the larger stones coming from severe thunderstorms. The METAR reporting code for hail 5 millimetres (0.20 in) or greater in diameter is GR, while smaller hailstones and graupel are coded GS.

Hail is possible within most thunderstorms as it is produced by cumulonimbi (thunderclouds),[1] and within 2 nautical miles (3.7 km) of the parent storm. Hail formation requires environments of strong, upward motion of air with the parent thunderstorm (similar to tornadoes) and lowered heights of the freezing level. Hail is most frequently formed in the interior of continents within the mid-latitudes of Earth, with hail generally confined to higher elevations within the tropics.

Hail forms in strong thunderstorm clouds, particularly those with intense updrafts, high liquid water content, great vertical extent, large water droplets, and where a good portion of the cloud layer is below freezing 0 °C (32 °F). These type of strong updrafts can also indicate the presence of a tornado. The growth rate is maximized where air is near a temperature of −13 °C (9 °F).




DAMAGES


Hail can cause serious damage, notably to automobiles, aircraft, skylights, glass-roofed structures, livestock, and most commonly, farmers' crops.Hail damage to roofs often goes unnoticed until further structural damage is seen, such as leaks or cracks. It is hardest to recognize hail damage on shingled roofs and flat roofs, but all roofs have their own hail damage detection problems. Metal roofs are fairly resistant to hail damage, but may accumulate cosmetic damage in the form of dents and damaged coatings.
Hail is one of the most significant thunderstorm hazards to aircraft. When hail stones exceed 0.5 inches (13 mm) in diameter, planes can be seriously damaged within seconds. The hailstones accumulating on the ground can also be hazardous to landing aircraft. Hail is also a common nuisance to drivers of automobiles, severely denting the vehicle and cracking or even shattering windshields and windows. Wheat, corn, soybeans, and tobacco are the most sensitive crops to hail damage. Hail is one of Canada's most expensive hazards.

Rarely, massive hailstones have been known to cause concussions or fatal head trauma. Hailstorms have been the cause of costly and deadly events throughout history. One of the earliest recorded incidents occurred around the 9th century in Roopkund, Uttarakhand, India. The largest hailstone in terms of diameter and weight ever recorded in the United States fell on July 23, 2010 in Vivian, South Dakota; it measured 8 inches (20 cm) in diameter and 18.62 inches (47.3 cm) in circumference, weighing in at 1.93 pounds (0.88 kg). This broke the previous record for diameter set by a hailstone 7 inches diameter and 18.75 inches circumference which fell in Aurora, Nebraska in the United States on June 22, 2003, as well as the record for weight, set by a hailstone of 1.67 pounds (0.76 kg) that fell in Coffeyville, Kansas in 1970.

PREVENTION

During the Middle Ages, people in Europe used to ring church bells and fire cannons to try to prevent hail. After World War II, cloud seeding was done to eliminate the hail threat,particularly across Russia. Russia claimed a 50 to 80 percent reduction in crop damage from hail storms by deploying silver iodide in clouds using rockets and artillery shells. Their results have not been able to be verified. Hail suppression programs have been undertaken by 15 countries between 1965 and 2005. To this day, no hail prevention method has been proven to work.



last is rain as you all know what is rain so i am not writing about rain do it yourself.

refards sabahat

__________________
GEO to aise k sab tumhara ha,MARO to aisa jaise tmhara kuch bhi nhi.

Maza deti han zindagi ki thokerin unko,jinhen NAAM-E-KHUDA le kar sanbhal janey ki adat ho.