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Climatology - Part 2
7.0 LAND AND SEA BREEZES AND MONSOONS
Land and sea breezes are, in fact, monsoons on a smaller scale. Both are basically caused by differential heating of land and sea, the former in a diurnal rhythm and the latter in a seasonal rhythm.
During the day, the land gets heated up much faster than the sea. Warm air rises forming a region of local low pressure. The sea remains comparatively cool with a higher pressure so a sea breeze blows in from sea to land. Its speed or strength is between 5-20 m.p.h and it is generally stronger in tropical than temperate regions. Its influence does not normally exceed 15miles from the coast. It is most deeply felt when one stands facing the sea in a coastal resort.
At night the reverse takes place. As the land cools down much faster than the sea, the cold and heavy air produces a region of local high pressure. The sea conserves its heat and remains quite warm. Its pressure is comparatively low. A land breeze thus blows out from land to sea. Fishermen in the tropics often take advantage of the out-going land breeze and sail out with it. They return the next morning with the in-coming sea breeze, complete with their catch.
In the same way, monsoons are caused. Rapid heating in the hot summer over most parts of India for example induces heated air to rise. The South-West Monsoon from the surrounding ocean is attracted by the low pressure over the land and blows in, bringing torrential rain to the sub-continent.
Similarly, in winter when the land is cold, the surrounding seas remain comparatively warm. High pressure is created over Indo-Pakistan and the North-East Monsoon blows out from the continent into the Indian Ocean and the Bay of Bengal.
7.1 Fohn Wind or Chinook Wind
Both the Fohn and Chinook winds are dry winds experienced on the leeward side of mountains when descending air becomes compressed with increased pressure. The Fohn win is experienced in the valleys of the northern Alps, particularly in Switzerland in spring. Chinook winds are experienced on the eastern slopes on the eastern slopes of the Rockies in U.S.A. and Canada in winter.
Air ascending the southern slopes of the Alps expands and cools. Condensation takes place when the air is saturated. Rain and even snow fall on the higher slopes.
In descending the northern slope, the wind experiences and increase in pressure and temperature. The air is compressed and warmed. Most of its moisture is lost and the wind reaches the valley bottom as a dry, hot wind - the Fohn. It may raise the temperature by 15° to 30°F., within an hour! It melts snow and causes avalanches. In North America it is called Chinook, meaning 'the snow-eater'.
One of the advantages of these winds is that it hastens the growth of crops and fruits and thaws the snow-covered pastures. In the Rockies, the Chinook has been known to raise temperature 35°F. within 15 minutes! The occurrence of frequent Chinooks means winter is mild.
8.0 CYCLONIC ACTIVITY
Tropical cyclones, typhoons, hurricanes and tornadoes are different kinds of tropical cyclones. They are well developed low pressure systems into which violent winds blow. Typhoons occur in the China Sea; tropical cyclones in the Indian Ocean; hurricanes in the West Indian islands in the Caribbean; tornadoes in the Guinea lands of West Africa, and the southern U.S.A. in which the local name of Whirl-wind is often applied and willy-willies occur in north-western Australia.
8.1 Typhoons
Typhoons occur mainly in regions between 6° and 20° north and south of the equator and are most frequent from July to October. In extent, they are smaller than temperate cyclones and have a diameter of only 50 to 200 miles, but they have a much steeper pressure gradient. Violent winds with a velocity of over 100 m.p.h are common. The sky is overcast and the torrential downpour is accompanied by thunder and lightning. In the wake of the typhoon, damage is widespread, e.g. in 1922, a typhoon that hurled huge waves on to the Swatow coast drowned 50,000 people.
8.2 Tornadoes
Tornadoes are small but very violent tropical and sub-tropical cyclones in which the air is spiraling at a tremendous speed of as much as 500 m.p.h! A tornado appears as a dark funnel cloud 250 to 1,400 feet in diameter. As a tornado passes through a region, it writhes and twists, causing complete devastation within the limits of its passage. There is such a great difference in pressure that houses virtually explode. Tornadoes are most frequent in spring but not common in many countries and their destructive effects are confined to a small area. Tornadoes are most typical of the U.S.A and occur mainly in the Mississippi basin.
8.3 Cyclones
Cyclones better known as depressions and are confined to temperate latitudes. The lowest pressure is in the centre and the isobars, as shown in climatic charts, are close together. Depressions vary from 150 to 2,000 miles in extent.
They remain quite stationary or move several hundred miles in a day. The approach of a cyclone is characterized by a fall in barometric reading, dull sky, Oppressive air and strong winds. Rain or snow falls and the weather is generally bad. Winds blow inwards into regions of low pressure in the centre, circulating in anticlockwise direction in the northern hemisphere and clockwise in the southern hemisphere . Precipitation resulting from cyclonic activities is due to the convergence of warm tropical air and cold polar air. Fronts are developed and condensation takes place, forming either rain , snow or sleet.
Other tropical cyclones have similar characteristics and differ, perhaps, only in intensity, duration and locality. Hurricanes have calm, rainless centres where the pressure is lowest (about 965 mb.) but around this 'eye' the wind strength exceeds force 12 of the Beaufort scale (75 m.p.h). Dense dark clouds gather and violent stormy weather lasts for several hours. A terrible hurricane struck Barbados in the West Indies in 1780, which nearly destroyed the whole island, tearing down buildings and uprooting trees. About 6,000 inhabitants were reported dead.
8.4 Anticyclones
Anticyclones are the opposite of cyclones, with high pressure in the centre and the isobars far apart. The pressure gradient is gentle and winds are light. Anticyclones normally herald fine weather. Skies are clear, the air is calm and temperatures are high in summer but cold in winter. In winter intense cooling of the lower atmosphere may result in thick fogs. Anticyclonic conditions may last for days or weeks and then fade out quietly. Winds in anticyclones blow outwards and are also subject to deflection, but they blow clockwise in the northern hemisphere and anticlockwise in the southern hemisphere.
9.0 CLIMATE CLASSIFICATION
9.1 Early attempts at global climatic classification
Very early attempts by the ancient Greeks at classifying climate were logic-based, and resulted in Paramenides' identification of three principal climate regions - the Frigid Zone, the Temperate Zone, and the Torrid Zone Other climatic classification schemes followed, including one by Hipparchus who updated the Paramenides classification by including information on the calculated day length for particular locations. Logic-based climatic classification systems ruled until the development and proliferation of weather recording instrumentation.
9.2 The classical age of climatic classifications
The Modified Köppen Climatic Classification System utilizes monthly temperature and precipitation data in making calculations upon which the classification scheme is based. Köppen identified five main climatic groups: A (tropical), B (arid), C (mesothermal or mid-latitude mild), D (microthermal or mid-latitude cold), and E (polar). In general, the A, C, and D climates support the growth of trees, whereas the B and E climates do not, being either too dry or too cold, respectively.
In the case of A, C, and D climates, the second-order subdivision refers to the precipitation seasonality (with "f" representing climates that are wet year-round, "s" indicating summer dry climates, "w" representing winter dry climates, and "m" representing tropical monsoon conditions). For B climates, the second-order subdivision is "S" if the dry climates are only semi-arid, and "W" if the dry climates are true deserts. In the case of "E" climates, the second-order subdivision is "T" for the milder Tundra sub-type of polar climate, while "F" (frozen) is used to represent the Ice Cap subtype
For the mesothermal and microthermal climates, third-order subdivisions identify the characteristics of summer temperatures, with "a" representing hot summers, "b" used for warm summers, "c" indicating mild summers, and the rare "d" indicating cool summers. Arid climates have a third-order subdivision of "h" and "k" which are used to denote "hot" and "cold" arid or semi-arid regions, respectively.
The Thornthwaite Climatic Classification System is built on the physical interactions between local moisture and temperature rather than only the precipitation and temperature data. It represents a more sophisticated and precise scheme of classification based on local surface water balances. Thornthwaite devised a number of specific indices to quantify necessary climatic components, including the moisture index (MI) and the potential evapotranspiration (PE) rate for a location. Thornthwaite also derived a Thermal Efficiency Index (T/ET) of the ratio of temperature (T) to a calculated evapotranspiration (ET) value, and a Dryness Index (DI) and Humidity Index (HI) to identify the times of the year with water deficit or surplus
9.3 Other global classification systems
The Holdridge Life Zones Climatic Classification System was intended to be for global application, but it became most widely used in tropical areas, where it has proved useful in ecological and alpine applications. The Budyko Climatic Classification System uses an energy budget approach to classifying climates.
9.4 Genetic classifications
The strategy is to classify climates solely on the basis of the major forcing mechanisms that make climate the way that it is. The Bergeron Climatic Classification System, devised by Swedish meteorologist Tor Bergeron in 1928, is one early-modern genetic classification system. This system categorizes the climate at a location based on the manual and somewhat subjective determination of frequency with which it is dominated by certain types of weather
9.5 Air masses and fronts
The Bjerknes model classified areas of a surface cyclone and associated pressure features into specific sectors with each section exhibiting particular weather characteristics. An air mass is a body of air that is relatively uniform in its characteristics for distances of hundreds to thousands of kilometers, with its characteristics resulting largely from the characteristics of the place where the air mass forms - the source region.
Air mass types include: the intensely-cold and dry Arctic (A) type; the slightly warmer and more humid continental polar (cP); cool, humid maritime polar (mP); warm, humid maritime tropical (mT), hot, dry continental tropical (cT), and the "E" air masses of equatorial origin - a more extreme version of the mT type. A cold front represents a situation in which a colder air mass is pushing a warmer air mass back toward its source region. A warm front is a region where a warmer air mass is displacing a colder one poleward.
In a stationary front, the cold and warm air mass may temporarily be at a stalemate, with neither air mass pushing the other backward. A cold front and warm front generally emanate from a central region of low pressure in a midlatitude wave cyclone. After the midlatitude wave cyclone travels eastward for a few days in the mid-latitude westerlies, the cold front will actually catch up with the warm front near the cyclonic center, where the two fronts are closest to each other, resulting in an occluded front
9.6 Local and Regional classifications
The Lamb Weather Types and Muller Weather Types are based on the mid-latitude wave cyclone model. These are manual systems - meaning that the researcher subjectively categorizes the types based on his/her interpretation of the weather map on a given day.
Quantitative Analysis to Derive Climatic Types: Eigenvector Analysis is helpful for identifying relationships between two different sets of variables (such as spatial data points, observations of an atmospheric variable through time, or atmospheric variables) to be analyzed simultaneously. In synoptic typing, eigenvector analysis may be used whereby the atmospheric variables may collectively represent the properties that dominate, and locations where similar characteristics of the atmosphere tend to occur simultaneously would be categorized as part of the same type for that entity of time.
Cluster Analysis: The actual classification process in any eigenvector analysis (including PCA) comes when the matrix of loadings or scores (which usually represents the spatial and temporal variability (respectively) in the data set) is subjected to a cluster analysis. Several types of cluster analysis can be used with each giving a slightly different classification result, but all have the common goal of selecting the groups so that the distance in n-dimensional space between the points within the cluster (within-group variability) is minimized.
Hybrid Techniques: Typically these techniques require the investigator to identify "prototype" atmospheric circulation patterns manually with the computerized cluster analysis used to categorize all other days (or months, if monthly mean data are used) automatically, quickly, and efficiently, into the group with the prototype day (or month) that the weather map for the day (or month) in question.
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