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GLOBAL ATMOSPHERIC CIRCULATION • GLOBAL ATMOSPHERIC CIRCULATION • AIR MASSES • FRONTS • PRESSURE SYSTEMS Simple Model of Global Circulation Ideal Global Circulation Initially, assumes that the Earth: • Has a uniform surface • It is not rotating • It is not tilted. Approximate actual global circulation on the Earth Actual global circulation is affected by - Rotation of the Earth - The Earth’s Tilt - The irregular surface of the land - The deference in land and sea surfaces Air masses Huge bodies of air within the troposphere characterized with uniform weather conditions and characteristics are called Air masses. Regions of air masses origin are known as source regions. Ideal source regions are: • Large continental areas • Major bodies of water. Air mass clasification Typically, air masses are classified according to the characteristics of their source of origin or area of formation. According this classification AM can be equatorial, tropical, polar or arctic. Air masses are also classified as continental or maritime in terms of moisture characteristics. The combination of these two characteristics give the common classification of AM maritime polar (mP) continental polar (cP) maritime tropical (mT) continental tropical (cT) Maritime arctic (mA) continental arctic (cA) Arctic AAM Originates in the polar ice cap in the winter. Antarctic is used for the Southern Hemisphere Polar PAM Originates between 40° and 60 ° latitude, in the summer up to 80 °latitude Tropical TAM Originates in the subtropical high pressure cells. In summer tropical regions can also form over the large continents. Equatorial EAM Forms in the region between the subtropical highs on both sides the inter tropical convergence zone. Thermodynamical classification of AM According thermodynamical properties AM can be classified as: - Cold AM (It is colder then underlying surface) - Warm AM (It is colder then underlying surface) - Stable AM (No conditions for convection) - Unstable AM (There are conditions for convection) - Neutral AM (do not change its properties significantly when travelling from the source region) As an air mass is moving from the source region, it is modified or change its properties. Weather and flight conditions Unstable and moist • CU,CB, TS • Moderate to heavy showers of rain, hail • Moderate to sever icing and turbulence in cloud • Visibility good outside cloud • Strong squalls and gusts Unstable and dry Cu (2-4 octas) Weather and flight conditions Stable and moist • St, Sc • Drizzle presipitation • Light to Moderate icing and light turbulence in cloud and presipitation • Poor visibility due to Fog, mist and precipitation • ice-covered ground Stable and dry No clouds, poor visibility due to dust, smoke, haze , low drifting, blowing (Snow,Dust,Sand)