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Section 05 Pressure Systems Lesson 14/15/16 Pressure Systems • The air pressure may vary considerably between different places on earth. • These pressure differences are of decisive importance to the Earth’s weather and winds. • On the charts of the weather service we find the pressure pattern, delineated by the isobars, enclosing areas of different pressure, e.g. – high pressures (anticyclones) and ridges, – low pressures (cyclones) and troughs. – cols Anticyclones, Ridges • A high pressure system is: – an area enclosed by isobars, – pressure values of which decrease with distance from the centre of the high or axis of the ridge. • vertical airflow in a high pressure cell comprises; – – – – – mass convergence at height and divergence at low levels creating a descending air movement or subsidence within the core of the anticyclone with an outflow at low level with a subsidence inversion. Anticyclonic Subsidence Subsidence Inversion Cloud Formation High Pressure Types • Warm Anticyclones/Ridges – Subtropical high pressure belt at 30° N/S • Cold Anticyclones/Ridges – Siberian High – Cold ridge ahead of a warm front Warm Anticyclone • High pressure at height sustained relative to surroundings • Convergence aloft fed by subtropical jet stream. Subtropical High • • • • • Permanent high pressure system Situated at 30° N/S Subsidence from aloft The centres consist of warm air. The subsidence inversion in these cells is sometimes called Trade Wind inversion. • Large areas of ST and SC on polar side • Trade wind CU in the equatorial side. • Source region for Tropical Maritime air. Cold Anticyclones • Pressure decreases with height COLD Cold Anticyclones/Ridges • Form over cold continental land masses in winter – Siberian High, Canadian High. • • • • • • Usually temporary Shallow in depth, max depth up to 700 mb. Turn into thermal lows with summer heating. Very stable air. Strong surface inversion. Source region for Polar Continental air. Cold High/Ridge with Travelling Depression Low Pressure System Cold Core Low (e.g. Icelandic Low) • Low pressure intensifies with height • Strong convergence at low level • CU/CB/RASH Warm Core Low • Forms over land in summer or, • Forms over warm water in winter. • Thermal low/Asiatic Monsoon Low. Thermal Low Origins of Non Frontal Low Pressure Areas • Orographic Lows (with mountain ranges) • Thermal Lows • • • • Instability Lows/Polar lows Summer lows over land/Monsoon Low Winter Lows over a warm sea(Mediterranean Low/Baltic Low) Equatorial Low/Trough (Intertropical Convergence Zone) Origins of Non Frontal Low Pressure Areas, cont’d • Cold Air Pool • Tropical Revolving Storms (see Chapter 17) • Easterly Waves • Tornadoes (see Chapter 16) • Whirlwinds/Dust Devils/Water spouts Types of NFD • Not all depressions are frontal • Over the UK most depressions are frontal. • However world-wide most depressions are non-frontal. Types of NFD • Most of these are thermal in origin, and arise from surface heating and the release of latent heat. • Some depressions are orographic such as the Genoa Low • The consequent cooling and condensation produce the associated bad weather. Orographic Lows • Wind flowing across a mountain barrier tends to curve anticyclonically over the high ground and cyclonically to its lee. • This leads to a ridge of high pressure over the mountain or hills and a trough on the lee side. • The inertial effect of air piling up on the windward side of high ground reinforces the pressure contrast, and closed circulations may develop on the lee side. Orographic/Lee Depression • Sometimes associated with Föhn wind Cold Warm Orographic Occlusion • Occlusion formed on wave Orographic Lows Cont. • Lee lows are not common in the U.K., though lee troughing may be observed. • High mountains favour the development of lee orographic lows. • Strong N.W. flows over the Alps lead to the formation of the "Genoa Low" in N. Italy. Orographic Lows Cont. • Lee lows also form to the lee of Greenland and Scandinavia. • Inactive due to descending air in the lee of the high ground. This causes the weather to be more like that found in an anticyclone, rather than a depression. Thermal Depressions • Expansion of the air due to warming over a warm surface leads to a rise in pressure aloft. • The greater the heating, the greater the pressure change, level for level. • The rise in pressure at the level above the surface leads to an outflow of air aloft. • This causes a fall in surface pressure and convergence. • Cyclonic circulation exists at the surface with divergence aloft. • Such thermal lows tend to be shallow. Thermal Depressions, cont’d • Thermal depressions are often sub-divided into: – Monsoon low – Summer lows over land – Equatorial low or trough Formation of Thermal Low Formation of Thermal Low Air flows away aloft Rising Air Pressure falls at the surface Formation of Thermal Low Horizontal divergence Horizontal convergence Polar/Instability Low • Polar lows may form in Pm air which is unstable • They usually form between the southern tip of Greenland and the Norwegian coast • They quickly dissipate over land Polar lows • Cold arctic or polar air (-20°C to -40°C) moving out from Northern Canada or Greenland is warmed by the relatively warm sea (+01°C). • The temperature contrast exists by night as well as day so that these polar lows tend to persist if the air is unstable. • The clouds are mainly CB. • They track SE in the Polar airstream. • If a polar low is close to the polar front, a wave may develop on the front, bringing tropical air into the polar low and greatly invigorating it. Polar Lows Along Norwegian Coast • Cyclonic airflow develops between two highs • When cold air reaches the warmer sea, instability lows can develop Baltic Sea Low • Energy is released into the existing low from warm waters • Heavy precipitation and squalls may develop Cold Pool Formation • Unstable meridional extension of upper air flow. • Wave collapses leaving a closed circulation of cold unstable air aloft. Cold Pool – A region of low mean temperature in an isobaric layer aloft. – Sometimes cold air outbreaks, cut off from the main stream, generate a pool of cold air at height in a position far south of the normal Polar front. – This cold pool can remain for several days constituting a potential area of instability at height. Cold Pool – In the summer warm lows form over the continents, and sometimes these may develop into instability lows. Cold Pool – This happens when cold air is carried in over the low (by the upper airflow) or when a cold pool already exists at height. – In these conditions the atmosphere becomes unstable, and a major area of thunderstorms may develop. – These thunderstorms interfere with aviation in the area, since it may be difficult to fly round them. • Waterspout on the Solent (Hampshire/Isle of Wight), on July 7, 1966 (O. Thomas) Water Spouts – There are two types of waterspouts. – The first is developed downwards from a convective cloud formed over warm water surfaces where ordinary towering cumulus clouds are developing. – The second type is formed by a rapidly converging airflow over a warm water surface and builds upward from the surface just like a whirlwind. Water Spouts – These columns of rotating air are known as waterspouts. – Southern Florida in summer. – Autumn waterspouts can be seen over the northern Baltic Sea. Tornadoes, Whirlwinds & Dust Devils • Tornadoes form inside cloud & come down • Dust devils develop at the surface and go upwards • Tornado (USA) – dia. 500m – associated with Cb/Ts • Violent whirlwinds – dia. 50 – 250m • Small whirlwinds (Dust Devils) – dia. 10m max. 20kt • In arid areas – dia. 30m height 300ft max. 50kt Average Global Surface Pressure - January Lows - January • Icelandic Low • Aleutian Islands Low • Thermal Lows – Southern Hemisphere Highs - January • Sub-Tropical High • Siberian High • N. American High Average Global Surface Pressure - July Lows - July • Icelandic Low • Asiatic/Baluchistan Low • N. American Low Asiatic Monsoon Low Highs - July • Azores High • N. Pacific High • Sub-Tropical High