<P> Extratropical cyclones form as waves along weather fronts due to a passing by shortwave aloft or upper level jet streak before occluding later in their life cycle as cold - core cyclones . Polar lows are small - scale, short - lived atmospheric low - pressure systems that occur over the ocean areas poleward of the main polar front in both the Northern and Southern Hemispheres . They are part of the larger class of mesoscale weather - systems . Polar lows can be difficult to detect using conventional weather reports and are a hazard to high - latitude operations, such as shipping and gas - and oil - platforms . They are vigorous systems that have near - surface winds of at least 17 metres per second (38 mph). </P> <P> Tropical cyclones form due to latent heat driven by significant thunderstorm activity, and are warm - core with well - defined circulations . Certain criteria need to be met for their formation . In most situations, water temperatures of at least 26.5 ° C (79.7 ° F) are needed down to a depth of at least 50 m (160 ft); waters of this temperature cause the overlying atmosphere to be unstable enough to sustain convection and thunderstorms . Another factor is rapid cooling with height, which allows the release of the heat of condensation that powers a tropical cyclone . High humidity is needed, especially in the lower - to - mid troposphere; when there is a great deal of moisture in the atmosphere, conditions are more favorable for disturbances to develop . Low amounts of wind shear are needed, as high shear is disruptive to the storm's circulation . Lastly, a formative tropical cyclone needs a pre-existing system of disturbed weather, although without a circulation no cyclonic development will take place . Mesocyclones form as warm core cyclones over land, and can lead to tornado formation . Waterspouts can also form from mesocyclones, but more often develop from environments of high instability and low vertical wind shear . </P> <P> In deserts, lack of ground and plant moisture that would normally provide evaporative cooling can lead to intense, rapid solar heating of the lower layers of air . The hot air is less dense than surrounding cooler air . This, combined with the rising of the hot air, results in a low - pressure area called a thermal low . Monsoon circulations are caused by thermal lows which form over large areas of land and their strength is driven by how land heats more quickly than the surrounding nearby ocean . This generates a steady wind blowing toward the land, bringing the moist near - surface air over the oceans with it . Similar rainfall is caused by the moist ocean - air being lifted upwards by mountains, surface heating, convergence at the surface, divergence aloft, or from storm - produced outflows at the surface . However the lifting occurs, the air cools due to expansion in lower pressure, which in turn produces condensation . In winter, the land cools off quickly, but the ocean keeps the heat longer due to its higher specific heat . The hot air over the ocean rises, creating a low - pressure area and a breeze from land to ocean while a large area of drying high pressure is formed over the land, increased by wintertime cooling . Monsoons resemble sea and land breezes, terms usually referring to the localized, diurnal (daily) cycle of circulation near coastlines everywhere, but they are much larger in scale - also stronger and seasonal . </P> <P> Large polar cyclones help determine the steering of systems moving through the mid-latitudes, south of the Arctic and north of the Antarctic . The Arctic oscillation provides an index used to gauge the magnitude of this effect in the Northern Hemisphere . Extratropical cyclones tend to form east of climatological trough positions aloft near the east coast of continents, or west side of oceans . A study of extratropical cyclones in the Southern Hemisphere shows that between the 30th and 70th parallels there are an average of 37 cyclones in existence during any 6 - hour period . A separate study in the Northern Hemisphere suggests that approximately 234 significant extratropical cyclones form each winter . In Europe, particularly in the United Kingdom and in the Netherlands, recurring extratropical low - pressure weather systems are typically known as depressions . These tend to bring wet weather throughout the year . Thermal lows also occur during the summer over continental areas across the subtropics - such as the Sonoran Desert, the Mexican plateau, the Sahara, South America, and Southeast Asia . The lows are most commonly located over the Tibetan plateau and in the lee of the Rocky mountains . </P>

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