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39-703: The Aleutian Low is a semi-permanent low-pressure system located near the Aleutian Islands in the Bering Sea during the Northern Hemisphere winter. It is a climatic feature centered near the Aleutian Islands measured based on mean sea-level pressure. It is one of the largest atmospheric circulation patterns in the Northern Hemisphere and represents one of the "main centers of action in atmospheric circulation." The Aleutian Low

78-612: A convective low acquires a well-hot circulation in the tropics it is termed a tropical cyclone . Tropical cyclones can form during any month of the year globally but can occur in either the northern or southern hemisphere during December. Atmospheric lift will also generally produce cloud cover through adiabatic cooling once the air temperature drops below the dew point as it rises, the cloudy skies typical of low-pressure areas act to dampen diurnal temperature extremes . Since clouds reflect sunlight , incoming shortwave solar radiation decreases, which causes lower temperatures during

117-413: 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

156-560: A regional scale, some exceptionally humid areas of Earth experience cloudy conditions virtually all time such as South America's Amazon Rainforest while some highly arid areas experience clear-sky conditions virtually all the time such as Africa's Sahara Desert . Although clouds can exist within a wide range of altitudes, typical cloud cover has a base at approximately 4,000m and extends up to an altitude of about 5,000m. Clouds height can vary depending on latitude; with cloud cover in polar latitudes being slightly lower and in tropical regions

195-522: Is a storm that occurs in the Atlantic Ocean and northeastern Pacific Ocean , a typhoon occurs in the northwestern Pacific Ocean, and a tropical cyclone occurs in the south Pacific or Indian Ocean . Friction with land slows down the wind flowing into low-pressure systems and causes wind to flow more inward, or flowing more ageostrophically , toward their centers. Tornadoes are often too small, and of too short duration, to be influenced by

234-478: Is a latitudinal variation in the cloud cover. Areas around 10-15% below the global mean can be found around 20°N and 20°S, due to an absence of equatorial effects and strong winds reducing cloud formation. On the other hand, in the storm regions of the Southern Hemisphere midlatitudes were found to have with 15–25% more cloudiness than the global mean at 60°S. On average, about 67% of the entire Earth

273-558: Is an umbrella term for several different processes, all of which result in the development of some sort of cyclone . Meteorologists use the term "cyclone" where circular pressure systems flow in the direction of the Earth's rotation, which normally coincides with areas of low pressure. The largest low-pressure systems are cold-core polar cyclones and extratropical cyclones which lie on the synoptic scale . Warm-core cyclones such as tropical cyclones, mesocyclones , and polar lows lie within

312-470: Is around 72% over the oceans, with low seasonal variation, and about 55% above land, with significant seasonal variation. Clouds play multiple critical roles in the climate system and diurnal cycle . In particular, being bright objects in the visible part of the solar spectrum , they efficiently reflect light to space and thus contribute to the cooling of the planet, as well as trapping remaining heat at night . Cloud cover thus plays an important role in

351-579: Is characterized by heavily influencing the path and strength of cyclones . Extratropical cyclones which form in the sub-polar latitudes in the North Pacific typically slow down and reach maximum intensity in the area of the Aleutian Low. Tropical cyclones that form in the tropical and equatorial regions of the Pacific can veer northward and get caught in the Aleutian Low. This is usually seen in

390-535: Is characterized by the development of lower air pressure over the warmest part of the various continents. The large-scale thermal lows over continents help create pressure gradients which drive monsoon circulations. In the southern hemisphere, the monsoon trough associated with the Australian monsoon reaches its most southerly latitude in February, oriented along a west-northwest/east-southeast axis. Many of

429-502: Is cloud-covered at any moment. On a continental scale, it can be noticed based upon a long-term satellite recording of cloudiness data that on a year-mean basis, Europe , North America , South America and Asia are dominated by cloudy skies due to the westerlies , monsoon or other effects. On the other hand, Africa , the Middle East and Australia are dominated by clear skies due to their continentality and aridity . On

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468-416: Is initially accelerated from areas of high pressure to areas of low pressure. This is due to density (or temperature and moisture) differences between two air masses . Since stronger high-pressure systems contain cooler or drier air, the air mass is denser and flows towards areas that are warm or moist, which are in the vicinity of low-pressure areas in advance of their associated cold fronts . The stronger

507-498: The British Isles and Netherlands ), recurring low-pressure weather systems are typically known as "low levels". Cyclogenesis is the development and strengthening of cyclonic circulations, or low-pressure areas, within the atmosphere. Cyclogenesis is the opposite of cyclolysis , and has an anticyclonic (high-pressure system) equivalent which deals with the formation of high-pressure areas — anticyclogenesis . Cyclogenesis

546-600: The Earth 's surface. Large-scale thermal lows over continents help drive monsoon circulations. Low-pressure areas can also form due to organized thunderstorm activity over warm water. When this occurs over the tropics in concert with the Intertropical Convergence Zone , it is known as a monsoon trough . Monsoon troughs reach their northerly extent in August and their southerly extent in February. When

585-572: The Hadley cell circulation. Monsoon troughing in the western Pacific reaches its zenith in latitude during the late summer when the wintertime surface ridge in the opposite hemisphere is the strongest. It can reach as far as the 40th parallel in East Asia during August and 20th parallel in Australia during February. Its poleward progression is accelerated by the onset of the summer monsoon which

624-753: 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

663-471: The troposphere below as air flows upwards away from the surface, which lowers surface pressures as this upward motion partially counteracts the force of gravity packing the air close to the ground. Thermal lows form due to localized heating caused by greater solar incidence over deserts and other land masses. Since localized areas of warm air are less dense than their surroundings, this warmer air rises, which lowers atmospheric pressure near that portion of

702-551: The Aleutian Low. This high-pressure circulation pattern strongly influences tropical cyclone paths. The presence of the Eurasian and North American continents prevent a continuous belt of low pressure from developing in the Northern Hemisphere sub-polar latitudes, which would mirror the circumpolar belt of low pressure and frequent storms in the Southern Ocean. However, the presence of the continents disrupts this motion, and

741-476: The Coriolis force, but may be so-influenced when arising from a low-pressure system. Cloud cover Cloud cover (also known as cloudiness , cloudage , or cloud amount ) refers to the fraction of the sky obscured by clouds on average when observed from a particular location. Okta is the usual unit for measurement of the cloud cover. The cloud cover is correlated to the sunshine duration as

780-705: 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 . In deserts , lack of ground and plant moisture that would normally provide evaporative cooling can lead to intense, rapid solar heating of

819-552: The day. At night the absorptive effect of clouds on outgoing longwave radiation , such as heat energy from the surface, allows for warmer night-time minimums in all seasons. The stronger the area of low pressure, the stronger the winds experienced in its vicinity. Globally, low-pressure systems are most frequently located over the Tibetan Plateau and in the lee of the Rocky Mountains . In Europe (particularly in

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858-405: The difference between temperatures aloft and sea surface temperatures is the greatest. However, each particular basin has its own seasonal patterns. On a worldwide scale, May is the least active month while September is the most active month. Nearly one-third of the world's tropical cyclones form within the western Pacific Ocean, making it the most active tropical cyclone basin on Earth . Wind

897-467: The energetic balance of the atmosphere and a variation of it is a factor and consequence of and to the climate change expected by recent studies. Cloud cover values only vary by 3% from year-to-year averages, whereas the local, day-to-day variability in cloud amounts typically rises to 30% over the globe. Land is generally covered by 10-15% less cloud than the oceans, because the seas are covered with water, allowing for more evaporation . Lastly, there

936-662: The flow around Rossby waves migrate equatorward of the polar cyclones located in both the Northern and Southern hemispheres. All share one important aspect, that of upward vertical motion within the troposphere. Such upward motions decrease the mass of local atmospheric columns of air, which lowers surface pressure. 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

975-545: 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. Large polar cyclones help determine

1014-403: 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,

1053-527: The later summer months. Both the November 2011 Bering Sea cyclone and the November 2014 Bering Sea cyclone were extratropical cyclones that had dissipated and restrengthened when the systems entered the Aleutian Low region. The storms are remembered and marked as two of the strongest storms to impact the Bering Sea and Aleutian Islands with pressure dropping below 950 mb in each system. The magnitude of

1092-471: The least cloudy locales are the sunniest ones while the cloudiest areas are the least sunny places, as clouds can block sunlight, especially at sunrise and sunset where sunlight is already limited. The global cloud cover averages around 67-68%, though it ranges from 56% to 73% depending on the minimum optical depth considered (lower when optical depth is large, and higher when it is low, such that subvisible cirrus clouds are counted). Average cloud cover

1131-497: The low is strongest in the winter and almost completely dissipates in the summer. The circulation pattern is measured based on averages of synoptic features help mark the locations of cyclones and their paths over a given time period. However, there is significant variability in these measurements. The circulation pattern shifts during the Northern Hemisphere summer when the North Pacific High takes over and breaks apart

1170-579: The low pressure creates an extreme atmospheric disturbance, which can cause other significant shifts in weather. Following the November 2014 Bering Sea cyclone, a huge cold wave, November 2014 North American cold wave , hit the US bringing record breaking low temperatures to many states. The low serves as an atmospheric driver for low-pressure systems, post-tropical cyclones and their remnants and can generate strong storms that impact Alaska and Canada. Intensity of

1209-407: 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

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1248-423: The northern hemisphere (as the wind moves inward and is deflected right from the center of high pressure) and clockwise circulation in the southern hemisphere (as the wind moves inward and is deflected left from the center of high pressure). A tropical cyclone differs from a hurricane or typhoon based only on geographic location. A tropical cyclone is fundamentally different from a mid-latitude cyclone. A hurricane

1287-722: 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 offshore platforms . They are vigorous systems that have near-surface winds of at least 17 metres per second (38 mph). 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

1326-414: The pressure difference, or pressure gradient , between a high-pressure system and a low-pressure system, the stronger the wind. Thus, stronger areas of low pressure are associated with stronger winds. The Coriolis force caused by the Earth 's rotation is what gives winds around low-pressure areas (such as in hurricanes , cyclones , and typhoons ) their counter-clockwise (anticlockwise) circulation in

1365-399: The smaller mesoscale . Subtropical cyclones are of intermediate size. Cyclogenesis can occur at various scales, from the microscale to the synoptic scale. Larger-scale troughs, also called Rossby waves, are synoptic in scale. Shortwave troughs embedded within the flow around larger scale troughs are smaller in scale, or mesoscale in nature. Both Rossby waves and shortwaves embedded within

1404-512: 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

1443-667: The subpolar belt of low pressure is well developed only in the North Pacific (the Aleutian Low) and the North Atlantic (the Icelandic Low , which is located between Greenland and Iceland). The strength of the Aleutian Low has been proposed as a driving factor in determining primary production in the water column and, in turn, impacting the catch in the salmon fishery. Low-pressure area Diverging winds aloft, ahead of these troughs, cause atmospheric lift within

1482-470: The world's rainforests are associated with these climatological low-pressure systems. Tropical cyclones generally need to form more than 555 km (345 mi) or poleward of the 5th parallel north and 5th parallel south , allowing the Coriolis effect to deflect winds blowing towards the low-pressure center and creating a circulation. Worldwide, tropical cyclone activity peaks in late summer, when

1521-724: 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. Elongated areas of low pressure form at the monsoon trough or Intertropical Convergence Zone as part of

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