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Essay on Anticyclones
Essay # 1. Definition of Anticyclones:
An anticyclone (that is, opposite to a cyclone) is a weather phenomenon defined by the National Weather Service’s glossary as “A large-scale circulation of winds around a central region of high atmospheric pressure, clockwise in the Northern Hemisphere, anticlockwise in the Southern Hemisphere”.
Effects of surface-based anticyclones include clearing skies as well as cooler, drier air. Fog can also form overnight within a region of higher pressure. Mid-tropospheric systems, such as the subtropical ridge, deflect tropical cyclones around their periphery and cause a temperature inversion inhibiting free convection near their center, building up surface-based haze under their base.
Anticyclones aloft can form within warm core lows, such as tropical cyclones, due to descending cool air from the backside of upper troughs, such as polar highs, or from large scale sinking, such as the subtropical ridge. Anti-cyclonic flow spirals in a clockwise direction in the Northern Hemisphere and anticlockwise in the Southern Hemisphere.
Essay # 2. Characteristics of Anticyclones:
Surrounded by circular isobars anticyclone is such a wind system which has highest air pressure at the centre and lowest at the outer margin and winds blow from the centre outward in clockwise direction in the northern hemisphere and anticlockwise in the southern hemisphere.
Thus, anticyclones are high pressure systems and more common in the subtropical high pressure belts but are practically absent in the equatorial regions. They are generally associated with rainless fair weather. This is why anticyclones are called weather-less phenomena.
They are characterized by the following properties:
(1) They are usually circular in shape but sometimes they also assume ‘V’ shape. There is maximum air pressure at the centre and it decreases outward. The difference of pressure between the centre and periphery of anticyclone ranges between 10-20mb and sometimes it becomes 35mb.
(2) They are much larger in size and area than temperate cyclones as their diameter is 75 per cent larger than that of the latter.
(3) Though anticylones follow cyclones but their track is highly variable and unpredictable. They move very sluggishly and sometimes they become stationary over a particular place for few days. The average velocity of anticyclones is 30-50km per hour.
(4) Because of high pressure at the centre winds blow outward clockwise in the northern hemisphere and anticlockwise in the southern hemisphere.
(5) Winds descend from above at the centre and thus weather becomes clear and rainless because the descending winds cause atmospheric stability.
(6) Temperature in anticyclones depends on weather, nature of air mass and humidity in the air. They record high temperature during summer season due to development of warm air masses whereas they carry low temperature during winter season due to polar cold air masses.
(7) Anticylones do not have fronts.
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3. Formation of Anticyclones:
Surface based high-pressure systems form due to downward motion through the troposphere, the atmospheric layer where weather occurs. Preferred areas within a synoptic flow pattern in higher levels of the troposphere are beneath the western side of troughs, or dips in the Rossby wave pattern. High-pressure systems are alternatively referred to as anticyclones. The subtropical ridge forms due to the Hadley cell circulation between the equator and the subtropics of the Northern Hemisphere and Southern Hemisphere. Upper-level high pressure areas lie over tropical cyclones due to their warm core nature.
Surface anticyclones form due to downward motion through the troposphere, the atmospheric layer where weather occurs. Preferred areas within a synoptic flow pattern in higher levels of the troposphere are beneath the western side of troughs. On weather maps, these areas show converging winds (isotachs), also known as confluence, or converging height lines near or above the level of non-divergence, which is near the 500 hPa pressure surface about midway up through the troposphere. Because they weaken in intensity with height, these high pressure systems are cold.
Subtropical Ridge:
Heating of the earth near the equator leads to large amounts of upward motion and convection along the monsoon trough or Inter-tropical convergence zone. The divergence over the near-equatorial trough leads to air rising and moving away from the equator aloft. As it moves towards the Mid-Latitudes, the air cools and sinks, which leads to subsidence near the 30th parallel of both hemispheres.
This circulation is known as the Hadley cell and leads to the formation of the subtropical ridge. Many of the world’s deserts are caused by these climatological high-pressure areas. Because these anticyclones strengthen with height, they are known as warm core ridges.
Formation Aloft:
The development of anticyclones aloft occurs in warm core cyclones, such as tropical cyclones, when latent heat caused by the formation of clouds is released aloft, which increases air temperatures and the resultant atmospheric thickness of the layer, which increases high pressure aloft which acts to evacuate their outflow.
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4. Structure of Anticyclones:
Wind flows from areas of high pressure to areas of low pressure. This is due to density differences between the two air masses. Since stronger high-pressure systems contain cooler or drier air, the air mass is more dense 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 the pressure difference, or pressure gradient, between a high- pressure system and a low pressure system, the stronger the wind.
The coriolis force caused by the Earth’s rotation is what gives winds within high-pressure systems their clockwise circulation in the northern hemisphere (as the wind moves outward and is deflected right from the center of high pressure) and anticlockwise circulation in the southern hemisphere (as the wind moves outward and is deflected left from the center of high pressure). Friction with land slows down the wind flowing out of high-pressure systems and causes wind to flow more outward, or flowing more ageostrophically, from their centers.
Shape and Size:
Anticyclones are generally of circular, near circular or wedge shape but are very large in size. Sometimes, they become so large in size that their diameters become 9,000km. There is little difference between the length and width of anticyclones. Temperate anticyclones are so extensive that a single anticyclone covers nearly half of the USA.
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5. Types of Anticyclones:
Anticyclones were classified into:
(i) Warm anticyclones, and
(ii) Cold anticyclones by Hanzilk in 1909,
While Humphreys divided them into 3 types viz.:
(i) Mechanical or dynamic anticyclones,
(ii) Thermal or heat anticyclones, and
(iii) Radiation anticyclones.
A.E.M. Gaddes has classified anticyclones into 3 types e.g.:
(i) Large anticyclone, covering the whole of the continent,
(ii) Temporary anticyclone, which has a diameter of 250 to 300km and covers only the marginal areas of the continents, and
(iii) Cyclone-originated anticyclone, which develops due to high pressure caused between two temperate cyclones. Normally, anticyclones are divided into (i) cold anticyclones, (ii) warm anticyclones, and (iii) blocking anticyclones.
(1) Cold anticyclones:
After originating in the arctic regions cold anticyclones advance in easterly and south-easterly directions. Though they are smaller than warm anticyclones in size but move more rapidly than the latter. They are of very low thickness. Very few cold anticyclones are higher than 3,000m.
Harwitz and Noble have observed upper atmospheric low pressure above surface anticyclones at US-Canada border but such situation is not a permanent feature.
Cold anticyclones are divided into two subtypes e.g.:
(i) Temporary anticyclones, which die out in the transit while moving forward, only a few reach tropical regions, and
(ii) Semi-permanent anticyclones, which cover longer distances and are more active.
Cold anticyclones are thermally induced because they do not develop due to descent of air from above. They are originated due to development of high pressure because of very low insolation during winter season in the arctic regions.
Cold anticyclones follow two tracks:
(i) Anticyclones after originating in the north of Canada move in easterly and south-easterly direction and affect the weather conditions of Canada and USA.
(ii) Anticyclones originating in the north of Siberia move towards China, Japan and Alaska. Anticyclones affecting north-west Europe originate with temperate cyclones (in their rear portion). While entering tropical region cold anticyclones die out due to increase in temperature.
(2) Warm Anticyclones:
Warm anticyclones originate in the belt of subtropical high pressure where winds diverge in opposite directions. Thus, warm anticyclones are originated due to descent of air from above and consequent divergence at the surface. They, thus, are dynamically induced. They are large in size but are very sluggish in movement.
Sometimes, they become stationary over a place for several days and weeks. They are associated with light wind, cloudless sky and clear weather. Warm anticyclones mostly influence the weather of S.E. USA and Western Europe.
(3) Blocking Anticyclones:
Blocking anticyclones develop due to obstruction in the air circulation in the upper troposphere. This is why they are called blocking anticyclones. They develop over N.W. Europe and adjoining Atlantic Ocean and the western part of the N. Pacific Ocean between 140°-170°W longitudes. They are similar to warm anticyclones as regards wind system, air pressure and weather but are small in size and move very slowly.
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6. Importance of Anticyclones:
When the subtropical ridge in the northwest Pacific is stronger than normal, it leads to a wet monsoon season for Asia. The subtropical ridge position is linked to how far northward monsoon moisture and thunderstorms extend into the United States.
Typically, the subtropical ridge across North America migrates far enough northward to begin monsoon conditions across the Desert Southwest from July to September. When the subtropical ridge is farther north than normal towards the Four Corners, monsoon thunderstorms can spread northward into Arizona. When suppressed to the south, the atmosphere dries out across the Desert Southwest, causing a break in the monsoon regime.
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7. Weather Conditions of Anticyclones:
Generally, anticyclones are rainless and sky is free of clouds because of the fact that descending air in the centre of anticyclone is warmed up at dry adiabatic rate due to subsidence. This causes rise in temperature which reduces normal lapse rate of temperature, with the result the stability of air increases resulting into marked increase in the aridity of air. This is why anticyclones are indicative of dry weather.
This does not mean that anticyclones are always rainless. While passing over oceans sometimes they pick up moisture and yield light rains with moderate clouds. The arrival of anticyclones is heralded by clearing of clouds, if already present in the sky, clear weather and decrease in wind velocity. The weather of Canada, USA, and north Eurasia is mostly affected by anticyclones.
Anticyclones are originated due to the descent of either polar cold air mass or warm tropical air mass. It is, thus, obvious that cold anticyclones are associated with extremely low temperature and they cause cold waves during winter season but when they come in summer season, weather becomes pleasant. On the other hand, warm anticyclones bring heat waves during summer season in the subtropical regions.
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8. Effects of Anticyclones:
There are following three effects like Surface-based systems, Mid-tropospheric systems and Upper tropospheric systems.
Surface-Based Systems:
High pressure systems are frequently associated with light winds at the surface and subsidence through the lower portion of the troposphere. Subsidence will generally dry out an air mass by adiabatic, or compressional, heating. Thus, high pressure typically brings clear skies. During the day, since no clouds are present to reflect sunlight, there is more incoming shortwave solar radiation and temperatures rise.
At night, the absence of clouds means that outgoing long-wave radiation (i.e. heat energy from the surface) is not absorbed, giving cooler diurnal low temperatures in all seasons. When surface winds become light, the subsidence produced directly under a high-pressure system can lead to a buildup of particulates in urban areas under the ridge, leading to widespread haze. If the low level relative humidity rises towards 100 per cent overnight, fog can form.
Strong, vertically shallow high-pressure systems moving from higher latitudes to lower latitudes in the northern hemisphere are associated with continental arctic air masses. The low, sharp inversion can lead to areas of persistent stratocumulus or stratus cloud, colloquially known as anti-cyclonic gloom. The type of weather brought about by an anticyclone depends on its origin.
For example, extensions of the Azores high pressure may bring about anti-cyclonic gloom during the winter, as they are warmed at the base and will trap moisture as they move over the warmer oceans. High pressures that build to the north and extend southwards will often bring clear weather. This is due to being cooled at the base (as opposed to warmed) which helps prevent clouds from forming.
Once arctic air moves over an unfrozen ocean, the air mass modifies greatly over the warmer water and takes on the character of a maritime air mass, which reduces the strength of the high-pressure system. When extremely cold air moves over relatively warm oceans, polar lows can develop. However, warm and moist (or maritime tropical) air masses which move pole-ward from tropical sources are slower to modify than arctic air masses.
Mid-Tropospheric Systems:
The circulation around mid-level ridges, and the subsidence at their center, act to steer tropical cyclones around their periphery. Due to the subsidence within this type of system, a cap can be set up which inhibits the development of free convection. This limits thunderstorm activity near their center, and traps low-level pollutants such as ozone as haze under their base, which is a significant problem in large urban centers during summer months such as Los Angeles, California and Mexico City, Mexico.
Upper Tropospheric Systems:
The existence of an upper level ridge allows upper level divergence which leads to surface convergence. If a capping mid-level ridge does not exist, this leads to free convection and the development of showers and thunderstorms if the lower atmosphere is humid.
Since tropical cyclones strengthen these ridges, a positive feedback loop develops between the convective tropical cyclone and the upper level high, where the strength of both systems intensifies. This loop stops once ocean temperatures under the system cool sufficiently, under 26.5°C (79.7°F), which forces the thunderstorm activity to wane, which then weakens the upper level ridge.