Types of Pressure Systems & Semi-Permanent Highs and Lows

The general circulation of the atmosphere refers to the flow of air over the entire globe. It serves to transport warm air from tropical regions poleward and maintains a return flow of cold air from the poles equatorward. The north/south temperature gradient is the cause for the jet streams in the upper atmosphere and the trade winds are responsible for the tropical storms in the summer months. The tilt of the earth causes the change of the seasons with differential heating at different latitudes. The polar front jet forms between the polar cell and the Ferrell cells in the picture below and the subtropical jet stream forms between the Hadley and Ferrell cells. This tilt and differential heating cause these semi-permanent systems to migrate to the north and south as the seasons change and then help steer tropical storms and hurricanes along with onshore flow feeding moisture from the oceans onto land and providing the moisture needed for convective severe weather. High pressure systems are the controlling factor and determine the direction low pressure systems follow. High pressure can push or stop low pressure, but low pressure can never steer or block high pressure even if it is a hurricane. Three Cell Theory Semi-permanent highs and lows are persistent pressure systems that appear over an area throughout the year. The winds around these pressure systems can steer weather patterns and affect how storms and weather systems enter the United States. The paths of winter weather systems and hurricanes are largely controlled by the location of these highs and lows. Latitudinal differential heating influences the formation of semipermanent pressure systems and, consequently, the direction of the prevailing winds. The equatorial trough is generally found between 10°N and 10°S. This is the region where the northeast trade winds of the northern hemisphere converge with the southeast trade winds of the southern hemisphere. It is a warm, moist region of rising air that produces a low pressure belt at the surface around the globe. Three Cell Systems Western Hemisphere semi-permanent highs and lows are atmospheric circulations located near North America that direct weather and climate patterns in the United States. There are similar semi-permanent pressure patterns located in the Eastern Hemisphere. The placement of these pressure systems is tied to the location of continents, mountain ranges, and atmospheric jet streams. They affect weather throughout the year by steering low pressure areas (mid-latitude cyclones) towards the United States. These systems also direct weather patterns into the US such as hurricanes, ENSO (El Niño-Southern Oscillation), and winter weather. They are considered semi-permanent because they are located in a generally constant geographic region, although they can shift their central location, grow or contract over time, and get stronger or weaker. Subtropical highs develop at the surface near 30°N/S; a direct result of the convergence aloft between the Hadley and Ferrell Cells. These highs separate the easterly trades from the prevailing westerlies. They contain warm, dry regions of sinking air (subsidence). They are typically found over open ocean regions. In regions associated with the maximum subsidence (southeast quadrant), major deserts may be found over continents. The two semipermanent highs located in the northern hemisphere are the North Pacific High and North Atlantic (Bermuda-Azores) High. Subpolar Lows occur where the polar easterlies and prevailing westerlies converge (near 60°N/S). They are found in cold, stormy regions of rising air. Two semipermanent lows prevail in the northern hemisphere: (1) the Aleutian Low and (2) the Icelandic Low. Polar Highs develop near the poles. These are a result of the convergence aloft and resulting subsidence between the polar cells. They are cold, dry and very stable regions of sinking air. The Arctic high, in the northern hemisphere, is strong in the winter and weakens during the summer. Semi-Permanent High Pressure Systems Semipermanent pressure systems influence climatic elements. They determine the prevailing winds which, in turn, influence the availability of moisture, heat transfer and stability. Generally, the western side of subtropical highs are unstable; the eastern side is stable. Semi-Permanent High Pressure Bermuda High The Bermuda High is a high pressure system located in the Atlantic Ocean. During the summer, this pressure system is located just off the east coast of the United States. The clockwise circulation around this high pressure area helps direct the path of hurricanes and helps determine where they will make landfall. The summer months along the East Coast are relatively wet and humid because the wind around the high blows moist air from the ocean inland. This explains the increased humidity throughout the summer months in the southeast US. During the winter months, the Bermuda High is located farther east of the US towards the middle of the Atlantic Ocean. This allows the jet stream to dip farther south into the southeast US, sometimes bringing wintry weather with it. The winds around the Bermuda High also push the surface water of the Atlantic Ocean around. This results in the strong Gulf Stream current off the East Coast, which flows northward along the western edge of the Atlantic Ocean, bringing warm water from the tropics to more northern locations. The center of the Bermuda High is an area of light winds and little currents. Sailing ships that were caught in the center of the Bermuda High often got stuck with no way out and sailors were forced to toss supplies and sometimes even stock animals overboard to lighten the ship enough to make headway out of the calm high pressure area. Figure B shows the location of the Bermuda High during the 2005 Hurricane Season. This led the hurricane tracks into the Gulf of Mexico where devastating hurricanes made landfall such as Hurricane Katrina and Hurricane Rita. Pacific High The Pacific High is located off the west coast of North America. During the summer months, it is located just off the coast of California. Winds blow clockwise around a high pressure system. This keeps the western coast of the United States relatively dry during the summer especially compared to locations along the East Coast. During the winter, the Pacific High moves farther south. This allows winds to blow into the west coast and the polar front to move south into the United States. This brings storms into the coast making the winter season wet across the western United States. Semi-Permanent Low Pressure Systems Icelandic Low The Icelandic Low is a semi-permanent, subpolar area of low pressure in the North Atlantic Ocean and found between Iceland and Southern Greenland and in the winter extends into the the Barents Sea. Because of its broad area and range of central pressure, it is an area where migratory lows tend to slow down and deepen. It is strongest during the winter. During the winter, the Icelandic Low is the dominate weather feature in the area. It is less weaker during the summer and may split into two low pressure systems. One over the Davis Strait between Greenland and Baffin Island and the other one west of Iceland. Then the Azores or Bermuda High becomes the dominate weather feature in the North Atlantic. Aleutian Low The Aleutian Low is a low pressure system located over the Gulf of Alaska and the Bering Sea. During winter, this is a zone of activity where numerous storms are located. Atmospheric disturbances often move into this area and intensify under the influence of the Aleutian Low. The circulation around the Aleutian Low then steers the storms into the Pacific Northwest, particularly during the winter months. During summer, the Aleutian Low is weak and often nearly nonexistent. Siberian High The Siberian High is a massive collection of cold dry air that accumulates in the northeastern part of Eurasia from September until April. It is usually centered on Lake Baikal. It reaches its greatest size and strength in the winter when the air temperature near the center of the high-pressure area is often lower than −40 °C (−40 °F). The atmospheric pressure is often above 1,040 millibars (31 inHg). The Siberian High is the strongest semi-permanent high in the northern hemisphere and is responsible for both the lowest temperature in the Northern Hemisphere, of −67.8 °C (−90.0 °F) on 15 January 1885 at Verkhoyansk, and the highest pressure, 1083.8 millibar (108.38 kPa, 32.01 inHg) at Agata, Krasnoyarsk Krai on 31 December 1968, ever recorded. The Siberian High is responsible both for severe winter cold and attendant dry conditions with little snow and few or no glaciers across Siberia, Mongolia, and China. During the summer, the Siberian High is largely replaced by the Asiatic low (a low-pressure summertime system over southwestern Asia). Barotropic Low Pressure Systems A barotropic pressure system is one that exhibits no thermal advection. The isobars and the isotherms are parallel with each other. These systems are not steered or influenced by the jet stream. Atmospheric pressure is measured at any weather station around the globe and there can still be a significant pressure variance between weather stations. As air circulates up and down through the atmosphere, mass is add and removed from a column of air and each hour during a weather observation the pressure is recorded in a METAR and sent out to a national database. This pressure is then collected and extrapolated to create surface pressure charts that are used by meteorologists to determine the intensities and deterioration of weather patterns. The standard atmospheric pressure at sea level is 29.92 inches (1013.23 millibars). As air circulates and mass is removed from the column of air above a weather station, the pressure drops since there is less mass over that station. The atmosphere is constantly circulation trying to balance then temperature contrast between air masses. If a column of air is warmed due to condensation, the warming causes the column of air to rise and expand causing a drop in surface pressure. A least intense example is when a thermal low forms over land and then causes a sea breeze to flow toward an island or peninsula. A better example would be if a thermal low formed over central Florida and the sea breezes from each coast collides in central Florida causing low level convergence at the surface and rapidly dropping surface pressure. This is typically the cause for the line of thunderstorms down the center of Florid in the summer. The most extreme example of this is the intense low pressure that forms in the eye of a hurricane, where latent heat release from rain formation causes warming of the air column within the eye. It the most intense hurricanes and typhoons, over 10% of the atmosphere can be removed from the eye of the storm through this process. The lowest pressure ever recorded was in the eye of Typhoon Tip, in the tropical western Pacific Ocean, on October 12, 1979: 25.69 inches of mercury (870 millibars). this record pressure was about 14% lower than normal. That’s 14% of the atmosphere’s mass had been removed from the column of air. Warm Core Low There are two types of warm core low pressure systems. Hurricane (Tropical Storm or Tropical Depression) Latent heat of condensation Intense low level cyclonic circulation that weakens with height Anticyclonic outflow above 200mb Cyclostrophic balance Thermal Low Weak low level cyclonic circulation Diffuse area Visible to 850mb and sometimes 700mb Upper level ridging Isolated precipitation. (Mainly afternoon thunderstorms) Forms in Continental Tropical (cT) air mass SW USA & Mexico during summer Other characteristics Greatest thickness is at the center The system is nearly vertical with little tilt Decreases in intensity with height Cyclonic circulation weakens with height and may be anticyclonic No fronts associated with these lows Equatorward of the polar front jet (PFJ) Temperature increases toward the center Warm Core lows are frequently stationary Warm Lows are shallow, have little height and no slope Intense warm lows found in the southwest and Hurricanes do slope toward warm air aloft Cold Core Low There are two types of cold core lows. Cut-off Low South of the main Polar Front Jet (PFJ) Common over the SW USA and coastal areas Common in the spring Decaying Wave North of the main Polar Front Jet (PFJ) Common near Iceland & Aleutians, at times Hudson Bay Deep center and slow moving Most common in the winter Other Characteristics Contains the coldest air at its center throughout the troposphere. Going outward from its center in any direction at any level in the troposphere, warmer air is encountered. Increases in intensity with height. Found in such cyclones are cold pools with relative minimums in thickness values. Temperature distribution is almost symmetrical. The axis of the low is nearly vertical. When they slope, they slope towards the coldest temperatures aloft. Cyclonic circulation aloft is usually reflected on the surface in an abnormally low daily mean temperature. Cold Core Low Barotropic High Pressure Systems Since surface air pressure is a measure of the weight of the atmosphere above any location, a high pressure area represents a region where there is somewhat more atmospheric mass overlying it. High pressure areas are usually caused by air masses being cooled, either from below (for instance, the subtropical high pressure zones that form over relatively cool ocean waters to the west of Califormia, Africa, and South America), or from above as infrared cooling of winter air masses over land exceeds the warming of those airmasses by sunlight. As the airmass cools, it shrinks, allowing air from the surroundings to fill in above it, thus increasing thte total mass of atmosphere above the surface, which then results in higher surface barometric pressures. The pressure difference between the high pressure area and its lower-pressure surroundings causes a wind to develop flowing from higher to lower pressure. But because of the rotation of the Earth, the wind is deflected to the right (in the Northern Hemisphere) which then causes the wind to flow in a clockwise direction around the high pressure zone. The highest surface air pressures occur in the winter over Asia and North America where cold air masses form and become very deep. The world record for highest surface pressure (adjusted to sea level) was at Agata Lake in Siberia on December 31, 1968 at 32.01 inches (1083.8 mb). Average sea level pressure is 29.92 inches (1013.23 mb), making this record pressure 7% higher than normal, which means there was 7% more atmospheric mass above that point than normal. Warm Core High There are two types of warm core highs Sub-Tropical High Caused by upper level convergence between the Hadley and Farrell Cells Well defined high pressure center on the surface Cut-Off High North of the Polar Front Jet (PFJ) Most common in the north Atlantic ocean Other Characteristics Temperature increases toward the center Increases in intensity with height Has anticyclonic circulation (clockwise rotation) at all levels Examples include the Bermuda High & Pacific High Pressure Systems Cold Core High There is only one type of cold core high. Polar High Polar high forms in the continental polar (cP) air mass Norther Canada & Siberian High Characteristics A Cold Core High is one in which the temperatures on the horizontal plane decrease toward the center. The Pressure deceases rapidly with height. Thicknesses are relatively low. In the vertical, cold core highs slope toward the colder air aloft. Because these highs are quiet shallow, it is common for an upper level low to exist above a cold core high. If precipitation is caused by the upper level low, depending in the height of the freezing level associated with the high on the surface; a cold rain or snow can be expected. Cold Core High