Horse latitudes, subtropical ridges or subtropical highs are the subtropical latitudes between 30 and 35 degrees both north and south where Earth's atmosphere is dominated by the subtropical high, an area of high pressure, which suppresses precipitation and cloud formation, has variable winds mixed with calm winds. It is the product of the global air circulation cell known as the Hadley Cell; the subtropical ridge is characterized by calm winds, which act to reduce air quality under its axis by causing fog overnight, haze during daylight hours as a result of the stable atmosphere found near its location. The air descending from the upper troposphere flows out from its center at surface level toward the upper and lower latitudes of each hemisphere, creating both the trade winds and the westerlies; the subtropical ridge moves poleward during the summer, reaching its most northern latitude in early fall, before moving equatorward during the cold season. The El Niño southern climate oscillation can displace the northern hemisphere subtropical ridge, with La Niñas allowing for a more northerly axis for the ridge, while El Niños show flatter, more southerly ridges.
The change of the ridge position during ENSO cycles changes the tracks of tropical cyclones that form around their equatorward and western peripheries. As the subtropical ridge varies in position and strength, it can enhance or depress monsoon regimes around their low-latitude periphery; the horse latitudes are associated with the subtropical anticyclone. The belt in the Northern Hemisphere is sometimes called the "calms of Cancer" and that in the Southern Hemisphere the "calms of Capricorn"; the warm and sunny conditions of the horse latitudes are the main cause for the existence of the world's major non-polar deserts, such as the Sahara Desert in Africa, the Arabian and Syrian deserts in the Middle East, the Mojave and Sonoran deserts in the southwestern United States and northern Mexico, all in the Northern Hemisphere. A and documented explanation is that the term is derived from the "dead horse" ritual of seamen. In this practice, the seaman paraded a straw-stuffed effigy of a horse around the deck before throwing it overboard.
Seamen were paid in advance before a long voyage, they spent their pay all at once, resulting in a period of time without income. If they got advances from the ship's paymaster, they would incur debt; this period was called the "dead horse" time, it lasted a month or two. The seaman's ceremony was to celebrate having worked off the "dead horse" debt; as west-bound shipping from Europe reached the subtropics at about the time the "dead horse" was worked off, the latitude became associated with the ceremony. An alternative theory, of sufficient popularity to serve as an example of folk etymology, is that the term horse latitudes originates from when the Spanish transported horses by ship to their colonies in the West Indies and Americas. Ships became becalmed in mid-ocean in this latitude, thus prolonging the voyage. A third explanation, which explains both the northern and southern horse latitudes and does not depend on the length of the voyage or the port of departure, is based on maritime terminology: a ship was said to be'horsed' when, although there was insufficient wind for sail, the vessel could make good progress by latching on to a strong current.
This was suggested by Edward Taube in his article "The Sense of "Horse" in the Horse Latitudes". He argued the maritime use of'horsed' described a ship, being carried along by an ocean current or tide in the manner of a rider on horseback; the term had been in use since the end of the seventeenth century. Furthermore, The India Directory in its entry for Fernando de Noronha, an island off the coast of Brazil, mentions it had been visited by ships "occasioned by the currents having horsed them to the westward". Heating of the earth near the equator leads to large amounts of convection along the monsoon trough or Intertropical convergence zone; this air mass rises to the lower stratosphere where it diverges, moving away from the equator in the upper troposphere in both northerly and southerly directions. As it moves towards the mid-latitudes on both sides of the equator, the air sinks; the resulting air mass subsidence creates a subtropical ridge of high pressure near the 30th parallel in both hemispheres.
At the surface level, the sinking air diverges again with some returning to the equator, completing the Hadley circulation. This circulation on each side of the equator 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; the subtropical ridge starts migrating poleward in late spring reaching its zenith in early autumn before retreating equatorward during the late fall and early spring. The equatorward migration of the subtropical ridge during the cold season is due to increasing north-south temperature differences between the poles and tropics; the latitudinal movement of the subtropical ridge is correlated with the progression of the monsoon trough or Intertropical Convergence Zone. Most tropical cyclones form on the side of the subtropical ridge closer to the equator move poleward past the ridge axis before recurving into the main belt of the Westerlies; when the subtropical ridge shifts due to ENSO, so will the preferred tropical cyclone tracks.
Intertropical Convergence Zone
The Intertropical Convergence Zone, known by sailors as the doldrums or the calms, is the area encircling Earth near the Equator, where the northeast and southeast trade winds converge. The ITCZ was identified from the 1920s to the 1940s as the "Intertropical Front", but after the recognition in the 1940s and the 1950s of the significance of wind field convergence in tropical weather production, the term ITCZ was applied; when it lies near the Equator, it is called the near-equatorial trough. Where the ITCZ is drawn into and merges with a monsoonal circulation, it is sometimes referred to as a monsoon trough, a usage more common in Australia and parts of Asia. In the seamen's speech, the zone is referred to as the doldrums because of its erratic weather patterns with stagnant calms and violent thunderstorms; the ITCZ appears as a band of clouds thunderstorms, that encircle the globe near the Equator. In the Northern Hemisphere, the trade winds move in a southwestward direction from the northeast, while in the Southern Hemisphere, they move northwestward from the southeast.
When the ITCZ is positioned north or south of the Equator, these directions change according to the Coriolis effect imparted by Earth's rotation. For instance, when the ITCZ is situated north of the Equator, the southeast trade wind changes to a southwest wind as it crosses the Equator; the ITCZ is formed by vertical motion appearing as convective activity of thunderstorms driven by solar heating, which draw air in. The ITCZ is a tracer of the ascending branch of the Hadley cell and is wet; the dry descending branch is the horse latitudes. The location of the ITCZ varies with the seasons corresponding with the location of the thermal equator; as the heat capacity of the oceans is greater than air over land, migration is more prominent over land. Over the oceans, where the convergence zone is better defined, the seasonal cycle is more subtle, as the convection is constrained by the distribution of ocean temperatures. Sometimes, a double ITCZ forms, with one located north and another south of the Equator, one of, stronger than the other.
When this occurs, a narrow ridge of high pressure forms between the two convergence zones. The South Pacific convergence zone is a reverse-oriented, or west-northwest to east-southeast aligned, trough extending from the west Pacific warm pool southeastwards towards French Polynesia, it lies just south of the equator during the Southern Hemisphere warm season, but can be more extratropical in nature east of the International Date Line. It is considered the largest and most important piece of the ITCZ, has the least dependence upon heating from a nearby land mass during the summer than any other portion of the monsoon trough; the southern ITCZ in the southeast Pacific and southern Atlantic, known as the SITCZ, occurs during the Southern Hemisphere fall between 3° and 10° south of the equator east of the 140th meridian west longitude during cool or neutral El Niño–Southern Oscillation patterns. When ENSO reaches its warm phase, otherwise known as El Niño, the tongue of lowered sea surface temperatures due to upwelling off the South American continent disappears, which causes this convergence zone to vanish as well.
Variation in the location of the intertropical convergence zone drastically affects rainfall in many equatorial nations, resulting in the wet and dry seasons of the tropics rather than the cold and warm seasons of higher latitudes. Longer term changes in the intertropical convergence zone can result in severe droughts or flooding in nearby areas. In some cases, the ITCZ may become narrow when it moves away from the equator. There appears to be a 15 to 25-day cycle in thunderstorm activity along the ITCZ, half the wavelength of the Madden–Julian oscillation. Within the ITCZ the average winds are slight, unlike the zones north and south of the equator where the trade winds feed. Early sailors named this belt of calm the doldrums because of the inactivity and stagnation they found themselves in after days of no wind. To find oneself becalmed in this region in a hot and muggy climate could mean death in an era when wind was the only effective way to propel ships across the ocean. Today and competitive sailors attempt to cross the zone as as possible as the erratic weather and wind patterns may cause unexpected delays.
Tropical cyclogenesis depends upon low-level vorticity as one of its six requirements, the ITCZ fills this role as it is a zone of wind change and speed, otherwise known as horizontal wind shear. As the ITCZ migrates to tropical - subtropical latitudes and beyond during the respective hemisphere's summer season, increasing Coriolis force makes the formation of tropical cyclones within this zone more possible. Surges of higher pressure from high latitudes can enhance tropical disturbances along its axis. In the north Atlantic and the northeastern Pacific oceans, tropical waves move along the axis of the ITCZ causing an increase in thunderstorm activity, under weak vertical wind shear, these clusters of thunderstorms can come. Thunderstorms along the Intertropical Convergence Zone played a role in the loss of Air France Flight 447, which left Rio de Janeiro–Galeão International Airport on Sunday, May 31, 2009, at about 7:00 p.m. local time and had been expected to land at Charles de Gaulle Airport near Paris on Monday, June 1, 2009, at 11:15 a.m.
The aircraft crashed with no survivors while flying through a series
In physics, angular momentum is the rotational equivalent of linear momentum. It is an important quantity in physics because it is a conserved quantity—the total angular momentum of a closed system remains constant. In three dimensions, the angular momentum for a point particle is a pseudovector r × p, the cross product of the particle's position vector r and its momentum vector p = mv; this definition can be applied to each point in physical fields. Unlike momentum, angular momentum does depend on where the origin is chosen, since the particle's position is measured from it. Just like for angular velocity, there are two special types of angular momentum: the spin angular momentum and the orbital angular momentum; the spin angular momentum of an object is defined as the angular momentum about its centre of mass coordinate. The orbital angular momentum of an object about a chosen origin is defined as the angular momentum of the centre of mass about the origin; the total angular momentum of an object is the sum of orbital angular momenta.
The orbital angular momentum vector of a particle is always parallel and directly proportional to the orbital angular velocity vector ω of the particle, where the constant of proportionality depends on both the mass of the particle and its distance from origin. However, the spin angular momentum of the object is proportional but not always parallel to the spin angular velocity Ω, making the constant of proportionality a second-rank tensor rather than a scalar. Angular momentum is additive. For a continuous rigid body, the total angular momentum is the volume integral of angular momentum density over the entire body. Torque can be defined as the rate of change of angular momentum, analogous to force; the net external torque on any system is always equal to the total torque on the system. Therefore, for a closed system, the total torque on the system must be 0, which means that the total angular momentum of the system is constant; the conservation of angular momentum helps explain many observed phenomena, for example the increase in rotational speed of a spinning figure skater as the skater's arms are contracted, the high rotational rates of neutron stars, the Coriolis effect, the precession of gyroscopes.
In general, conservation does limit the possible motion of a system, but does not uniquely determine what the exact motion is. In quantum mechanics, angular momentum is an operator with quantized eigenvalues. Angular momentum is subject to the Heisenberg uncertainty principle, meaning that at any time, only one component can be measured with definite precision; because of this, it turns out that the notion of an elementary particle "spinning" about an axis does not exist. For technical reasons, elementary particles still possess a spin angular momentum, but this angular momentum does not correspond to spinning motion in the ordinary sense. Angular momentum is a vector quantity that represents the product of a body's rotational inertia and rotational velocity about a particular axis. However, if the particle's trajectory lies in a single plane, it is sufficient to discard the vector nature of angular momentum, treat it as a scalar. Angular momentum can be considered a rotational analog of linear momentum.
Thus, where linear momentum p is proportional to mass m and linear speed v, p = m v, angular momentum L is proportional to moment of inertia I and angular speed ω, L = I ω. Unlike mass, which depends only on amount of matter, moment of inertia is dependent on the position of the axis of rotation and the shape of the matter. Unlike linear speed, which does not depend upon the choice of origin, angular velocity is always measured with respect to a fixed origin; therefore speaking, L should be referred to as the angular momentum relative to that center. Because I = r 2 m for a single particle and ω = v r for circular motion, angular momentum can be expanded, L = r 2 m ⋅ v r, reduced to, L = r m v, the product of the radius of rotation r and the linear momentum of the particle p = m v, where v in this case is the equivalent linear speed at the radius; this simple analysis can apply to non-circular motion if only the component of the motion, perpendicular to the radius vector is considered. In that case, L
Edmond Halley, FRS was an English astronomer, mathematician and physicist. He was the second Astronomer Royal in Britain, succeeding John Flamsteed in 1720. From an observatory he constructed on Saint Helena, Halley recorded a transit of Mercury across the Sun, he realised. He used his observations to expand contemporary star maps, he aided in observationally proving Isaac Newton's laws of motion, funded the publication of Newton's influential Philosophiæ Naturalis Principia Mathematica. From his September 1682 observations, he used the laws of motion to compute the periodicity of Halley's Comet in his 1705 Synopsis of the Astronomy of Comets, it was named after him upon its predicted return in 1758. Beginning in 1698, he made sailing expeditions and made observations on the conditions of terrestrial magnetism. In 1718, he discovered the proper motion of the "fixed" stars. Halley was born in east London, his father, Edmond Halley Sr. was a wealthy soap-maker in London. As a child, Halley was interested in mathematics.
He studied at St Paul's School where he developed his initial interest in astronomy, from 1673 at The Queen's College, Oxford. While still an undergraduate, Halley published papers on sunspots. While at the University of Oxford, Halley was introduced to the Astronomer Royal. Influenced by Flamsteed's project to compile a catalog of northern stars, Halley proposed to do the same for the Southern Hemisphere. In 1676, Halley visited the south Atlantic island of Saint Helena and set up an observatory with a large sextant with telescopic sights to catalogue the stars of the Southern Hemisphere. While there he observed a transit of Mercury across the Sun, realised that a similar transit of Venus could be used to determine the absolute size of the Solar System, he returned to England in May 1678. In the following year he went to Danzig on behalf of the Royal Society to help resolve a dispute; because astronomer Johannes Hevelius did not use a telescope, his observations had been questioned by Robert Hooke.
Halley stayed with Hevelius and he observed and verified the quality of Hevelius' observations. In 1679, Halley published the results from his observations on St. Helena as Catalogus Stellarum Australium which included details of 341 southern stars; these additions to contemporary star maps earned him comparison with Tycho Brahe: e.g. "the southern Tycho" as described by Flamsteed. Halley was awarded his M. A. degree at Oxford and elected as a Fellow of the Royal Society at the age of 22. In September 1682 he carried out a series of observations of what became known as Halley's Comet, though his name became associated with it because of his work on its orbit and predicting its return in 1758. In 1686, Halley published the second part of the results from his Helenian expedition, being a paper and chart on trade winds and monsoons; the symbols he used to represent trailing winds still exist in most modern day weather chart representations. In this article he identified solar heating as the cause of atmospheric motions.
He established the relationship between barometric pressure and height above sea level. His charts were an important contribution to the emerging field of information visualisation. Halley spent most of his time on lunar observations, but was interested in the problems of gravity. One problem that attracted his attention was the proof of Kepler's laws of planetary motion. In August 1684, he went to Cambridge to discuss this with Isaac Newton, much as John Flamsteed had done four years earlier, only to find that Newton had solved the problem, at the instigation of Flamsteed with regard to the orbit of comet Kirch, without publishing the solution. Halley asked to see the calculations and was told by Newton that he could not find them, but promised to redo them and send them on which he did, in a short treatise entitled, On the motion of bodies in an orbit. Halley recognised the importance of the work and returned to Cambridge to arrange its publication with Newton, who instead went on to expand it into his Philosophiæ Naturalis Principia Mathematica published at Halley's expense in 1687.
Halley's first calculations with comets were thereby for the orbit of comet Kirch, based on Flamsteed's observations in 1680-1. Although he was to calculate the orbit of the comet of 1682, he was inaccurate in his calculations of the orbit of comet Kirch, they indicated a periodicity of 575 years, thus appearing in the years 531 and 1106, heralding the death of Julius Caesar in a like fashion in −44. It is now known to have an orbital period of circa 10,000 years. In 1691, Halley built a diving bell, a device in which the atmosphere was replenished by way of weighted barrels of air sent down from the surface. In a demonstration and five companions dived to 60 feet in the River Thames, remained there for over an hour and a half. Halley's bell was of little use for practical salvage work, as it was heavy, but he made improvements to it over time extending his underwater exposure time to over 4 hours. Halley suffered one of the earliest recorded cases of middle ear barotrauma; that same year, at a meeting of the Royal Society, Halley introduced a rudimentary working model of a magnetic compass using a liquid-filled housing to damp the swing and wobble of the magnetised needle.
In 1691, Halley sought the post of Savilian Professor of Astronomy at Oxford. While a candidate for the position, Halley faced the animosity of th
International Standard Book Number
The International Standard Book Number is a numeric commercial book identifier, intended to be unique. Publishers purchase ISBNs from an affiliate of the International ISBN Agency. An ISBN is assigned to each variation of a book. For example, an e-book, a paperback and a hardcover edition of the same book would each have a different ISBN; the ISBN is 13 digits long if assigned on or after 1 January 2007, 10 digits long if assigned before 2007. The method of assigning an ISBN is nation-based and varies from country to country depending on how large the publishing industry is within a country; the initial ISBN identification format was devised in 1967, based upon the 9-digit Standard Book Numbering created in 1966. The 10-digit ISBN format was developed by the International Organization for Standardization and was published in 1970 as international standard ISO 2108. Published books sometimes appear without an ISBN; the International ISBN agency sometimes assigns such books ISBNs on its own initiative.
Another identifier, the International Standard Serial Number, identifies periodical publications such as magazines and newspapers. The International Standard Music Number covers musical scores; the Standard Book Numbering code is a 9-digit commercial book identifier system created by Gordon Foster, Emeritus Professor of Statistics at Trinity College, for the booksellers and stationers WHSmith and others in 1965. The ISBN identification format was conceived in 1967 in the United Kingdom by David Whitaker and in 1968 in the United States by Emery Koltay; the 10-digit ISBN format was developed by the International Organization for Standardization and was published in 1970 as international standard ISO 2108. The United Kingdom continued to use the 9-digit SBN code until 1974. ISO has appointed the International ISBN Agency as the registration authority for ISBN worldwide and the ISBN Standard is developed under the control of ISO Technical Committee 46/Subcommittee 9 TC 46/SC 9; the ISO on-line facility only refers back to 1978.
An SBN may be converted to an ISBN by prefixing the digit "0". For example, the second edition of Mr. J. G. Reeder Returns, published by Hodder in 1965, has "SBN 340 01381 8" – 340 indicating the publisher, 01381 their serial number, 8 being the check digit; this can be converted to ISBN 0-340-01381-8. Since 1 January 2007, ISBNs have contained 13 digits, a format, compatible with "Bookland" European Article Number EAN-13s. An ISBN is assigned to each variation of a book. For example, an ebook, a paperback, a hardcover edition of the same book would each have a different ISBN; the ISBN is 13 digits long if assigned on or after 1 January 2007, 10 digits long if assigned before 2007. An International Standard Book Number consists of 4 parts or 5 parts: for a 13-digit ISBN, a prefix element – a GS1 prefix: so far 978 or 979 have been made available by GS1, the registration group element, the registrant element, the publication element, a checksum character or check digit. A 13-digit ISBN can be separated into its parts, when this is done it is customary to separate the parts with hyphens or spaces.
Separating the parts of a 10-digit ISBN is done with either hyphens or spaces. Figuring out how to separate a given ISBN is complicated, because most of the parts do not use a fixed number of digits. ISBN is most used among others special identifiers to describe references in Wikipedia and can help to find the same sources with different description in various language versions. ISBN issuance is country-specific, in that ISBNs are issued by the ISBN registration agency, responsible for that country or territory regardless of the publication language; the ranges of ISBNs assigned to any particular country are based on the publishing profile of the country concerned, so the ranges will vary depending on the number of books and the number and size of publishers that are active. Some ISBN registration agencies are based in national libraries or within ministries of culture and thus may receive direct funding from government to support their services. In other cases, the ISBN registration service is provided by organisations such as bibliographic data providers that are not government funded.
A full directory of ISBN agencies is available on the International ISBN Agency website. Partial listing: Australia: the commercial library services agency Thorpe-Bowker.
The lapse rate is the rate at which an atmospheric variable temperature in Earth's atmosphere, changes with altitude. Lapse rate arises in the sense of a gradual change, it corresponds to the vertical component of the spatial gradient of temperature. Although this concept is most applied to the Earth's troposphere, it can be extended to any gravitationally supported parcel of gas. A formal definition from the Glossary of Meteorology is: The decrease of an atmospheric variable with height, the variable being temperature unless otherwise specified. In general, a lapse rate is the negative of the rate of temperature change with altitude change, thus: Γ = − d T d z where Γ is the lapse rate given in units of temperature divided by units of altitude, T is temperature, z is altitude; the temperature profile of the atmosphere is a result of an interaction between radiation and convection. Sunlight heats it; the ground heats the air at the surface. If radiation were the only way to transfer heat from the ground to space, the greenhouse effect of gases in the atmosphere would keep the ground at 333 K, the temperature would decay exponentially with height.
However, when air is hot, it tends to expand. Thus, hot air tends to transfer heat upward; this is the process of convection. Convection comes to equilibrium when a parcel of air at a given altitude has the same density as the other air at the same elevation; when a parcel of air expands, it pushes on the air around it. Since the parcel does work but gains no heat, it loses internal energy so that its temperature decreases; the process of expanding and contracting without exchanging heat is an adiabatic process. The term adiabatic means that no heat transfer occurs out of the parcel. Air has low thermal conductivity, the bodies of air involved are large, so transfer of heat by conduction is negligibly small; the adiabatic process for air has a characteristic temperature-pressure curve, so the process determines the lapse rate. When the air contains little water, this lapse rate is known as the dry adiabatic lapse rate: the rate of temperature decrease is 9.8 °C/km. The reverse occurs for a sinking parcel of air.
Only the troposphere in the Earth's atmosphere undergoes convection: the stratosphere does not convect. However, some exceptionally energetic convection processes—notably volcanic eruption columns and overshooting tops associated with severe supercell thunderstorms—may locally and temporarily inject convection through the tropopause and into the stratosphere; these calculation use a simple model of an atmosphere, either dry or moist, within a still vertical column at equilibrium. Thermodynamics defines an adiabatic process as: P d V = − V d P γ the first law of thermodynamics can be written as m c v d T − V d P γ = 0 Also, since α = V / m and γ = c p / c v, we can show that: c p d T − α d P = 0 where c p is the specific heat at constant pressure and α is the specific volume. Assuming an atmosphere in hydrostatic equilibrium: d P = − ρ g d z where g is the standard gravity and ρ is the density. Combining these two equations to eliminate the pressure, one arrives at the result for the dry adiabatic lapse rate, Γ d = − d T d z = g c p = 9.8 ∘ C / km The presence of water within the atmosphere complicates the process of convection.
Water vapor contains latent heat of vaporization. As a parcel of air rises and cools, it becomes saturated. With further decrease in temperature the water vapor in excess of the equilibrium amount condenses, forming cloud, releasing heat. Before saturation, the rising air follows the dry adiabatic lapse rate. After saturation, the rising air follows the moist adiabatic lapse rate; the release of latent heat is an important source of energy in the development of thunderstorms. While the dry adiabatic lap
In meteorology, an air mass is a volume of air defined by its temperature and water vapor content. Air masses cover many hundreds or thousands of miles, adapt to the characteristics of the surface below them, they are classified according to their continental or maritime source regions. Colder air masses are termed arctic, while warmer air masses are deemed tropical. Continental and superior air masses are dry while monsoon air masses are moist. Weather fronts separate air masses with different density characteristics. Once an air mass moves away from its source region, underlying vegetation and water bodies can modify its character. Classification schemes tackle an air mass' characteristics, as well as modification; the Bergeron classification is the most accepted form of air mass classification, though others have produced more refined versions of this scheme over different regions of the globe. Air mass classification involves three letters; the first letter describes its moisture properties, with c used for continental air masses and m for maritime air masses.
Its source region: T for Tropical, P for Polar, A for arctic or Antarctic, M for monsoon, E for Equatorial, S for superior air. For instance, an air mass originating over the desert southwest of the United States in summer may be designated "cT". An air mass originating over northern Siberia in winter may be indicated as "cA"; the stability of an air mass may be shown using a third letter, either "k" or "w". An example of this might be a polar air mass blowing over the Gulf Stream, denoted as "cPk". One may encounter the use of an apostrophe or "degree tick" denoting that a given air mass having the same notation as another it is replacing is colder than the replaced air mass. For example, a series of fronts over the Pacific might show an air mass denoted mPk followed by another denoted mPk'. Another convention utilizing these symbols is the indication of modification or transformation of one type to another. For instance, an Arctic air mass blowing out over the Gulf of Alaska may be shown as "cA-mPk".
Yet another convention indicates the layering of air masses in certain situations. For instance, the overrunning of a polar air mass by an air mass from the Gulf of Mexico over the Central United States might be shown with the notation "mT/cP". Tropical and equatorial air masses are hot; those that develop over land are drier and hotter than those that develop over oceans, travel poleward on the western periphery of the subtropical ridge. Maritime tropical air masses are sometimes referred to as trade air masses. Maritime tropical air masses that affect the United States originate in the Caribbean Sea, southern Gulf of Mexico, tropical Atlantic east of Florida through the Bahamas. Monsoon air masses are unstable. Superior air masses are dry, reach the ground, they reside over maritime tropical air masses, forming a warmer and drier layer over the more moderate moist air mass below, forming what is known as a trade wind inversion over the maritime tropical air mass. Continental Polar air masses are air masses that are cold and dry due to their continental source region.
Continental polar air masses that affect North America form over interior Canada. Continental Tropical air masses are a type of tropical air produced by the subtropical ridge over large areas of land and originate from low-latitude deserts such as the Sahara Desert in northern Africa, the major source of these air masses. Other less important sources producing cT air masses are the Arabian Peninsula, the central arid/semi-arid part of Australia and deserts lying in the Southwestern United States. Continental tropical air masses are hot and dry. Arctic and polar air masses are cold; the qualities of arctic air are developed over snow-covered ground. Arctic air is cold, colder than polar air masses. Arctic air can be shallow in the summer, modify as it moves equatorward. Polar air masses develop over higher latitudes over the land or ocean, are stable, shallower than arctic air. Polar air over the ocean loses its stability. A weather front is a boundary separating two masses of air of different densities, is the principal cause of meteorological phenomena.
In surface weather analyses, fronts are depicted using various colored lines and symbols, depending on the type of front. The air masses separated by a front differ in temperature and humidity. Cold fronts may feature narrow bands of thunderstorms and severe weather, may on occasion be preceded by squall lines or dry lines. Warm fronts are preceded by stratiform precipitation and fog; the weather clears after a front's passage. Some fronts produce no precipitation and little cloudiness, although there is invariably a wind shift. Cold fronts and occluded fronts move from west to east, while warm fronts move poleward; because of the greater density of air in their wake, cold fronts and cold occlusions move faster than warm fronts and warm occlusions. Mountains and warm bodies of water can slow the movement of fronts; when a front becomes stationary, the density contrast across the frontal boundary vanishes, the front can degenerate into a line which separates regions of differing wind velocity, known as a sh