A firestorm is a conflagration which attains such intensity that it creates and sustains its own wind system. It is most a natural phenomenon, created during some of the largest bushfires and wildfires. Although the term has been used to describe certain large fires, the phenomenon's determining characteristic is a fire with its own storm-force winds from every point of the compass; the Black Saturday bushfires and the Great Peshtigo Fire are possible examples of forest fires with some portion of combustion due to a firestorm, as is the Great Hinckley Fire. Firestorms have occurred in cities as a deliberate effect of targeted explosives, such as occurred as a result of the aerial firebombings of Hamburg, firebombing of Tokyo and the atomic bombing of Hiroshima. A firestorm is created as a result of the stack effect as the heat of the original fire draws in more and more of the surrounding air; this draft can be increased if a low-level jet stream exists over or near the fire. As the updraft mushrooms, strong inwardly-directed gusty winds develop around the fire, supplying it with additional air.
This would seem to prevent the firestorm from spreading on the wind, but the tremendous turbulence created may cause the strong surface inflow winds to change direction erratically. Firestorms resulting from the bombardment of urban areas in the Second World War were confined to the areas seeded with incendiary devices, the firestorm did not appreciably spread outward. A firestorm may develop into a mesocyclone and induce true tornadoes/fire whirls; this occurred with the 2002 Durango fire, with the much greater Peshtigo Fire. The greater draft of a firestorm draws in greater quantities of oxygen, which increases combustion, thereby substantially increasing the production of heat; the intense heat of a firestorm manifests as radiated heat, which may ignite flammable material at a distance ahead of the fire itself. This serves to expand the area and the intensity of the firestorm. Violent, erratic wind drafts suck movables into the fire and as is observed with all intense conflagrations, radiated heat from the fire can melt asphalt, some metals, glass, turn street tarmac into flammable hot liquid.
The high temperatures ignite anything that might burn, until the firestorm runs low on fuel. A firestorm does not appreciably ignite material at a distance ahead of itself. During the formation of a firestorm many fires merge to form a single convective column of hot gases rising from the burning area and strong, fire-induced, radial winds are associated with the convective column, thus the fire front is stationary and the outward spread of fire is prevented by the in-rushing wind. A firestorm is characterized by strong to gale-force winds blowing toward the fire, everywhere around the fire perimeter, an effect, caused by the buoyancy of the rising column of hot gases over the intense mass fire, drawing in cool air from the periphery; these winds from the perimeter blow the fire brands into the burning area and tend to cool the unignited fuel outside the fire area so that ignition of material outside the periphery by radiated heat and fire embers is more difficult, thus limiting fire spread.
At Hiroshima, this inrushing to feed the fire is said to have prevented the firestorm perimeter from expanding, thus the firestorm was confined to the area of the city damaged by the blast. Large wildfire conflagrations are distinct from firestorms if they have moving fire fronts which are driven by the ambient wind and do not develop their own wind system like true firestorms. Furthermore, non-firestorm conflagrations can develop from a single ignition, whereas firestorms have only been observed where large numbers of fires are burning over a large area, with the important caveat that the density of burning fires needs to be above a critical threshold for a firestorm to form; the high temperatures within the firestorm zone ignite most everything that might burn, until a tipping point is reached, that is, upon running low on fuel, which occurs after the firestorm has consumed so much of the available fuel within the firestorm zone that the necessary fuel density required to keep the firestorm's wind system active drops below the threshold level, at which time the firestorm breaks up into isolated conflagrations.
In Australia, the prevalence of eucalyptus trees that have oil in their leaves results in forest fires that are noted for their tall and intense flame front. Hence the bush fires appear more as a firestorm than a simple forest fire. Sometimes, emission of combustible gases from swamps has a similar effect. For instance, methane explosions enforced the Peshtigo Fire. Firestorms will produce hot buoyant smoke clouds of water vapor that will form condensation clouds as it enters the cooler upper atmosphere, generating what is known as pyrocumulus clouds or, if large enough, pyrocumulonimbus clouds. For example, the black rain that began to fall 20 minutes after the atomic bombing of Hiroshima produced in total 5–10
Spring is one of the four temperate seasons, following winter and preceding summer. There are various technical definitions of spring, but local usage of the term varies according to local climate and customs; when it is spring in the Northern Hemisphere, it is autumn in the Southern Hemisphere and vice versa. At the spring equinox and nights are twelve hours long, with day length increasing and night length decreasing as the season progresses. Spring and "springtime" refer to the season, to ideas of rebirth, renewal and regrowth. Subtropical and tropical areas have climates better described in terms of other seasons, e.g. dry or wet, monsoonal or cyclonic. Cultures may have local names for seasons which have little equivalence to the terms originating in Europe. Meteorologists define four seasons in many climatic areas: spring, summer and winter; these are demarcated by the values of their average temperatures on a monthly basis, with each season lasting three months. The three warmest months are by definition summer, the three coldest months are winter and the intervening gaps are spring and autumn.
Spring, when defined in this manner, can start on different dates in different regions. Thus, in the US and UK, spring months are March and May, while in New Zealand and Australia, spring conventionally begins on September 1 and ends November 30. Swedish meteorologists define the beginning of spring as the first occasion on which the average daytime temperature exceeds zero degrees Celsius for seven consecutive days, thus the date varies with latitude and elevation. In some cultures in the Northern Hemisphere, the astronomical vernal equinox is taken to mark the first day of spring, the summer solstice is taken as the first day of summer. In Persian culture the first day of spring is the first day of the first month which begins on 20 or 21 March. In other traditions, the equinox is taken as mid-spring. In the traditional Chinese calendar, the "spring" season consists of the days between Lichun, taking Chunfen as its midpoint ending at Lixia. According to the Celtic tradition, based on daylight and the strength of the noon sun, spring begins in early February and continues until early May.
The beginning of spring is not always determined by fixed calendar dates. The phenological or ecological definition of spring relates to biological indicators, such as the blossoming of a range of plant species, the activities of animals, the special smell of soil that has reached the temperature for micro flora to flourish; these indicators, along with the beginning of spring, vary according to the local climate and according to the specific weather of a particular year. Most ecologists divide the year into six seasons. In addition to spring, ecological reckoning identifies an earlier separate prevernal season between the hibernal and vernal seasons; this is a time when only the hardiest flowers like the crocus are in bloom, sometimes while there is still some snowcover on the ground. During early spring, the axis of the Earth is increasing its tilt relative to the Sun, the length of daylight increases for the relevant hemisphere; the hemisphere begins to warm causing new plant growth to "spring forth," giving the season its name.
Any snow begins to melt, swelling streams with runoff and any frosts become less severe. In climates that have no snow, rare frosts and ground temperatures increase more rapidly. Many flowering plants bloom at this time of year, in a long succession, sometimes beginning when snow is still on the ground and continuing into early summer. In snowless areas, "spring" may begin as early as February or August, heralded by the blooming of deciduous magnolias and quince. Many temperate areas have a dry spring, wet autumn, which brings about flowering in this season, more consistent with the need for water, as well as warmth. Subarctic areas may not experience "spring" at all until May. While spring is a result of the warmth caused by the changing orientation of the Earth's axis relative to the Sun, the weather in many parts of the world is affected by other, less predictable events; the rainfall in spring follows trends more related to longer cycles—such as the solar cycle—or events created by ocean currents and ocean temperatures—for example, the El Niño effect and the Southern Oscillation Index.
Unstable spring weather may occur more when warm air begins to invade from lower latitudes, while cold air is still pushing from the Polar regions. Flooding is most common in and near mountainous areas during this time of year, because of snow-melt, accelerated by warm rains. In North America, Tornado Alley is most active at this time of year since the Rocky Mountains prevent the surging hot and cold air masses from spreading eastward, instead force them into direct conflict. Besides tornadoes, supercell thunderstorms can produce dangerously large hail and high winds, for which a severe thunderstorm warning or tornado warning is issued. More so than in winter, the jet streams play an important role in unstable and severe Northern Hemisphere weather in springtime. In recent decades, season creep has been observed, which means that many phenological signs of spring are occurring earlier in many regions by around two days per decade. Spring in the Southern Hemisphere is different in several significant ways to that of the Northern Hemisphere
A season is a division of the year marked by changes in weather and amount of daylight. On Earth, seasons result from Earth's orbit around the Sun and Earth's axial tilt relative to the ecliptic plane. In temperate and polar regions, the seasons are marked by changes in the intensity of sunlight that reaches the Earth's surface, variations of which may cause animals to undergo hibernation or to migrate, plants to be dormant. Various cultures define the nature of seasons based on regional variations. During May and July, the Northern Hemisphere is exposed to more direct sunlight because the hemisphere faces the Sun; the same is true of the Southern Hemisphere in November and January. It is Earth's axial tilt that causes the Sun to be higher in the sky during the summer months, which increases the solar flux. However, due to seasonal lag, June and August are the warmest months in the Northern Hemisphere while December and February are the warmest months in the Southern Hemisphere. In temperate and subpolar regions, four seasons based on the Gregorian calendar are recognized: spring, autumn or fall, winter.
The definition of seasons is cultural. In India from the ancient times, six seasons or Ritu based on south Asian religious or cultural calendars are recognised and identified today for the purposes such as agriculture and trade. Ecologists use a six-season model for temperate climate regions which are not tied to any fixed calendar dates: prevernal, estival, serotinal and hibernal. Many tropical regions have monsoon season and the dry season; some have a third mild, or harmattan season. Seasons held special significance for agrarian societies, whose lives revolved around planting and harvest times, the change of seasons was attended by ritual. In some parts of the world, some other "seasons" capture the timing of important ecological events such as hurricane season, tornado season, wildfire season; the most important of these are the three seasons—flood and low water—which were defined by the former annual flooding of the Nile in Egypt. The seasons result from the Earth's axis of rotation being tilted with respect to its orbital plane by an angle of 23.4 degrees.
Regardless of the time of year, the northern and southern hemispheres always experience opposite seasons. This is because during summer or winter, one part of the planet is more directly exposed to the rays of the Sun than the other, this exposure alternates as the Earth revolves in its orbit. For half of the year, the Northern Hemisphere tips toward the Sun, with the maximum amount occurring on about June 21. For the other half of the year, the same happens, but in the Southern Hemisphere instead of the Northern, with the maximum around December 21; the two instants when the Sun is directly overhead at the Equator are the equinoxes. At that moment, both the North Pole and the South Pole of the Earth are just on the terminator, hence day and night are divided between the two hemispheres. Around the March equinox, the Northern Hemisphere will be experiencing spring as the hours of daylight increase, the Southern Hemisphere is experiencing autumn as daylight hours shorten; the effect of axial tilt is observable as the change in day length and altitude of the Sun at solar noon during the year.
The low angle of Sun during the winter months means that incoming rays of solar radiation are spread over a larger area of the Earth's surface, so the light received is more indirect and of lower intensity. Between this effect and the shorter daylight hours, the axial tilt of the Earth accounts for most of the seasonal variation in climate in both hemispheres. Compared to axial tilt, other factors contribute little to seasonal temperature changes; the seasons are not the result of the variation in Earth's distance to the Sun because of its elliptical orbit. In fact, Earth reaches perihelion in January, it reaches aphelion in July, so the slight contribution of orbital eccentricity opposes the temperature trends of the seasons in the Northern Hemisphere. In general, the effect of orbital eccentricity on Earth's seasons is a 7% variation in sunlight received. Orbital eccentricity can influence temperatures, but on Earth, this effect is small and is more than counteracted by other factors; this is because the Northern Hemisphere has more land than the Southern, land warms more than sea.
Any noticeable intensification of southern winters and summers due to Earth's elliptical orbit is mitigated by the abundance of water in the Southern Hemisphere. Seasonal weather fluctuations depend on factors such as proximity to oceans or other large bodies of water, currents in those oceans, El Niño/ENSO and other oceanic cycles, prevailing winds. In the temperate and polar regions, seasons are marked by changes in the amount of sunlight, which in turn causes cycles of dormancy in plants and hibernation in animals; these effects vary with proximity to bodies of water. For example, the South Pole is in the middle of the continent of Antarctica and therefore a considerable distance from the moderating influence of the southern oceans; the North Pole is in the Arctic Ocean, thus its temperature extremes are buffered by the water. The result is that the South Pole is colder during the southern winter than the North Pole dur
A waterspout is an intense columnar vortex that occurs over a body of water. Some are connected to a cumulus congestus cloud, some to a cumuliform cloud and some to a cumulonimbus cloud. In the common form, it is a non-supercell tornado over water. While it is weaker than most of its land counterparts, stronger versions spawned by mesocyclones do occur. Most waterspouts do not suck up water. While waterspouts form in the tropics and subtropical areas, other areas report waterspouts, including Europe, New Zealand, the Great Lakes, Antarctica and on rare occasions, the Great Salt Lake; some found on the East Coast of the United States, the coast of California. Although rare, waterspouts have been observed in connection with lake-effect snow precipitation bands. Waterspouts have a five-part life cycle: formation of a dark spot on the water surface, spiral pattern on the water surface, formation of a spray ring, development of the visible condensation funnel, decay. Waterspouts exist on a microscale.
The cloud from which they develop can be as innocuous as a moderate cumulus, or as great as a supercell. While some waterspouts are strong and tornadic in nature, most are much weaker and caused by different atmospheric dynamics, they develop in moisture-laden environments as their parent clouds are in the process of development, it is theorized they spin as they move up the surface boundary from the horizontal shear near the surface, stretch upwards to the cloud once the low level shear vortex aligns with a developing cumulus cloud or thunderstorm. Some weak tornadoes, known as landspouts, have been shown to develop in a similar manner. More than one waterspout can occur in the same vicinity at the same time; as many as nine simultaneous waterspouts have been reported on Lake Michigan. Waterspouts that are not associated with a rotating updraft of a supercell thunderstorm are known as "non-tornadic" or "fair-weather waterspouts", are by far the most common type. Fair-weather waterspouts occur in coastal waters and are associated with dark, flat-bottomed, developing convective cumulus towers.
Waterspouts of this type develop and dissipate, having life cycles shorter than 20 minutes. They rate no higher than EF0 on the Enhanced Fujita scale exhibiting winds of less than 30 m/s, they are most seen in tropical and sub-tropical climates, with upwards of 400 per year observed in the Florida Keys. They move if at all, since the cloud to which they are attached is horizontally static, being formed by vertical convective action instead of the subduction/adduction interaction between colliding fronts. Fair-weather waterspouts are similar in both appearance and mechanics to landspouts, behave as such if they move ashore. "Tornadic waterspouts" accurately referred to as "tornadoes over water", are formed from mesocyclones in a manner identical to land-based tornadoes in connection with severe thunderstorms, but occurring over water. A tornado which travels from land to a body of water would be considered a tornadic waterspout. Since the vast majority of mesocyclonic thunderstorms occur in land-locked areas of the United States, true tornadic waterspouts are correspondingly rarer than their fair-weather counterparts in that country.
However, in some areas, such as the Adriatic and Ionian seas, tornadic waterspouts can make up half of the total number. A winter waterspout known as a snow devil, an icespout, an ice devil, a snownado, or a snowspout, is an rare instance of a waterspout forming under the base of a snow squall; the term "winter waterspout" is used to differentiate between the common warm season waterspout and this rare winter season event. Little is known about this phenomenon and only six known pictures of this event exist to date, four of which were taken in Ontario, Canada. There are a couple of critical criteria for the formation of a winter waterspout. Cold temperatures need to be present over a body of water warm enough to produce fog resembling steam above the water's surface. Like the more efficient lake-effect snow events, winds focusing down the axis of long lakes enhance wind convergence and enhance their development. Though the majority of waterspouts occur in the tropics, they can seasonally appear in temperate areas throughout the world, are common across the western coast of Europe as well as the British Isles and several areas of the Mediterranean and Baltic Sea.
They are not restricted to saltwater. Waterspouts are common on the Great Lakes during late summer and early fall, with a record 66+ waterspouts reported over just a seven-day period in 2003, they are more frequent within 100 kilometers from the coast than farther out at sea. Waterspouts are common along the southeast U. S. coast off southern Florida and the Keys and can happen over seas and lakes worldwide. 160 waterspouts are reported per year across Europe, with the Netherlands reporting the most at 60, followed by Spain and Italy at 25, the United Kingdom at 15. They are most common in late summer. In the Northern Hemisphere, September has been pinpointed as the prime month of formation. Waterspouts are observed off the east coast of Australia, with several being described by Joseph Banks during the voyage of the Endeavour in 1770. There are five stages to the waterspout life cycle. A prominent circular, light-colored d
A dust devil is a strong, well-formed, long-lived whirlwind, ranging from small to large. The primary vertical motion is upward. Dust devils are harmless, but can on rare occasions grow large enough to pose a threat to both people and property, they are comparable to tornadoes in that both are a weather phenomenon involving a vertically oriented rotating column of wind. Most tornadoes are associated with a larger parent circulation, the mesocyclone on the back of a supercell thunderstorm. Dust devils form as a swirling updraft under sunny conditions during fair weather coming close to the intensity of a tornado. Dust devils form when a pocket of hot air near the surface rises through cooler air above it, forming an updraft. If conditions are just right, the updraft may begin to rotate; as the air rises, the column of hot air is stretched vertically, thereby moving mass closer to the axis of rotation, which causes intensification of the spinning effect by conservation of angular momentum. The secondary flow in the dust devil causes other hot air to speed horizontally inward to the bottom of the newly forming vortex.
As more hot air rushes in toward the developing vortex to replace the air, rising, the spinning effect becomes further intensified and self-sustaining. A dust devil formed, is a funnel-like chimney through which hot air moves, both upwards and in a circle; as the hot air rises, it cools, loses its buoyancy and ceases to rise. As it rises, it displaces air; this cool air returning acts as a balance against the spinning hot-air outer wall and keeps the system stable. The spinning effect, along with surface friction will produce a forward momentum; the dust devil is able to sustain itself longer by moving over nearby sources of hot surface air. As available hot air near the surface is channeled up the dust devil surrounding cooler air will be sucked in. Once this occurs, the effect is dramatic, the dust devil dissipates in seconds; this occurs when the dust devil is not moving fast enough or begins to enter a terrain where the surface temperatures are cooler. Certain conditions increase the likelihood of dust devil formation.
Flat barren terrain, desert or tarmac: Flat conditions increase the likelihood of the hot-air "fuel" being a near constant. Dusty or sandy conditions will cause particles to become caught up in the vortex, making the dust devil visible, but are not necessary for the formation of the vortex. Clear skies or cloudy conditions: The surface needs to absorb significant amounts of solar energy to heat the air near the surface and create ideal dust devil conditions. Light or no wind and cool atmospheric temperature: The underlying factor for sustainability of a dust devil is the extreme difference in temperature between the near-surface air and the atmosphere. Windy conditions will destabilize the spinning effect of a dust devil. On Earth, many dust devils are small and weak less than 3 feet in diameter with maximum winds averaging about 45 miles per hour, they dissipate less than a minute after forming. On rare occasions, a dust devil can grow large and intense, sometimes reaching a diameter of up to 300 feet with winds in excess of 60 mph and can last for upwards of 20 minutes before dissipating.
Dust devils do not cause injuries, but rare, severe dust devils have caused damage and deaths in the past. One such dust devil struck the Coconino County Fairgrounds in Flagstaff, Arizona, on September 14, 2000, causing extensive damage to several temporary tents and booths, as well as some permanent fairgrounds structures. Several injuries were reported. Based on the degree of damage left behind, it is estimated that the dust devil produced winds as high as 75 mph, equivalent to an EF-0 tornado. On May 19, 2003, a dust devil lifted the roof off a two-story building in Lebanon, causing it to collapse and kill a man inside. In East El Paso, Texas in 2010, three children in an inflatable jump house were picked up by a dust devil and lifted over 10 feet, traveling over a fence and landing in a backyard three houses away. In Commerce City, Colorado in 2018, a powerful dust devil hurtled two porta-potties into the air. No one was injured in the incident. In 2019 a large dust devil in Yucheng county, Henan province, China killed 2 children and injured 18 children and 2 adults when a bouncy castle was lifted into the air.
Dust devils have been implicated in around 100 aircraft accidents. While many incidents have been simple taxiing problems, a few have had fatal consequences. Dust devils are considered major hazards among skydivers and paragliding pilots as they can cause a parachute or a glider to collapse with little to no warning, at altitudes considered too low to cut away, contribute to the serious injury or death of parachutists. There is an endemic disease in some arid areas, such as the southwestern United States, northwestern Mexico, Central America, South America, called Valley fever. “Cocci” fungus grows in alkaline soil and the fungus spores that lie dormant and can be picked up by dust devils, are blown around. When a person or animal breathes these spores in, they cause fungal pneumonia. While not a serious threat, this disease is still dangerous to some people and animals if they are old or young. Dust devils small ones, can produce radio noise and electrical fields greater than 10,000 volts per meter.
A dust devil picks up small dust particles. As the particles
Extratropical cyclones, sometimes called mid-latitude cyclones or wave cyclones, are low-pressure areas which, along with the anticyclones of high-pressure areas, drive the weather over much of the Earth. Extratropical cyclones are capable of producing anything from cloudiness and mild showers to heavy gales, thunderstorms and tornadoes; these types of cyclones are defined as large scale low pressure weather systems that occur in the middle latitudes of the Earth. In contrast with tropical cyclones, extratropical cyclones produce rapid changes in temperature and dew point along broad lines, called weather fronts, about the center of the cyclone; the term "cyclone" applies to numerous types of low pressure areas, one of, the extratropical cyclone. The descriptor extratropical signifies that this type of cyclone occurs outside the tropics and in the middle latitudes of Earth between 30° and 60° latitude, they are termed mid-latitude cyclones if they form within those latitudes, or post-tropical cyclones if a tropical cyclone has intruded into the mid latitudes.
Weather forecasters and the general public describe them as "depressions" or "lows". Terms like frontal cyclone, frontal depression, frontal low, extratropical low, non-tropical low and hybrid low are used as well. Extratropical cyclones are classified as baroclinic, because they form along zones of temperature and dewpoint gradient known as frontal zones, they can become barotropic late in their life cycle, when the distribution of heat around the cyclone becomes uniform with its radius. Extratropical cyclones form anywhere within the extratropical regions of the Earth, either through cyclogenesis or extratropical transition. 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 234 significant extratropical cyclones form each winter. Extratropical cyclones form along linear bands of temperature/dewpoint gradient with significant vertical wind shear, are thus classified as baroclinic cyclones.
Cyclogenesis, or low pressure formation, occurs along frontal zones near a favorable quadrant of a maximum in the upper level jetstream known as a jet streak. The favorable quadrants are at the right rear and left front quadrants, where divergence ensues; the divergence causes air to rush out from the top of the air column. As mass in the column is reduced, atmospheric pressure at surface level is reduced; the lowered pressure strengthens the cyclone. The lowered pressure acts creating convergence in the low-level wind field. Low-level convergence and upper-level divergence imply upward motion within the column, making cyclones tend to be cloudy; as the cyclone strengthens, the cold front sweeps towards the equator and moves around the back of the cyclone. Meanwhile, its associated warm front progresses more as the cooler air ahead of the system is denser, therefore more difficult to dislodge; the cyclones occlude as the poleward portion of the cold front overtakes a section of the warm front, forcing a tongue, or trowal, of warm air aloft.
The cyclone will become barotropically cold and begin to weaken. Atmospheric pressure can fall rapidly when there are strong upper level forces on the system; when pressures fall more than 1 millibar per hour, the process is called explosive cyclogenesis, the cyclone can be described as a bomb. These bombs drop in pressure to below 980 millibars under favorable conditions such as near a natural temperature gradient like the Gulf Stream, or at a preferred quadrant of an upper level jet streak, where upper level divergence is best; the stronger the upper level divergence over the cyclone, the deeper the cyclone can become. Hurricane-force extratropical cyclones are most to form in the northern Atlantic and northern Pacific oceans in the months of December and January. On 14 and 15 December 1986, an extratropical cyclone near Iceland deepened to below 920 hectopascals, a pressure equivalent to a category 5 hurricane. In the Arctic, the average pressure for cyclones is 980 millibars during the winter, 1,000 millibars during the summer.
Tropical cyclones transform into extratropical cyclones at the end of their tropical existence between 30° and 40° latitude, where there is sufficient forcing from upper-level troughs or shortwaves riding the Westerlies for the process of extratropical transition to begin. During this process, a cyclone in extratropical transition, will invariably form or connect with nearby fronts and/or troughs consistent with a baroclinic system. Due to this, the size of the system will appear to increase, while the core weakens. However, after transition is complete, the storm may re-strengthen due to baroclinic energy, depending on the environmental conditions surrounding the system; the cyclone will distort in shape, becoming less symmetric with time. During extratropical transition, the cyclone begins to tilt back into the colder airmass with height, the cyclone's primary energy source converts from the release of latent heat from condensation to baroclinic processes; the low pressure system loses its warm core and becomes a cold-core system.
The peak time of subtropical cyclogenesis in the North Atlantic is in the months of Septem
An ice storm is a type of winter storm characterized by freezing rain known as a glaze event or, in some parts of the United States, as a silver thaw. The U. S. National Weather Service defines an ice storm as a storm which results in the accumulation of at least 0.25-inch of ice on exposed surfaces. From 1982 to 1994, ice storms were more common than blizzards in the U. S. averaging 16 per year. They are not violent storms but instead are perceived as gentle rains occurring at temperatures just below freezing; the formation of ice begins with a layer of above-freezing air above a layer of sub-freezing temperatures closer to the surface. Frozen precipitation melts to rain while falling into the warm air layer, begins to refreeze in the cold layer below. If the precipitate refreezes while still in the air, it will land on the ground as sleet. Alternatively, the liquid droplets can continue to fall without freezing, passing through the cold air just above the surface; this thin layer of air cools the rain to a temperature below freezing.
However, the drops themselves do not freeze, a phenomenon called supercooling. When the supercooled drops strike ground or anything else below 0 °C, a layer of ice accumulates as the cold water drips off, forming a thickening film of ice, hence freezing rain. While meteorologists can predict when and where an ice storm will occur, some storms still occur with little or no warning. In the United States, most ice storms are in the northeastern part of the country, but damaging storms have occurred farther south. An ice storm in February 1994 resulted in tremendous ice accumulation as far south as Mississippi, caused reported damage in nine states. More timber was damaged than that caused by Hurricane Camille. An ice storm in eastern Washington in November 1996 directly followed heavy snowfall; the combined weight of the snow and 25 to 37 millimeters of ice caused widespread damage and was considered the most severe ice storm in the Spokane area since 1940. The freezing rain from an ice storm covers everything with smooth glaze ice.
In addition to hazardous driving or walking conditions, branches or whole trees may break from the weight of ice. Falling branches can block roads, tear down power and telephone lines, cause other damage. Without falling trees and tree branches, the weight of the ice itself can snap power lines and break and bring down power/utility poles; this can leave people without power for anywhere from several days to a month. According to most meteorologists, just one quarter of an inch of ice accumulation can add about 500 pounds of weight per line span. Damage from ice storms is capable of shutting down entire metropolitan areas. Additionally, the loss of power during ice storms has indirectly caused numerous illnesses and deaths due to unintentional carbon monoxide poisoning. At lower levels, CO poisoning causes symptoms such as nausea, dizziness and headache, but high levels can cause unconsciousness, heart failure, death; the high incidence of CO poisoning during ice storms occurs due to the use of alternative methods of heating and cooking during prolonged power outages, common after severe ice storms.
Gas generators and propane barbecues, kerosene heaters contribute to CO poisoning when they operate in confined locations. CO is produced when appliances burn fuel without enough oxygen present, such as basements and other indoor locations. Loss of electricity during ice storms can indirectly lead to hypothermia and death from hypothermia, it can lead to ruptured pipes due to water freezing inside the pipes. November 26-29, 1921. Worst ice storm in known New England history. 4" of ice hit eastern and central Massachusetts. A ice storm had ice up to 6" thick in northwestern Texas during January 22-24, 1940. A ice storm had ice up to 6" thick in upstate New York during December 29-30, 1942. An ice storm which struck Northern Idaho January 1961 set a record for thickest recorded ice accumulation from a single storm in the United States, at 8 inches. In March 1991, a major ice storm in the area of Rochester, New York caused $375 million in damages, placing it among the worst natural disasters in New York State history.
In February 1994, a severe ice storm caused over $1 billion in damage in the Southern United States in Mississippi and Alabama. The North American ice storm of 1998 occurred during January 5–10, 1998, it was one of the most devastating and costly ice storms in North American history and one of the most devastating ice storms in modern history. Reported ice accumulations of 4" in some areas; the storm caused massive power failures in several large cities on the East Coast of the United States. The most affected area was eastern Ontario and southwestern Quebec in Canada, where over 3 million people were without power for up to a month and a half. Whole trees snapped and electrical pylons were flattened under the weight of the accumulated ice, it caused $340 million dollars of damage in Maine and became one of Canada's costliest natural disasters. The Northeastern United States was impacted by a major ice storm on December 11–12, 2008, which left about 1.25 million homes and businesses without power.
Areas impacted with 3⁄4 to 1 in of ice accumulation included eastern New York in the Albany area and western Massachusetts, southern New Hampshire and south-central Maine, Pennsylvania in the Pocono Mountains region, northwestern Connecticut, southern Vermont. Southern New Hampshire and northernmost Massachusetts got hit the worst with the s