A microburst is an intense small-scale downdraft produced by a thunderstorm or rain shower. There are two types of microbursts: dry microbursts, they go through three stages in their cycle, the downburst and cushion stages called "Suriano's Stroke". A microburst can be dangerous to aircraft during landing, due to the wind shear caused by its gust front. Several fatal and historic crashes have been attributed to the phenomenon over the past several decades, flight crew training goes to great lengths on how to properly recover from a microburst/wind shear event. A microburst has high winds that can knock over grown trees, they last for seconds to minutes. The term was defined by mesoscale meteorology expert Ted Fujita as affecting an area 4 km in diameter or less, distinguishing them as a type of downburst and apart from common wind shear which can encompass greater areas. Fujita coined the term macroburst for downbursts larger than 4 km. A distinction can be made between a wet microburst which consists of precipitation and a dry microburst which consists of virga.
They are formed by precipitation-cooled air rushing to the surface, but they also could be powered by strong winds aloft being deflected toward the surface by dynamical processes in a thunderstorm. When rain falls below the cloud base or is mixed with dry air, it begins to evaporate and this evaporation process cools the air; the cool air accelerates as it approaches the ground. When the cool air approaches the ground, it spreads out in all directions. High winds spread out in this type of pattern showing little or no curvature are known as straight-line winds. Dry microbursts produced by high based thunderstorms that generate little to no surface rainfall, occur in environments characterized by a thermodynamic profile exhibiting an inverted-V at thermal and moisture profile, as viewed on a Skew-T log-P thermodynamic diagram. Wakimoto developed a conceptual model of a dry microburst environment that comprised three important variables: mid-level moisture, a deep and dry adiabatic lapse rate in the sub-cloud layer, low surface relative humidity.
Wet microbursts are downbursts accompanied by significant precipitation at the surface. These downbursts rely more on the drag of precipitation for downward acceleration of parcels as well as the negative buoyancy which tend to drive "dry" microbursts; as a result, higher mixing ratios are necessary for these downbursts to form. Melting of ice hail, appears to play an important role in downburst formation in the lowest 1 km above ground level; these factors, among others, make forecasting wet microbursts difficult. The evolution of microbursts is broken down into three stages: the contact stage, the outburst stage, the cushion stage. Start by using the vertical momentum equation: d w d t = − 1 ρ ∂ p ∂ z − g By decomposing the variables into a basic state and a perturbation, defining the basic states, using the ideal gas law the equation can be written in the form B ≡ − ρ ′ ρ ¯ g = g T v ′ − T ¯ v T ¯ v where B is buoyancy; the virtual temperature correction is rather small and to a good approximation.
The effects of precipitation loading on the vertical motion are parametrized by including a term that decreases buoyancy as the liquid water mixing ratio increases, leading to the final form of the parcel's momentum equation: d w ′ d t = 1 ρ ¯ ∂ p ′ ∂ z + B − g ℓ The first term is the effect of perturbation pressure gradients on vertical motion. In some storms this term has a large effect on updrafts but there is not much reason to believe it has much of an impact on downdrafts and therefore will be ignored; the second term is the effect of buoyancy on vertical motion. In the case of microbursts, one expects to find that B is negative meaning the parcel is cooler than its environment; this cooling takes place as a result of phase changes. Precipitation particles that are small, but are in great quantity, promote a maximum contribution to cooling and, hence, to creation of negative buoyancy; the major contribution to this process is from evaporation. The last term is the effect of water loading.
Whereas evaporation is promoted by large numbers of small droplets, it only requires a few large drops to c
Winter is the coldest season of the year in polar and temperate zones. It occurs before spring in each year. Winter is caused by the axis of the Earth. Different cultures define different dates as the start of winter, some use a definition based on weather; when it is winter in the Northern Hemisphere, it is summer in the Southern Hemisphere, vice versa. In many regions, winter is associated with freezing temperatures; the moment of winter solstice is when the Sun's elevation with respect to the North or South Pole is at its most negative value. The day on which this occurs has the shortest day and the longest night, with day length increasing and night length decreasing as the season progresses after the solstice; the earliest sunset and latest sunrise dates outside the polar regions differ from the date of the winter solstice and these depend on latitude, due to the variation in the solar day throughout the year caused by the Earth's elliptical orbit. The English word "winter" comes from the Proto-Indo-European root "wend," relating to water.
The tilt of the Earth's axis relative to its orbital plane plays a large role in the formation of weather. The Earth is tilted at an angle of 23.44° to the plane of its orbit, causing different latitudes to directly face the Sun as the Earth moves through its orbit. This variation brings about seasons; when it is winter in the Northern Hemisphere, the Southern Hemisphere faces the Sun more directly and thus experiences warmer temperatures than the Northern Hemisphere. Conversely, winter in the Southern Hemisphere occurs when the Northern Hemisphere is tilted more toward the Sun. From the perspective of an observer on the Earth, the winter Sun has a lower maximum altitude in the sky than the summer Sun. During winter in either hemisphere, the lower altitude of the Sun causes the sunlight to hit the Earth at an oblique angle, thus a lower amount of solar radiation strikes the Earth per unit of surface area. Furthermore, the light must travel a longer distance through the atmosphere, allowing the atmosphere to dissipate more heat.
Compared with these effects, the effect of the changes in the distance of the Earth from the Sun is negligible. The manifestation of the meteorological winter in the northerly snow–prone latitudes is variable depending on elevation, position versus marine winds and the amount of precipitation. For instance, within Canada, Winnipeg on the Great Plains, a long way from the ocean, has a January high of −11.3 °C and a low of −21.4 °C. In comparison, Vancouver on the west coast with a marine influence from moderating Pacific winds has a January low of 1.4 °C with days well above freezing at 6.9 °C. Both places are at 49°N latitude, in the same western half of the continent. A similar but less extreme effect is found in Europe: in spite of their northerly latitude, the British Isles have not a single non-mountain weather station with a below-freezing mean January temperature. Meteorological reckoning is the method of measuring the winter season used by meteorologists based on "sensible weather patterns" for record keeping purposes, so the start of meteorological winter varies with latitude.
Winter is defined by meteorologists to be the three calendar months with the lowest average temperatures. This corresponds to the months of December and February in the Northern Hemisphere, June and August in the Southern Hemisphere; the coldest average temperatures of the season are experienced in January or February in the Northern Hemisphere and in June, July or August in the Southern Hemisphere. Nighttime predominates in the winter season, in some regions winter has the highest rate of precipitation as well as prolonged dampness because of permanent snow cover or high precipitation rates coupled with low temperatures, precluding evaporation. Blizzards develop and cause many transportation delays. Diamond dust known as ice needles or ice crystals, forms at temperatures approaching −40 °C due to air with higher moisture from above mixing with colder, surface-based air, they are made of simple hexagonal ice crystals. The Swedish meteorological institute defines winter as when the daily mean temperatures are below 0 °C for five consecutive days.
According to the SMHI, winter in Scandinavia is more pronounced when Atlantic low-pressure systems take more southerly and northerly routes, leaving the path open for high-pressure systems to come in and cold temperatures to occur. As a result, the coldest January on record in Stockholm, in 1987, was the sunniest. Accumulations of snow and ice are associated with winter in the Northern Hemisphere, due to the large land masses there. In the Southern Hemisphere, the more maritime climate and the relative lack of land south of 40°S makes the winters milder. In this region, snow occurs every year in elevated regions such as the Andes, the Great Dividing Range in Australia, the mountains of New Zealand, occurs in the southerly Patagonia region of South Argentina. Snow occurs year-round in Antarctica. In the Northern Hemisphere, some authorities define the period of winter based on astronomical fixed points, regardless of weather conditions. In one version of this definition, winter begins at the winter solstice and ends at the ver
In meteorology, a cloud is an aerosol consisting of a visible mass of minute liquid droplets, frozen crystals, or other particles suspended in the atmosphere of a planetary body or similar space. Water or various other chemicals may compose the crystals. On Earth, clouds are formed as a result of saturation of the air when it is cooled to its dew point, or when it gains sufficient moisture from an adjacent source to raise the dew point to the ambient temperature, they are seen in the Earth's homosphere. Nephology is the science of clouds, undertaken in the cloud physics branch of meteorology. There are two methods of naming clouds in their respective layers of the atmosphere. Cloud types in the troposphere, the atmospheric layer closest to Earth's surface, have Latin names due to the universal adaptation of Luke Howard's nomenclature. Formally proposed in 1802, it became the basis of a modern international system that divides clouds into five physical forms that appear in any or all of three altitude levels.
These physical types, in approximate ascending order of convective activity, include stratiform sheets, cirriform wisps and patches, stratocumuliform layers, cumuliform heaps, large cumulonimbiform heaps that show complex structure. The physical forms are divided by altitude level into ten basic genus-types; the Latin names for applicable high-level genera carry a cirro- prefix, an alto- prefix is added to the names of the mid-level genus-types. Most of the genera can be further subdivided into varieties. Low stratiform clouds that extend down to the Earth's surface are given the common names fog and mist, but have no Latin names. Several clouds that form higher up in the stratosphere and mesosphere have common names for their main types, they are seen infrequently in the polar regions of Earth. Clouds have been observed in the atmospheres of other planets and moons in the Solar System and beyond. However, due to their different temperature characteristics, they are composed of other substances such as methane and sulfuric acid as well as water.
Taken as a whole, homospheric clouds can be cross-classified by form and level to derive the ten tropospheric genera, the fog and mist that forms at surface level, several additional major types above the troposphere. The cumulus genus includes three species. Clouds with sufficient vertical extent to occupy more than one altitude level are classified as low- or mid-level according to the altitude range at which each forms; however they are more informally classified as multi-level or vertical. The origin of the term cloud can be found in the old English clud or clod, meaning a hill or a mass of rock. Around the beginning of the 13th century, the word came to be used as a metaphor for rain clouds, because of the similarity in appearance between a mass of rock and cumulus heap cloud. Over time, the metaphoric usage of the word supplanted the old English weolcan, the literal term for clouds in general. Ancient cloud studies were not made in isolation, but were observed in combination with other weather elements and other natural sciences.
In about 340 BC the Greek philosopher Aristotle wrote Meteorologica, a work which represented the sum of knowledge of the time about natural science, including weather and climate. For the first time and the clouds from which precipitation fell were called meteors, which originate from the Greek word meteoros, meaning'high in the sky'. From that word came the modern term meteorology, the study of clouds and weather. Meteorologica was based on intuition and simple observation, but not on what is now considered the scientific method, it was the first known work that attempted to treat a broad range of meteorological topics. After centuries of speculative theories about the formation and behavior of clouds, the first scientific studies were undertaken by Luke Howard in England and Jean-Baptiste Lamarck in France. Howard was a methodical observer with a strong grounding in the Latin language and used his background to classify the various tropospheric cloud types during 1802, he believed. Lamarck had worked independently on cloud classification the same year and had come up with a different naming scheme that failed to make an impression in his home country of France because it used unusual French names for cloud types.
His system of nomenclature included twelve categories of clouds, with such names as hazy clouds, dappled clouds and broom-like clouds. By contrast, Howard used universally accepted Latin, which caught on after it was published in 1803; as a sign of the popularity of the naming scheme, the German dramatist and poet Johann Wolfgang von Goethe composed four poems about clouds, dedicating them to Howard. An elaboration of Howard's system was formally adopted by the International Meteorological Conference in 1891; this system covered only the tropospheric cloud types, but the discovery of clouds above the troposphere during the late 19th century led to the creation separate classification schemes for these high clouds. Terrestrial clouds can be found throughout most of the homosphere, which includes the troposphere and mesosphere. Within these layers of the atmosphere, air can become saturated as a result of being cooled to its dew point or by having moisture added from an adjacent source. In the latter case, saturation occurs when the dew po
A tropical cyclone is a rotating storm system characterized by a low-pressure center, a closed low-level atmospheric circulation, strong winds, a spiral arrangement of thunderstorms that produce heavy rain. Depending on its location and strength, a tropical cyclone is referred to by different names, including hurricane, tropical storm, cyclonic storm, tropical depression, cyclone. A hurricane is a tropical cyclone that occurs in the Atlantic Ocean and northeastern Pacific Ocean, a typhoon occurs in the northwestern Pacific Ocean. "Cyclone" refers to their winds moving in a circle, whirling round their central clear eye, with their winds blowing counterclockwise in the Northern Hemisphere and clockwise in the Southern Hemisphere. The opposite direction of circulation is due to the Coriolis effect. Tropical cyclones form over large bodies of warm water, they derive their energy through the evaporation of water from the ocean surface, which recondenses into clouds and rain when moist air rises and cools to saturation.
This energy source differs from that of mid-latitude cyclonic storms, such as nor'easters and European windstorms, which are fueled by horizontal temperature contrasts. Tropical cyclones are between 100 and 2,000 km in diameter; the strong rotating winds of a tropical cyclone are a result of the conservation of angular momentum imparted by the Earth's rotation as air flows inwards toward the axis of rotation. As a result, they form within 5° of the equator. Tropical cyclones are unknown in the South Atlantic due to a strong wind shear and a weak Intertropical Convergence Zone; the African easterly jet and areas of atmospheric instability which give rise to cyclones in the Atlantic Ocean and Caribbean Sea, along with the Asian monsoon and Western Pacific Warm Pool, are features of the Northern Hemisphere and Australia. Coastal regions are vulnerable to the impact of a tropical cyclone, compared to inland regions; the primary energy source for these storms is warm ocean waters, therefore these forms are strongest when over or near water, weaken quite over land.
Coastal damage may be caused by strong winds and rain, high waves, storm surges, the potential of spawning tornadoes. Tropical cyclones draw in air from a large area—which can be a vast area for the most severe cyclones—and concentrate the precipitation of the water content in that air into a much smaller area; this continual replacement of moisture-bearing air by new moisture-bearing air after its moisture has fallen as rain, which may cause heavy rain and river flooding up to 40 kilometres from the coastline, far beyond the amount of water that the local atmosphere holds at any one time. Though their effects on human populations are devastating, tropical cyclones can relieve drought conditions, they carry heat energy away from the tropics and transport it toward temperate latitudes, which may play an important role in modulating regional and global climate. Tropical cyclones are areas of low pressure in the troposphere, with the largest pressure perturbations occurring at low altitudes near the surface.
On Earth, the pressures recorded at the centers of tropical cyclones are among the lowest observed at sea level. The environment near the center of tropical cyclones is warmer than the surroundings at all altitudes, thus they are characterized as "warm core" systems; the near-surface wind field of a tropical cyclone is characterized by air rotating around a center of circulation while flowing radially inwards. At the outer edge of the storm, air may be nearly calm; as air flows radially inward, it begins to rotate cyclonically in order to conserve angular momentum. At an inner radius, air begins to ascend to the top of the troposphere; this radius is coincident with the inner radius of the eyewall, has the strongest near-surface winds of the storm. Once aloft, air flows away from the storm's center; the mentioned processes result in a wind field, nearly axisymmetric: Wind speeds are low at the center, increase moving outwards to the radius of maximum winds, decay more with radius to large radii.
However, the wind field exhibits additional spatial and temporal variability due to the effects of localized processes, such as thunderstorm activity and horizontal flow instabilities. In the vertical direction, winds are strongest near the surface and decay with height within the troposphere. At the center of a mature tropical cyclone, air sinks rather than rises. For a sufficiently strong storm, air may sink over a layer deep enough to suppress cloud formation, thereby creating a clear "eye". Weather in the eye is calm and free of clouds, although the sea may be violent; the eye is circular in shape, is 30–65 km in diameter, though eyes as small as 3 km and as large as 370 km have been observed. The cloudy outer edge of the eye is called the "eyewall"; the eyewall expands outward with height, resembling an arena foo
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
An arcus cloud is a low, horizontal cloud formation appearing as an accessory cloud to a cumulonimbus. Roll clouds and shelf clouds are the two main types of arcus. Arcus clouds most form along the leading edge or "gust fronts" of thunderstorm outflow. Roll clouds may arise in the absence of thunderstorms, forming along the shallow cold air currents of some sea breeze boundaries and cold fronts. A shelf cloud is a low, wedge-shaped arcus cloud. A shelf cloud is attached to the base of the parent cloud, a thunderstorm cumulonimbus, but could form on any type of convective clouds. Rising cloud motion can be seen in the leading part of the shelf cloud, while the underside appears turbulent and wind-torn. Cool, sinking air from a storm cloud's downdraft spreads out across the land surface, with the leading edge called a gust front; this outflow cuts under warm air being drawn into the storm's updraft. As the lower cooler air lifts the warm moist air, its water condenses, creating a cloud which rolls with the different winds above and below.
People seeing a shelf cloud may believe. This is a mistake, since an approaching shelf cloud appears to form a wall made of cloud. A shelf cloud appears on the leading edge of a storm, a wall cloud will be at the rear of the storm. A sharp, strong gust front will cause the lowest part of the leading edge of a shelf cloud to be ragged and lined with rising fractus clouds. In a severe case there will be vortices along the edge, with twisting masses of scud that may reach to the ground or be accompanied by rising dust. A low shelf cloud accompanied by these signs is the best indicator that a violent wind squall is approaching. An extreme example of this phenomenon looks like a tornado and is known as a gustnado. A roll cloud is a low, tube-shaped, rare type of arcus cloud, they differ from shelf clouds by being detached from other cloud features. Roll clouds appear to be "rolling" about a horizontal axis, they are a solitary wave called a soliton, a wave that has a single crest and moves without changing speed or shape.
One of the most famous frequent occurrences is the Morning Glory cloud in Queensland, which can occur up to four out of ten days in October. One of the main causes of the Morning Glory cloud is the mesoscale circulation associated with sea breezes that develop over the Cape York Peninsula and the Gulf of Carpentaria. However, similar features can be created by downdrafts from thunderstorms and are not associated with coastal regions. Coastal roll clouds have been seen in many places, including California, the English Channel, Shetland Islands, the North Sea coast, coastal regions of Australia, Nome, Alaska. Atmospheric convection Horizontal convective rolls International Cloud Atlas Morning Glory cloud, an long variety of roll cloud World Meteorological Organization Meteorological Service of Canada. "Spotter training: Identifying Clouds". Environment Canada. Archived from the original on July 15, 2013. Retrieved 2010-10-07. Roll Cloud vs. Shelf Cloud
A storm is any disturbed state of an environment or in an astronomical body's atmosphere affecting its surface, implying severe weather. It may be marked by significant disruptions to normal conditions such as strong wind, hail and lightning, heavy precipitation, heavy freezing rain, strong winds, or wind transporting some substance through the atmosphere as in a dust storm, sandstorm, etc. Storms have the potential to harm lives and property via storm surge, heavy rain or snow causing flooding or road impassibility, lightning and vertical wind shear. Systems with significant rainfall and duration help alleviate drought in places. Heavy snowfall can allow special recreational activities to take place which would not be possible otherwise, such as skiing and snowmobiling; the English word comes from Proto-Germanic *sturmaz meaning "noise, tumult". Storms are created when a center of low pressure develops with the system of high pressure surrounding it; this combination of opposing forces can create winds and result in the formation of storm clouds such as cumulonimbus.
Small localized areas of low pressure can form from hot air rising off hot ground, resulting in smaller disturbances such as dust devils and whirlwinds. There are many varieties and names for storms: Blizzard — There are varying definitions for blizzards, both over time and by location. In general, a blizzard is accompanied by gale-force winds, heavy snow, cold conditions; the temperature criterion has fallen out of the definition across the United States Bomb cyclone - A rapid deepening of a mid-latitude cyclonic low-pressure area occurring over the ocean, but can occur over land. The winds experienced during these storms can be as powerful as that of a hurricane. Coastal Storm — large wind waves and/or storm surge that strike the coastal zone, their impacts include coastal erosion and coastal flooding Derecho — A derecho is a widespread, long-lived, straight-line wind storm, associated with a land-based, fast-moving group of severe thunderstorms. Dust devil — a small, localized updraft of rising air.
Dust storm - A situation in which winds pick up large quantities of sand or soil reducing the visibility Firestorm — Firestorms are conflagrations which attain such intensity that they create and sustain their own wind systems. It is most a natural phenomenon, created during some of the largest bushfires, forest fires, wildfires; the Peshtigo Fire is one example of a firestorm. Firestorms can be deliberate effects of targeted explosives such as occurred as a result of the aerial bombings of Dresden. Nuclear detonations generate firestorms. Gale — An extratropical storm with sustained winds between 34–48 knots. Hailstorm — a type of storm that precipitates round chunks of ice. Hailstorms occur during regular thunderstorms. While most of the hail that precipitates from the clouds is small and harmless, there are occasional occurrences of hail greater than 2 inches in diameter that can cause much damage and injuries. Hypercane -a hypothetical tropical cyclone that could form over 50 °C water; such a storm would produce winds of over 800 km/h.
A series of hypercanes may have formed during the astroid or comet impact that killed the non-avian dinosaurs 66 million years ago. Such a phenomenon could occur during a supervolcanic eruption, or extreme global warming. Ice storm — Ice storms are one of the most dangerous forms of winter storms; when surface temperatures are below freezing, but a thick layer of above-freezing air remains aloft, rain can fall into the freezing layer and freeze upon impact into a glaze of ice. In general, 8 millimetres of accumulation is all, required in combination with breezy conditions, to start downing power lines as well as tree limbs. Ice storms make unheated road surfaces too slick to drive upon. Ice storms can vary in time range from hours to days and can cripple small towns and large metropolitan cities alike. Microburst - a powerful windstorm produced during a thunderstorm that only lasts a few minutes. Ocean Storm or sea storm — Storm conditions out at sea are defined as having sustained winds of 48 knots or greater.
Just referred to as a storm, these systems can sink vessels of all types and sizes. Snowstorm — A heavy fall of snow accumulating at a rate of more than 5 centimeters per hour that lasts several hours. Snow storms ones with a high liquid equivalent and breezy conditions, can down tree limbs, cut off power connections and paralyze travel over large regions. Squall — sudden onset of wind increase of at least 16 knots or greater sustained for at least one minute. Thunderstorm -- A thunderstorm is a type of storm that generates both thunder, it is accompanied by heavy precipitation. Thunderstorms occur throughout the world, with the highest frequency in tropical rainforest regions where there are conditions of high humidity and temperature along with atmospheric instability; these storms occur when high levels of condensation form in a volume of unstable air that generates deep, upward motion in the atmosphere. The heat energy creates powerful rising air currents. Cool descending air currents produce strong downdraughts below the storm.
After the storm has spent its energy, the rising currents die away and downdraughts break up the cloud. Individual s