Nature, in the broadest sense, is the natural, physical, or material world or universe. "Nature" can refer to the phenomena of the physical world, to life in general. The study of nature is a large, part of science. Although humans are part of nature, human activity is understood as a separate category from other natural phenomena; the word nature is derived from the Latin word natura, or "essential qualities, innate disposition", in ancient times meant "birth". Natura is a Latin translation of the Greek word physis, which related to the intrinsic characteristics that plants and other features of the world develop of their own accord; the concept of nature as a whole, the physical universe, is one of several expansions of the original notion. This usage continued during the advent of modern scientific method in the last several centuries. Within the various uses of the word today, "nature" refers to geology and wildlife. Nature can refer to the general realm of living plants and animals, in some cases to the processes associated with inanimate objects—the way that particular types of things exist and change of their own accord, such as the weather and geology of the Earth.
It is taken to mean the "natural environment" or wilderness—wild animals, forest, in general those things that have not been altered by human intervention, or which persist despite human intervention. For example, manufactured objects and human interaction are not considered part of nature, unless qualified as, for example, "human nature" or "the whole of nature"; this more traditional concept of natural things which can still be found today implies a distinction between the natural and the artificial, with the artificial being understood as that, brought into being by a human consciousness or a human mind. Depending on the particular context, the term "natural" might be distinguished from the unnatural or the supernatural. Earth is the only planet known to support life, its natural features are the subject of many fields of scientific research. Within the solar system, it is third closest to the sun, its most prominent climatic features are its two large polar regions, two narrow temperate zones, a wide equatorial tropical to subtropical region.
Precipitation varies with location, from several metres of water per year to less than a millimetre. 71 percent of the Earth's surface is covered by salt-water oceans. The remainder consists of continents and islands, with most of the inhabited land in the Northern Hemisphere. Earth has evolved through geological and biological processes that have left traces of the original conditions; the outer surface is divided into several migrating tectonic plates. The interior remains active, with a thick layer of plastic mantle and an iron-filled core that generates a magnetic field; this iron core is composed of a solid inner phase, a fluid outer phase. Convective motion in the core generates electric currents through dynamo action, these, in turn, generate the geomagnetic field; the atmospheric conditions have been altered from the original conditions by the presence of life-forms, which create an ecological balance that stabilizes the surface conditions. Despite the wide regional variations in climate by latitude and other geographic factors, the long-term average global climate is quite stable during interglacial periods, variations of a degree or two of average global temperature have had major effects on the ecological balance, on the actual geography of the Earth.
Geology is the study of the solid and liquid matter that constitutes the Earth. The field of geology encompasses the study of the composition, physical properties and history of Earth materials, the processes by which they are formed and changed; the field is a major academic discipline, is important for mineral and hydrocarbon extraction, knowledge about and mitigation of natural hazards, some Geotechnical engineering fields, understanding past climates and environments. The geology of an area evolves through time as rock units are deposited and inserted and deformational processes change their shapes and locations. Rock units are first emplaced either by deposition onto the surface or intrude into the overlying rock. Deposition can occur when sediments settle onto the surface of the Earth and lithify into sedimentary rock, or when as volcanic material such as volcanic ash or lava flows, blanket the surface. Igneous intrusions such as batholiths, laccoliths and sills, push upwards into the overlying rock, crystallize as they intrude.
After the initial sequence of rocks has been deposited, the rock units can be deformed and/or metamorphosed. Deformation occurs as a result of horizontal shortening, horizontal extension, or side-to-side motion; these structural regimes broadly relate to convergent boundaries, divergent boundaries, transform boundaries between tectonic plates. Earth is estimated to have formed 4.54 billion years ago from the solar nebula, along with the Sun and other planets. The moon formed 20 million years later. Molten, the outer layer of the Earth cooled, resulting in the solid crust. Outgassing and volcanic activity produced the primordial atmosphere. Condensing water vapor, most or all of which came from ice delivered by comets, produced the oceans and other water sources; the energetic chemistry is believed to have produced a self-replicat
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
Lightning is a violent and sudden electrostatic discharge where two electrically charged regions in the atmosphere temporarily equalize themselves during a thunderstorm. Lightning creates a wide range of electromagnetic radiations from the hot plasma created by the electron flow, including visible light in the form of black-body radiation. Thunder is the sound formed by the shock wave formed as gaseous molecules experience a rapid pressure increase; the three main kinds of lightning are: created either inside one thundercloud, or between two clouds, or between a cloud and the ground. The 15 recognized observational variants include "heat lightning", seen but not heard, dry lightning, which causes many forest fires, ball lightning, observed scientifically. Humans have deified lightning for millennia, lightning inspired expressions like "Bolt from the blue", "Lightning never strikes twice", "blitzkrieg" are common. In some languages, "Love at first sight" translates as "lightning strike"; the details of the charging process are still being studied by scientists, but there is general agreement on some of the basic concepts of thunderstorm electrification.
The main charging area in a thunderstorm occurs in the central part of the storm where air is moving upward and temperatures range from −15 to −25 °C, see figure to the right. At that place, the combination of temperature and rapid upward air movement produces a mixture of super-cooled cloud droplets, small ice crystals, graupel; the updraft carries the super-cooled cloud droplets and small ice crystals upward. At the same time, the graupel, larger and denser, tends to fall or be suspended in the rising air; the differences in the movement of the precipitation cause collisions to occur. When the rising ice crystals collide with graupel, the ice crystals become positively charged and the graupel becomes negatively charged. See figure to the left; the updraft carries. The larger and denser graupel is either suspended in the middle of the thunderstorm cloud or falls toward the lower part of the storm; the result is that the upper part of the thunderstorm cloud becomes positively charged while the middle to lower part of the thunderstorm cloud becomes negatively charged.
The upward motions within the storm and winds at higher levels in the atmosphere tend to cause the small ice crystals in the upper part of the thunderstorm cloud to spread out horizontally some distance from thunderstorm cloud base. This part of the thunderstorm cloud is called the anvil. While this is the main charging process for the thunderstorm cloud, some of these charges can be redistributed by air movements within the storm. In addition, there is a small but important positive charge buildup near the bottom of the thunderstorm cloud due to the precipitation and warmer temperatures. A typical cloud-to-ground lightning flash culminates in the formation of an electrically conducting plasma channel through the air in excess of 5 km tall, from within the cloud to the ground's surface; the actual discharge is the final stage of a complex process. At its peak, a typical thunderstorm produces three or more strikes to the Earth per minute. Lightning occurs when warm air is mixed with colder air masses, resulting in atmospheric disturbances necessary for polarizing the atmosphere.
However, it can occur during dust storms, forest fires, volcanic eruptions, in the cold of winter, where the lightning is known as thundersnow. Hurricanes generate some lightning in the rainbands as much as 160 km from the center; the science of lightning is called fulminology, the fear of lightning is called astraphobia. Lightning is not distributed evenly around the planet. On Earth, the lightning frequency is 44 times per second, or nearly 1.4 billion flashes per year and the average duration is 0.2 seconds made up from a number of much shorter flashes of around 60 to 70 microseconds. Many factors affect the frequency, distribution and physical properties of a typical lightning flash in a particular region of the world; these factors include ground elevation, prevailing wind currents, relative humidity, proximity to warm and cold bodies of water, etc. To a certain degree, the ratio between IC, CC and CG lightning may vary by season in middle latitudes; because human beings are terrestrial and most of their possessions are on the Earth where lightning can damage or destroy them, CG lightning is the most studied and best understood of the three types though IC and CC are more common types of lightning.
Lightning's relative unpredictability limits a complete explanation of how or why it occurs after hundreds of years of scientific investigation. About 70 % of lightning occurs over land in the tropics; this occurs from both the mixture of warmer and colder air masses, as well as differences in moisture concentrations, it happens at the boundaries between them. The flow of warm ocean currents past drier land masses, such as the Gulf Stream explains the elevated frequency of lightning in the Southeast United States; because the influence of small or absent land masses in the vast stretches of the world's oceans limits the differences between these variants in the atmosphere, lightning is notably less frequent there than over larger landforms. The North and South Poles are limited in their coverage of thunderstorms and theref
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
In meteorology, a cyclone is a large scale air mass that rotates around a strong center of low atmospheric pressure. Cyclones are characterized by inward spiraling winds; the largest low-pressure systems are polar vortices and extratropical cyclones of the largest scale. Warm-core cyclones such as tropical cyclones and subtropical cyclones lie within the synoptic scale. Mesocyclones and dust devils lie within smaller mesoscale. Upper level cyclones can exist without the presence of a surface low, can pinch off from the base of the tropical upper tropospheric trough during the summer months in the Northern Hemisphere. Cyclones have been seen on extraterrestrial planets, such as Mars and Neptune. Cyclogenesis is the process of cyclone intensification. Extratropical cyclones begin as waves in large regions of enhanced mid-latitude temperature contrasts called baroclinic zones; these zones contract and form weather fronts as the cyclonic circulation intensifies. In their life cycle, extratropical cyclones occlude as cold air masses undercut the warmer air and become cold core systems.
A cyclone's track is guided over the course of its 2 to 6 day life cycle by the steering flow of the subtropical jet stream. Weather fronts mark the boundary between two masses of air of different temperature and densities, are associated with the most prominent meteorological phenomena. Strong cold fronts feature narrow bands of thunderstorms and severe weather, may on occasion be preceded by squall lines or dry lines; such fronts form west of the circulation center and move from west to east. Warm fronts move poleward ahead of the cyclone path. Occluded fronts form late in the cyclone life cycle near the center of the cyclone and wrap around the storm center. Tropical cyclogenesis describes the process of development of tropical cyclones. Tropical cyclones form due to latent heat driven by significant thunderstorm activity, are warm core. Cyclones can transition between extratropical and tropical phases. Mesocyclones form as warm core cyclones over land, can lead to tornado formation. Waterspouts can form from mesocyclones, but more develop from environments of high instability and low vertical wind shear.
In the Atlantic and the northeastern Pacific oceans, a tropical cyclone is referred to as a hurricane, in the Indian and south Pacific oceans it is called a cyclone, in the northwestern Pacific it is called a typhoon. The growth of instability in the vortices is not universal. For example, the size, moist-convection, surface evaporation, the value of potential temperature at each potential height can affect the nonlinear evolution of a vortex. Henry Piddington published 40 papers dealing with tropical storms from Calcutta between 1836 and 1855 in The Journal of the Asiatic Society, he coined the term cyclone, meaning the coil of a snake. In 1842, he published Laws of the Storms. There are a number of structural characteristics common to all cyclones. A cyclone is a low-pressure area. A cyclone's center, is the area of lowest atmospheric pressure in the region. Near the center, the pressure gradient force and the force from the Coriolis effect must be in an approximate balance, or the cyclone would collapse on itself as a result of the difference in pressure.
Because of the Coriolis effect, the wind flow around a large cyclone is counterclockwise in the Northern Hemisphere and clockwise in the Southern Hemisphere. In the Northern Hemisphere, the fastest winds relative to the surface of the Earth therefore occur on the eastern side of a northward-moving cyclone and on the northern side of a westward-moving one. In contrast to low pressure systems, the wind flow around high pressure systems are clockwise in the northern hemisphere, counterclockwise in the southern hemisphere. Cyclogenesis is the development or strengthening of cyclonic circulation in the atmosphere. Cyclogenesis is an umbrella term for several different processes that all result in the development of some sort of cyclone, it can occur from the microscale to the synoptic scale. Extratropical cyclones begin as waves along weather fronts before occluding in their life cycle as cold-core systems. However, some intense extratropical cyclones can become warm-core systems when a warm seclusion occurs.
Tropical cyclones form as a result of significant convective activity, are warm core. Mesocyclones form as warm core cyclones over land, can lead to tornado formation. Waterspouts can form from mesocyclones, but more develop from environments of high instability and low vertical wind shear. Cyclolysis is the opposite of cyclogenesis, is the high-pressure system equivalent, which deals with the formation of high-pressure areas—Anticyclogenesis. A surface low can form in a variety of ways. Topography can create a surface low. Mesoscale convective systems can spawn surface lows that are warm core; the disturbance can grow into a wave-like formation along the front and the low is positioned at the crest. Around the low, the flow becomes cyclonic; this rotational flow moves polar air towards the equator on the west side of the low, while warm air move towards the pole on the east side. A cold front appears on the west side. Usua
Summer is the hottest of the four temperate seasons, falling after spring and before autumn. At the summer solstice, the days are longest and the nights are shortest, with day length decreasing as the season progresses after the solstice; the date of the beginning of summer varies according to climate and culture. When it is summer in the Northern Hemisphere, it is winter in the Southern Hemisphere, vice versa. From an astronomical view, the equinoxes and solstices would be the middle of the respective seasons, but sometimes astronomical summer is defined as starting at the solstice, the time of maximal insolation identified with the 21st day of June or December. A variable seasonal lag means that the meteorological center of the season, based on average temperature patterns, occurs several weeks after the time of maximal insolation; the meteorological convention is to define summer as comprising the months of June and August in the northern hemisphere and the months of December and February in the southern hemisphere.
Under meteorological definitions, all seasons are arbitrarily set to start at the beginning of a calendar month and end at the end of a month. This meteorological definition of summer aligns with the viewed notion of summer as the season with the longest days of the year, in which daylight predominates; the meteorological reckoning of seasons is used in Australia, Denmark and Japan. It is used by many in the United Kingdom. In Ireland, the summer months according to the national meteorological service, Met Éireann, are June and August. However, according to the Irish Calendar, summer ends on 1 August. School textbooks in Ireland follow the cultural norm of summer commencing on 1 May rather than the meteorological definition of 1 June. Days continue to lengthen from equinox to solstice and summer days progressively shorten after the solstice, so meteorological summer encompasses the build-up to the longest day and a diminishing thereafter, with summer having many more hours of daylight than spring.
Reckoning by hours of daylight alone, summer solstice marks the midpoint, not the beginning, of the seasons. Midsummer takes place over the shortest night of the year, the summer solstice, or on a nearby date that varies with tradition. Where a seasonal lag of half a season or more is common, reckoning based on astronomical markers is shifted half a season. By this method, in North America, summer is the period from the summer solstice to the autumn equinox. Reckoning by cultural festivals, the summer season in the United States is traditionally regarded as beginning on Memorial Day weekend and ending on Labor Day, more in line with the meteorological definition for the parts of the country that have four-season weather; the similar Canadian tradition starts summer on Victoria Day one week prior and ends, as in the United States, on Labour Day. In Chinese astronomy, summer starts on or around 5 May, with the jiéqì known as lìxià, i.e. "establishment of summer", it ends on or around 6 August.
In southern and southeast Asia, where the monsoon occurs, summer is more defined as lasting from March, April and June, the warmest time of the year, ending with the onset of the monsoon rains. Because the temperature lag is shorter in the oceanic temperate southern hemisphere, most countries in this region use the meteorological definition with summer starting on 1 December and ending on the last day of February. Summer is traditionally associated with warm weather. In the Mediterranean regions, it is associated with dry weather, while in other places it is associated with rainy weather; the wet season is the main period of vegetation growth within the savanna climate regime. Where the wet season is associated with a seasonal shift in the prevailing winds, it is known as a monsoon. In the northern Atlantic Ocean, a distinct tropical cyclone season occurs from 1 June to 30 November; the statistical peak of the Atlantic hurricane season is 10 September. The Northeast Pacific Ocean has a broader period of activity, but in a similar time frame to the Atlantic.
The Northwest Pacific sees tropical cyclones year-round, with a minimum in February and March and a peak in early September. In the North Indian basin, storms are most common from April to December, with peaks in May and November. In the Southern Hemisphere, the tropical cyclone season runs from 1 November until the end of April with peaks in mid-February to early March. Thunderstorm season in the United States and Canada runs in the spring through summer; these storms can produce hail, strong winds and tornadoes during the afternoon and evening. Schools and universities have a summer break to take advantage of the warmer weather and longer days. In all countries, children are out of school during this time of year for summer break, although dates vary. In the United States, public schools end in late May in Memorial Day weekend, while colleges finish in early May, although some schools get out on the last or second last Thursday in May. In England and Wales, school resumes again in early September.
In Canada the summer holiday starts on the last or second-last Friday in June and ends in late August or on the first Monday of September, with the exception of when that date falls before Labour Day, in which case, ends on the second Monday of the month. In Russia the summer