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
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
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
An anticyclone is a weather phenomenon defined by the United States National Weather Service's glossary as "a large-scale circulation of winds around a central region of high atmospheric pressure, clockwise in the Northern Hemisphere, counterclockwise in the Southern Hemisphere". Effects of surface-based anticyclones include clearing skies as well as drier air. Fog can form overnight within a region of higher pressure. Mid-tropospheric systems, such as the subtropical ridge, deflect tropical cyclones around their periphery and cause a temperature inversion inhibiting free convection near their center, building up surface-based haze under their base. Anticyclones aloft can form within warm core lows such as tropical cyclones, due to descending cool air from the backside of upper troughs such as polar highs, or from large scale sinking such as the subtropical ridge; the evolution of an anticyclone depends on a few variables such as its size, moist-convection, Coriolis force etc. Sir Francis Galton first discovered anticyclones in the 1860s.
Preferred areas within a synoptic flow pattern in higher levels of the hydrosphere are beneath the western side of troughs, or dips in the Rossby wave pattern. High-pressure systems are alternatively referred to as anticyclones, their circulation is sometimes referred to as cum sole. Subtropical high pressure zones form under the descending portion of the Hadley cell circulation. Upper-level high-pressure areas lie over tropical cyclones due to their warm core nature. Surface anticyclones form due to downward motion through the troposphere, the atmospheric layer where weather occurs. Preferred areas within a synoptic flow pattern in higher levels of the troposphere are beneath the western side of troughs. On weather maps, these areas show converging winds known as confluence, or converging height lines near or above the level of non-divergence, near the 500 hPa pressure surface about midway up the troposphere; because they weaken with height, these high-pressure systems are cold. Heating of the earth near the equator forces upward motion and convection along the monsoon trough or intertropical convergence zone.
The divergence over the near-equatorial trough leads to air rising and moving away from the equator aloft. As air moves towards the mid-latitudes, it cools and sinks leading to subsidence near the 30° parallel of both hemispheres; this circulation known as the Hadley cell forms the subtropical ridge. Many of the world's deserts are caused by these climatological high-pressure areas; because these anticyclones strengthen with height, they are known as warm core ridges. The development of anticyclones aloft occurs in warm core cyclones such as tropical cyclones when latent heat caused by the formation of clouds is released aloft increasing the air temperature. In the absence of rotation, the wind tends to blow from areas of high pressure to areas of low pressure; the stronger the pressure difference between a high-pressure system and a low-pressure system, the stronger the wind. The coriolis force caused by Earth's rotation gives winds within high-pressure systems their clockwise circulation in the northern hemisphere and anticlockwise circulation in the southern hemisphere.
Friction with land slows down the wind flowing out of high-pressure systems and causes wind to flow more outward from the center. High-pressure systems are associated with light winds at the surface and subsidence of air from higher portions of the troposphere. Subsidence will warm an air mass by adiabatic heating. Thus, high pressure brings clear skies; because no clouds are present to reflect sunlight during the day, there is more incoming solar radiation and temperatures rise near the surface. At night, the absence of clouds means that outgoing longwave radiation is not blocked, giving cooler diurnal low temperatures in all seasons; when surface winds become light, the subsidence produced directly under a high-pressure system can lead to a buildup of particulates in urban areas under the high pressure, leading to widespread haze. If the surface level relative humidity rises towards 100 percent overnight, fog can form; the movement of continental arctic air masses to lower latitudes produces strong but vertically shallow high-pressure systems.
The surface level, sharp temperature inversion can lead to areas of persistent stratocumulus or stratus cloud, colloquially known as anticyclonic gloom. The type of weather brought about by an anticyclone depends on its origin. For example, extensions of the Azores high pressure may bring about anticyclonic gloom during the winter because they pick up moisture as they move over the warmer oceans. High pressures that build to the north and move southwards bring clear weather because they are cooled at the base which helps prevent clouds from forming. Once arctic air moves over an unfrozen ocean, the air mass modifies over the warmer water and takes on the character of a maritime air mass, which reduces the strength of the high-pressure system; when cold air moves over warm oceans, polar lows can develop. However and moist air masses which move poleward from tropical sources are slower to modify than arctic air masses; the circulation around mid-level ridges, the air subsidence at their center, act to steer tropical cyclones ar
Rain is liquid water in the form of droplets that have condensed from atmospheric water vapor and become heavy enough to fall under gravity. Rain is a major component of the water cycle and is responsible for depositing most of the fresh water on the Earth, it provides suitable conditions for many types of ecosystems, as well as water for hydroelectric power plants and crop irrigation. The major cause of rain production is moisture moving along three-dimensional zones of temperature and moisture contrasts known as weather fronts. If enough moisture and upward motion is present, precipitation falls from convective clouds such as cumulonimbus which can organize into narrow rainbands. In mountainous areas, heavy precipitation is possible where upslope flow is maximized within windward sides of the terrain at elevation which forces moist air to condense and fall out as rainfall along the sides of mountains. On the leeward side of mountains, desert climates can exist due to the dry air caused by downslope flow which causes heating and drying of the air mass.
The movement of the monsoon trough, or intertropical convergence zone, brings rainy seasons to savannah climes. The urban heat island effect leads to increased rainfall, both in amounts and intensity, downwind of cities. Global warming is causing changes in the precipitation pattern globally, including wetter conditions across eastern North America and drier conditions in the tropics. Antarctica is the driest continent; the globally averaged annual precipitation over land is 715 mm, but over the whole Earth it is much higher at 990 mm. Climate classification systems such as the Köppen classification system use average annual rainfall to help differentiate between differing climate regimes. Rainfall is measured using rain gauges. Rainfall amounts can be estimated by weather radar. Rain is known or suspected on other planets, where it may be composed of methane, sulfuric acid, or iron rather than water. Air contains water vapor, the amount of water in a given mass of dry air, known as the mixing ratio, is measured in grams of water per kilogram of dry air.
The amount of moisture in air is commonly reported as relative humidity. How much water vapor a parcel of air can contain before it becomes saturated and forms into a cloud depends on its temperature. Warmer air can contain more water vapor than cooler air before becoming saturated. Therefore, one way to saturate a parcel of air is to cool it; the dew point is the temperature. There are four main mechanisms for cooling the air to its dew point: adiabatic cooling, conductive cooling, radiational cooling, evaporative cooling. Adiabatic cooling occurs when air expands; the air can rise due to convection, large-scale atmospheric motions, or a physical barrier such as a mountain. Conductive cooling occurs when the air comes into contact with a colder surface by being blown from one surface to another, for example from a liquid water surface to colder land. Radiational cooling occurs due to the emission of infrared radiation, either by the air or by the surface underneath. Evaporative cooling occurs when moisture is added to the air through evaporation, which forces the air temperature to cool to its wet-bulb temperature, or until it reaches saturation.
The main ways water vapor is added to the air are: wind convergence into areas of upward motion, precipitation or virga falling from above, daytime heating evaporating water from the surface of oceans, water bodies or wet land, transpiration from plants, cool or dry air moving over warmer water, lifting air over mountains. Water vapor begins to condense on condensation nuclei such as dust and salt in order to form clouds. Elevated portions of weather fronts force broad areas of upward motion within the Earth's atmosphere which form clouds decks such as altostratus or cirrostratus. Stratus is a stable cloud deck which tends to form when a cool, stable air mass is trapped underneath a warm air mass, it can form due to the lifting of advection fog during breezy conditions. Coalescence occurs. Air resistance causes the water droplets in a cloud to remain stationary; when air turbulence occurs, water droplets collide. As these larger water droplets descend, coalescence continues, so that drops become heavy enough to overcome air resistance and fall as rain.
Coalescence happens most in clouds above freezing, is known as the warm rain process. In clouds below freezing, when ice crystals gain enough mass they begin to fall; this requires more mass than coalescence when occurring between the crystal and neighboring water droplets. This process is temperature dependent, as supercooled water droplets only exist in a cloud, below freezing. In addition, because of the great temperature difference between cloud and ground level, these ice crystals may melt as they fall and become rain. Raindrops have sizes ranging from 0.1 to 9 mm mean diameter. Smaller drops are called cloud droplets, their shape is spherical; as a raindrop increases in size, its shape becomes more oblate, with its largest cross-section facing the oncoming airflow. Large rain drops become flattened on the bottom, like hamburger buns. Contrary to popular beli
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
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