Misplaced Pages

#136863

57-451: There are several energy fluxes involved in the melting of snow. These fluxes can act in opposing directions, that is either delivering heat to or removing heat from the snowpack . Ground heat flux is the energy delivered to the snowpack from the soil below by conduction. Radiation inputs to the snowpack include net shortwave (solar radiation including visible and ultraviolet light) and longwave ( infrared ) radiation. Net shortwave radiation

114-633: A glacier may form. Otherwise, snow typically melts seasonally, causing runoff into streams and rivers and recharging groundwater . Major snow-prone areas include the polar regions , the northernmost half of the Northern Hemisphere and mountainous regions worldwide with sufficient moisture and cold temperatures. In the Southern Hemisphere , snow is confined primarily to mountainous areas, apart from Antarctica . Snow affects such human activities as transportation : creating

171-401: A rainband ), when temperature is near freezing at the surface. The strong convection that develops has enough moisture to produce whiteout conditions at places which the line passes over as the wind causes intense blowing snow. This type of snowsquall generally lasts less than 30 minutes at any point along its path, but the motion of the line can cover large distances. Frontal squalls may form

228-433: A cirque (corrie or cwm), a typically armchair-shaped geological feature, which collects snow and where the snowpack compacts under the weight of successive layers of accumulating snow, forming névé. Further crushing of the individual snow crystals and reduction of entrapped air in the snow turns it into glacial ice. This glacial ice will fill the cirque until it overflows through a geological weakness or an escape route, such as

285-649: A crystal morphology diagram, relating crystal shapes to the temperature and moisture conditions under which they formed, which is summarized in the following table. Dendrites Hollow prisms Needles Solid plates Dendrites Solid plates Prisms Nakaya discovered that the shape is also a function of whether the prevalent moisture is above or below saturation. Forms below the saturation line tend more toward solid and compact while crystals formed in supersaturated air tend more toward lacy, delicate, and ornate. Many more complex growth patterns also form, which include side-planes, bullet-rosettes, and planar types, depending on

342-447: A droplet has frozen, it grows in the supersaturated environment—one where air is saturated with respect to ice when the temperature is below the freezing point. The droplet then grows by diffusion of water molecules in the air (vapor) onto the ice crystal surface where they are collected. Because water droplets are so much more numerous than the ice crystals, the crystals are able to grow to hundreds of micrometers or millimeters in size at

399-589: A few molecules in the droplet need to get together by chance to form an arrangement similar to that in an ice lattice. The droplet freezes around this "nucleus". In warmer clouds, an aerosol particle or "ice nucleus" must be present in (or in contact with) the droplet to act as a nucleus. Ice nuclei are very rare compared to cloud condensation nuclei on which liquid droplets form. Clays, desert dust, and biological particles can be nuclei. Artificial nuclei include particles of silver iodide and dry ice , and these are used to stimulate precipitation in cloud seeding . Once

456-568: A hemisphere's fall , winter, and spring, the atmosphere over continents can be cold enough through the depth of the troposphere to cause snowfall. In the Northern Hemisphere, the northern side of the low-pressure area produces the most snow. For the southern mid-latitudes , the side of a cyclone that produces the most snow is the southern side. A cold front , the leading edge of a cooler mass of air, can produce frontal snowsqualls —an intense frontal convective line (similar to

513-485: A highly porous, sintered material made up of a continuous ice structure and a continuously connected pore space, forming together the snow microstructure". Almost always near its melting temperature, a snowpack is continually transforming these properties wherein all three phases of water may coexist, including liquid water partially filling the pore space. After deposition, snow progresses on one of two paths that determine its fate, either by ablation (mostly by melting) from

570-470: A larger weather system. The physics of snow crystal development in clouds results from a complex set of variables that include moisture content and temperatures. The resulting shapes of the falling and fallen crystals can be classified into a number of basic shapes and combinations thereof. Occasionally, some plate-like, dendritic and stellar-shaped snowflakes can form under clear sky with a very cold temperature inversion present. Snow clouds usually occur in

627-435: A minimum extent of 2 million square kilometres (0.77 × 10 ^  sq mi) each August to a maximum extent of 45 million square kilometres (17 × 10 ^  sq mi) each January or nearly half of the land surface in that hemisphere. A study of Northern Hemisphere snow cover extent for the period 1972–2006 suggests a reduction of 0.5 million square kilometres (0.19 × 10 ^  sq mi) over

SECTION 10

#1732765923137

684-530: A mountain range results in adiabatic cooling, and ultimately condensation and precipitation. Moisture is gradually removed from the air by this process, leaving drier and warmer air on the descending, or leeward , side. The resulting enhanced snowfall, along with the decrease in temperature with elevation, combine to increase snow depth and seasonal persistence of snowpack in snow-prone areas. Mountain waves have also been found to help enhance precipitation amounts downwind of mountain ranges by enhancing

741-488: A series of snow events, punctuated by freezing and thawing, over areas that are cold enough to retain snow seasonally or perennially. Major snow-prone areas include the Arctic and Antarctic , the Northern Hemisphere, and alpine regions. The liquid equivalent of snowfall may be evaluated using a snow gauge or with a standard rain gauge , adjusted for winter by removal of a funnel and inner cylinder. Both types of gauges melt

798-409: A short distance ahead of the surface cold front or behind the cold front where there may be a deepening low-pressure system or a series of trough lines which act similar to a traditional cold frontal passage. In situations where squalls develop post-frontally, it is not unusual to have two or three linear squall bands pass in rapid succession separated only by 25 miles (40 kilometers), with each passing

855-419: A snow fall or seasonal snowpack, or by transitioning from firn (multi-year snow) into glacier ice . Over the course of time, a snowpack may settle under its own weight until its density is approximately 30% of water. Increases in density above this initial compression occur primarily by melting and refreezing, caused by temperatures above freezing or by direct solar radiation. In colder climates, snow lies on

912-608: A snowpack can create flow fingers and ponding or flow along capillary barriers, which can refreeze into horizontal and vertical solid ice formations within the snowpack. Among the measurements of the properties of snowpacks that the International Classification for Seasonal Snow on the Ground includes are: snow height, snow water equivalent, snow strength, and extent of snow cover. Each has a designation with code and detailed description. The classification extends

969-416: A snowslide or snowslip) is a rapid flow of snow down a sloping surface. Avalanches are typically triggered in a starting zone from a mechanical failure in the snowpack (slab avalanche) when the forces on the snow exceed its strength but sometimes only with gradually widening (loose snow avalanche). After initiation, avalanches usually accelerate rapidly and grow in mass and volume as they entrain more snow. If

1026-630: A substance denser than névé , yet less dense and hard than glacial ice . Firn resembles caked sugar and is very resistant to shovelling. Its density generally ranges from 550 to 830 kilograms per cubic metre (34 to 52 lb/cu ft), and it can often be found underneath the snow that accumulates at the head of a glacier . The minimum altitude that firn accumulates on a glacier is called the firn limit , firn line or snowline . There are four main mechanisms for movement of deposited snow: drifting of unsintered snow, avalanches of accumulated snow on steep slopes, snowmelt during thaw conditions, and

1083-472: Is "the transformation that the snow undergoes in the period from deposition to either melting or passage to glacial ice". Starting as a powdery deposition, snow becomes more granular when it begins to compact under its own weight, be blown by the wind, sinter particles together and commence the cycle of melting and refreezing. Water vapor plays a role as it deposits ice crystals, known as hoar frost , during cold, still conditions. During this transition, snow "is

1140-486: Is an accumulation of snow that compresses with time and melts seasonally, often at high elevation or high latitude. Snowpacks are an important water resource that feed streams and rivers as they melt, sometimes leading to flooding. Snowpacks provide water to down-slope communities for drinking and agriculture. High-latitude or high-elevation snowpacks contribute mass to glaciers in their accumulation zones , where annual snow deposition exceeds annual melting. Assessing

1197-949: Is due to climate change firmly remains a center of debate. Increased water runoff due to snowmelt was a cause of many famous floods. One well-known example is the Red River Flood of 1997, when the Red River of the North in the Red River Valley of the United States and Canada flooded. Flooding in the Red River Valley is augmented by the fact that the river flows north through Winnipeg, Manitoba and into Lake Winnipeg . As snow in Minnesota , North Dakota , and South Dakota begins to melt and flow into

SECTION 20

#1732765923137

1254-724: Is in turn related to the variability of key patterns of atmospheric circulation. A study of the mountains in the western United States show a region wide decline in spring snow-pack since the mid-1900s, dominated by loss at low elevations where winter temperatures are near freezing. These losses are an indication of increased temperatures which lead to snow loss via some combination of increased regularity of rain versus snow and increased melting during winter months. These natural variations make it challenging to quantify trends with confidence, to deduce observed changes to predict future climate, or to clearly detect changes in snow-pack due to human impact on warming trends. Snowpack Snowpack

1311-429: Is not a requirement, as blowing snow can create a ground blizzard . Snowstorm intensity may be categorized by visibility and depth of accumulation. Snowfall's intensity is determined by visibility , as follows: Snowsqualls may deposit snow in bands that extend from bodies of water as lake-event weather or result from the passage of an upper-level front. The International Classification for Seasonal Snow on

1368-464: Is termed ocean-effect or bay-effect snow . The effect is enhanced when the moving air mass is uplifted by the orographic influence of higher elevations on the downwind shores. This uplifting can produce narrow but very intense bands of precipitation which may deposit at a rate of many inches of snow each hour, often resulting in a large amount of total snowfall. The areas affected by lake-effect snow are called snowbelts . These include areas east of

1425-427: Is the difference in energy received from the sun and that reflected by the snowpack because of the snowpack albedo . Longwave radiation is received by the snowpack from many sources, including ozone, carbon dioxide, and water vapor present in all levels of the atmosphere. Longwave radiation is also emitted by the snowpack in the form near- Black-body radiation , where snow has an emissivity between 0.97 and 1.0. Generally

1482-645: The Great Lakes , the west coasts of northern Japan, the Kamchatka Peninsula in Russia, and areas near the Great Salt Lake , Black Sea , Caspian Sea , Baltic Sea , and parts of the northern Atlantic Ocean. Orographic or relief snowfall is created when moist air is forced up the windward side of mountain ranges by a large-scale wind flow. The lifting of moist air up the side of

1539-577: The air and cause earlier melting of snow around them. The snow does not melt slower gradually with distance from the trunk, but creates a wall surrounding snow-free ground around it rather. According to some of sources, North American spring ephermal plants like spring beauty ( Claytonia caroliniana ), trout lily ( Erythronium americanum ) and red trillium (Trillium erectum L.) benefit from such thaw circle. They can emerge earlier inside these circles, what gives them more time before development of tree canopy foliage cutting off significant portion of

1596-552: The atmosphere —usually within clouds—and then fall, accumulating on the ground where they undergo further changes. It consists of frozen crystalline water throughout its life cycle, starting when, under suitable conditions, the ice crystals form in the atmosphere, increase to millimeter size, precipitate and accumulate on surfaces, then metamorphose in place, and ultimately melt, slide or sublimate away. Snowstorms organize and develop by feeding on sources of atmospheric moisture and cold air. Snowflakes nucleate around particles in

1653-411: The movement of glaciers after snow has persisted for multiple years and metamorphosed into glacier ice. When powdery snow drifts with the wind from the location where it originally fell, forming deposits with a depth of several meters in isolated locations. After attaching to hillsides, blown snow can evolve into a snow slab, which is an avalanche hazard on steep slopes. An avalanche (also called

1710-648: The 35-year period. The following are world records regarding snowfall and snowflakes: The cities (more than 100,000 inhabitants) with the highest annual snowfall are Aomori (792 cm), Sapporo (485 cm) and Toyama (363 cm) in Japan , followed by St. John's (332 cm) and Quebec City (315 cm) in Canada , and Syracuse, NY (325 cm). According to the International Association of Cryospheric Sciences, snow metamorphism

1767-460: The Ground defines "height of new snow" as the depth of freshly fallen snow, in centimeters as measured with a ruler, that accumulated on a snowboard during an observation period of 24 hours, or other observation interval. After the measurement, the snow is cleared from the board and the board is placed flush with the snow surface to provide an accurate measurement at the end of the next interval. Melting, compacting, blowing and drifting contribute to

Snowmelt - Misplaced Pages Continue

1824-552: The Red River, the presence of downstream ice can act as a dam and force upstream water to rise. Colder temperatures downstream can also potentially lead to freezing of water as it flows north, thus augmenting the ice dam problem. Some areas in British Columbia are also prone to snowmelt flooding as well. The date of annual melt is of great interest as a potential indicator of climate change. In order to determine whether

1881-413: The accumulated snow and report the amount of water collected. At some automatic weather stations an ultrasonic snow depth sensor may be used to augment the precipitation gauge. Snow flurry , snow shower , snow storm and blizzard describe snow events of progressively greater duration and intensity. A blizzard is a weather condition involving snow and has varying definitions in different parts of

1938-425: The atmosphere by attracting supercooled water droplets, which freeze in hexagonal-shaped crystals. Snowflakes take on a variety of shapes, basic among these are platelets, needles, columns and rime . As snow accumulates into a snowpack , it may blow into drifts. Over time, accumulated snow metamorphoses, by sintering , sublimation and freeze-thaw . Where the climate is cold enough for year-to-year accumulation,

1995-458: The avalanche moves fast enough some of the snow may mix with the air forming a powder snow avalanche, which is a type of gravity current . They occur in three major mechanisms: Many rivers originating in mountainous or high-latitude regions receive a significant portion of their flow from snowmelt. This often makes the river's flow highly seasonal resulting in periodic flooding during the spring months and at least in dry mountainous regions like

2052-448: The conditions and ice nuclei. If a crystal has started forming in a column growth regime at around −5 °C (23 °F) and then falls into the warmer plate-like regime, plate or dendritic crystals sprout at the end of the column, producing so called "capped columns". Magono and Lee devised a classification of freshly formed snow crystals that includes 80 distinct shapes. They documented each with micrographs. Snow accumulates from

2109-475: The context of larger weather systems, the most important of which is the low-pressure area, which typically incorporate warm and cold fronts as part of their circulation. Two additional and locally productive sources of snow are lake-effect (also sea-effect) storms and elevation effects, especially in mountains. Mid-latitude cyclones are low-pressure areas which are capable of producing anything from cloudiness and mild snow storms to heavy blizzards . During

2166-544: The contribution of snowmelt to river hydraulics and ground hydrology . In doing so, they employ a variety of instruments to observe and measure the phenomena studied. Their findings contribute to knowledge applied by engineers , who adapt vehicles and structures to snow, by agronomists , who address the availability of snowmelt to agriculture , and those, who design equipment for sporting activities on snow. Scientists develop and others employ snow classification systems that describe its physical properties at scales ranging from

2223-450: The crystal facets and hollows/imperfections mean that the crystals often appear white in color due to diffuse reflection of the whole spectrum of light by the small ice particles. Micrography of thousands of snowflakes from 1885 onward, starting with Wilson Alwyn Bentley , revealed the wide diversity of snowflakes within a classifiable set of patterns. Closely matching snow crystals have been observed. Ukichiro Nakaya developed

2280-446: The difficulty of measuring snowfall. Glaciers with their permanent snowpacks cover about 10% of the earth's surface, while seasonal snow covers about nine percent, mostly in the Northern Hemisphere, where seasonal snow covers about 40 million square kilometres (15 × 10 ^  sq mi), according to a 1987 estimate. A 2007 estimate of snow cover over the Northern Hemisphere suggested that, on average, snow cover ranges from

2337-407: The earlier disappearance of spring snow cover in northern Alaska is related to global warming versus an appearance of a more natural, continual cycle of the climate, further study and monitoring is necessary. Large year-to-year variability complicates the picture and furthers the debate. Inter-annual variability of springtime snow pack comes largely from variability of winter month precipitation which

Snowmelt - Misplaced Pages Continue

2394-483: The expense of the water droplets by the Wegener–Bergeron–Findeisen process . These large crystals are an efficient source of precipitation, since they fall through the atmosphere due to their mass, and may collide and stick together in clusters, or aggregates. These aggregates are snowflakes , and are usually the type of ice particle that falls to the ground. Although the ice is clear, scattering of light by

2451-444: The formation and stability of snowpacks is important in the study and prediction of avalanches . Scientists study the physical properties of snow under different conditions and their evolution, and more specifically snow metamorphism , snow hydrology (that is, the contribution of snow melt to catchment hydrology ), the evolution of snow cover with climate change and its effect on the ice–albedo feedback and hydrology, both on

2508-442: The gap between two mountains. When the mass of snow and ice is sufficiently thick, it begins to move due to a combination of surface slope, gravity and pressure. On steeper slopes, this can occur with as little as 15 m (49 ft) of snow-ice. Scientists study snow at a wide variety of scales that include the physics of chemical bonds and clouds ; the distribution, accumulation, metamorphosis, and ablation of snowpacks; and

2565-453: The ground all winter. By late spring, snow densities typically reach a maximum of 50% of water. Snow that persists into summer evolves into névé , granular snow, which has been partially melted, refrozen and compacted. Névé has a minimum density of 500 kilograms per cubic metre (31 lb/cu ft), which is roughly half of the density of liquid water. Firn is snow that has persisted for multiple years and has been recrystallized into

2622-630: The ground and by using remote sensing . Snow is also studied in a more global context of impact on animal habitats and plant succession . An important effort is put into snow classification, both as a hydrometeor and on the ground. Snowpack modeling is done for snow stability, flood forecasting, water resource management, and climate studies. Snowpack modeling is either done by simple, statistical methods such as degree day or complex, physically based energy balance models such as SNOWPACK, CROCUS or SNOWMODEL. Snow Snow comprises individual ice crystals that grow while suspended in

2679-445: The individual crystal to the aggregated snowpack. A sub-specialty is avalanches , which are of concern to engineers and outdoors sports people, alike. Snow science addresses how snow forms, its distribution, and processes affecting how snowpacks change over time. Scientists improve storm forecasting, study global snow cover and its effect on climate, glaciers, and water supplies around the world. The study includes physical properties of

2736-615: The lift needed for condensation and precipitation. A snowflake consists of roughly 10 water molecules which are added to its core at different rates and in different patterns depending on the changing temperature and humidity within the atmosphere that the snowflake falls through on its way to the ground. As a result, snowflakes differ from each other though they follow similar patterns. Snow crystals form when tiny supercooled cloud droplets (about 10  μm in diameter) freeze . These droplets are able to remain liquid at temperatures lower than −18 °C (0 °F), because to freeze,

2793-491: The light. They perform nearly all of their yearly photosynthesis during this period. Evergreen trees tend to produce larger thaw circles than deciduous trees. This involves largely a different mechanism and spring ephemeral plants don't occur there. The snow melts earlier in forest also for example on microtopographic mounds (small elevations) or in wet places like edges of creeks or in seeps . These microsites affect distribution of many herbs too. In northern Alaska,

2850-563: The material as it changes, bulk properties of in-place snow packs, and the aggregate properties of regions with snow cover. In doing so, they employ on-the-ground physical measurement techniques to establish ground truth and remote sensing techniques to develop understanding of snow-related processes over large areas. In the field snow scientists often excavate a snow pit within which to make basic measurements and observations. Observations can describe features caused by wind, water percolation, or snow unloading from trees. Water percolation into

2907-469: The melt-date has advanced by 8 days since the mid-1960s. Decreased snowfall in winter followed by warmer spring conditions seems to be the cause for the advance. In Europe, the 2012 heat wave has especially been anomalous at higher altitudes. For the first time on record, some of the highest Alpine peaks in Europe were snow-free. Although it would seem that the two were related, the question of how much of this

SECTION 50

#1732765923137

2964-502: The mountain West of the US or most of Iran and Afghanistan , very low flow for the rest of the year. In contrast, if much of the melt is from glaciated or nearly glaciated areas, the melt continues through the warm season, with peak flows occurring in mid to late summer. Glaciers form where the accumulation of snow and ice exceeds ablation. The area in which an alpine glacier forms is called

3021-409: The need for keeping roadways, wings, and windows clear; agriculture : providing water to crops and safeguarding livestock; sports such as skiing , snowboarding , and snowmachine travel; and warfare . Snow affects ecosystems , as well, by providing an insulating layer during winter under which plants and animals are able to survive the cold. Snow develops in clouds that themselves are part of

3078-407: The net longwave radiation term is negative, meaning a net loss of energy from the snowpack. Latent temperature flux is the energy removed from or delivered to the snowpack which accompanies the mass transfers of evaporation , sublimation , or condensation . Sensible heat flux is the heat flux due to convection between the air and snowpack. Tree trunks absorbing sunlight become warmer than

3135-401: The same point roughly 30 minutes apart. In cases where there is a large amount of vertical growth and mixing, the squall may develop embedded cumulonimbus clouds resulting in lightning and thunder which is dubbed thundersnow . A warm front can produce snow for a period as warm, moist air overrides below-freezing air and creates precipitation at the boundary. Often, snow transitions to rain in

3192-406: The warm sector behind the front. Lake-effect snow is produced during cooler atmospheric conditions when a cold air mass moves across long expanses of warmer lake water, warming the lower layer of air which picks up water vapor from the lake, rises up through the colder air above, freezes, and is deposited on the leeward (downwind) shores. The same effect occurring over bodies of salt water

3249-489: The world. In the United States , a blizzard occurs when two conditions are met for a period of three hours or more: a sustained wind or frequent gusts to 35 miles per hour (16 m/s), and sufficient snow in the air to reduce visibility to less than 0.4 kilometers (0.25 mi). In Canada and the United Kingdom , the criteria are similar. While heavy snowfall often occurs during blizzard conditions, falling snow

#136863