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Lettris is a curious tetris-clone game where all the bricks have the same square shape but different content. Each square carries a letter. To make squares disappear and save space for other squares you have to assemble English words (left, right, up, down) from the falling squares.
Boggle gives you 3 minutes to find as many words (3 letters or more) as you can in a grid of 16 letters. You can also try the grid of 16 letters. Letters must be adjacent and longer words score better. See if you can get into the grid Hall of Fame !
Change the target language to find translations.
Tips: browse the semantic fields (see From ideas to words) in two languages to learn more.
1.of or pertaining to atmospheric phenomena, especially weather and weather conditions"meteorological factors" "meteorological chart" "meteoric (or meteorological) phenomena"
2.towards the side exposed to wind
1.the atmospheric conditions that comprise the state of the atmosphere in terms of temperature and wind and clouds and precipitation"they were hoping for good weather" "every day we have weather conditions and yesterday was no exception" "the condi..."
1.change under the action or influence of the weather"A weathered old hut"
2.sail to the windward of
3.cause to slope
4.face and withstand with courage"She braved the elements"
1.(geology)condition in which the earth's surface is worn away by the action of water and wind
WeatherWeath"er (?), n. [OE. weder, AS. weder; akin to OS. wedar, OFries. weder, D. weder, weêr, G. wetter, OHG. wetar, Icel. veðr, Dan. veir, Sw. väder wind, air, weather, and perhaps to OSlav. vedro fair weather; or perhaps to Lith. vetra storm, Russ. vieter', vietr', wind, and E. wind. Cf. Wither.]
1. The state of the air or atmosphere with respect to heat or cold, wetness or dryness, calm or storm, clearness or cloudiness, or any other meteorological phenomena; meteorological condition of the atmosphere; as, warm weather; cold weather; wet weather; dry weather, etc.
Not amiss to cool a man's stomach this hot weather. Shak.
Fair weather cometh out of the north. Job xxxvii. 22.
2. Vicissitude of season; meteorological change; alternation of the state of the air. Bacon.
3. Storm; tempest.
What gusts of weather from that gathering cloud
My thoughts presage! Dryden.
4. A light rain; a shower. [Obs.] Wyclif.
Stress of weather, violent winds; force of tempests. -- To make fair weather, to flatter; to give flattering representations. [R.] -- To make good weather, or To make bad weather (Naut.), to endure a gale well or ill; -- said of a vessel. Shak. -- Under the weather, ill; also, financially embarrassed. [Colloq. U. S.] Bartlett. -- Weather box. Same as Weather house, below. Thackeray. -- Weather breeder, a fine day which is supposed to presage foul weather. -- Weather bureau, a popular name for the signal service. See Signal service, under Signal, a. [U. S.] -- Weather cloth (Naut.), a long piece of canvas of tarpaulin used to preserve the hammocks from injury by the weather when stowed in the nettings. -- Weather door. (Mining) See Trapdoor, 2. -- Weather gall. Same as Water gall, 2. [Prov. Eng.] Halliwell. -- Weather house, a mechanical contrivance in the form of a house, which indicates changes in atmospheric conditions by the appearance or retirement of toy images.
Devised the weather house, that useful toy! Cowper.
WeatherWeath"er (?), v. t. [imp. & p. p. Weathered (?); p. pr. & vb. n. Weathering.]
1. To expose to the air; to air; to season by exposure to air.
[An eagle] soaring through his wide empire of the air
To weather his broad sails. Spenser.
This gear lacks weathering. Latimer.
2. Hence, to sustain the trying effect of; to bear up against and overcome; to sustain; to endure; to resist; as, to weather the storm.
For I can weather the roughest gale. Longfellow.
You will weather the difficulties yet. F. W. Robertson.
3. (Naut.) To sail or pass to the windward of; as, to weather a cape; to weather another ship.
4. (Falconry) To place (a hawk) unhooded in the open air. Encyc. Brit.
To weather a point. (a) (Naut.) To pass a point of land, leaving it on the lee side. (b) Hence, to gain or accomplish anything against opposition. -- To weather out, to encounter successfully, though with difficulty; as, to weather out a storm.
WeatherWeath"er, v. i. To undergo or endure the action of the atmosphere; to suffer meteorological influences; sometimes, to wear away, or alter, under atmospheric influences; to suffer waste by weather.
The organisms . . . seem indestructible, while the hard matrix in which they are imbedded has weathered from around them. H. Miller.
WeatherWeath"er, a. (Naut.) Being toward the wind, or windward -- opposed to lee; as, weather bow, weather braces, weather gauge, weather lifts, weather quarter, weather shrouds, etc.
Weather gauge. (a) (Naut.) The position of a ship to the windward of another. (b) Fig.: A position of advantage or superiority; advantage in position.
Against the weather gauge of laws. Hudibras.
WeatheringWeath"er*ing, n. (Geol.) The action of the elements on a rock in altering its color, texture, or composition, or in rounding off its edges.
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Freeman Weather Museum • John Hammond (weather forecaster) • King City weather radar station • Like Weather • Lisa Gallagher (weather forecaster) • List of Dish Network weather / business channels • List of extreme weather events • List of fictional characters who can manipulate weather • List of meteorologists for The Weather Channel • List of severe weather phenomena • List of weather instruments • List of weather records • List of weather-related disasters • Listening for the Weather • Live in Tokyo (Weather Report album) • Magic, Murder and the Weather • Matt Taylor (weather forecaster) • Monthly Weather Review • Mount Weather Emergency Operations Center • NBC Weather Plus • NOAA Weather Radio All Hazards • National Collegiate Weather Forecasting Contest • National Weather Association • National Weather Center • National Weather Digest • National Weather Service • National Weather Service Duties Act of 2005 • National Weather Service Employees Organization • National Weather Service bulletin for New Orleans region • Nick Miller (weather forecaster) • Night Passage (Weather Report album) • North American Weather Services • Overcast weather • Penkridge weather station • Pitsford Hall weather station • Pogonip (weather) • Sailing weather prediction • Severe weather terminology • Severe weather terminology (Canada) • Severe weather terminology (United States) • Significant weather alert • Social Weather Stations • Southern Weather • Space Weather Prediction Center • Space weather • Stormy Weather • Stormy Weather (Carl Hiaasen novel) • Stormy Weather (film) • Stormy Weather (song) • Strange Weather Lately • Surface weather analysis • Susan Powell (weather forecaster) • Take the Weather with You • Talk about the Weather • The Weather • The Weather Channel • The Weather Channel (Australia) • The Weather Channel (Australia) Programming • The Weather Company • The Weather Girls • The Weather Inside • The Weather Makers • The Weather Man • The Weather Prophets • The Weather Underground • Traffic and Weather • Trevor the Weather • U.S. Weather Bureau • UK Weather Records • Under the Weather • Universal Weather and Aviation, Inc. • WAVY Weather Station • Weather (disambiguation) • Weather Competition • Weather Data Mining • Weather Eye • Weather Is Good on Deribasovskaya, It Rains Again on Brighton Beach • Weather Modification Operations and Research Board • Weather Plus • Weather Report • Weather Report (1971 album) • Weather Report (1982 album) • Weather Report (album) • Weather Report Girl • Weather Research and Forecasting model • Weather Star • Weather Star 4000 • Weather Star III • Weather Star Jr • Weather Star XL • Weather Station Kurt • Weather Systems • Weather Underground • Weather Underground (weather service) • Weather Underground Organization • Weather Wizard • Weather and Forecasting • Weather balloon • Weather beacon • Weather buoy • Weather control • Weather extremes in Canada • Weather forecasting • Weather front • Weather gage • Weather helm • Weather hole • Weather house • Weather in 2006 • Weather in 2007 • Weather lore • Weather map • Weather observer • Weather of Armenia • Weather or No • Weather outlook • Weather proverb • Weather radio • Weather records in Windsor, Ontario • Weather satellite • Weather shore • Weather spotter • Weather station • Weather stick • Weather the Storm • Weather vane • Weather warfare • Weather wars • Weather with You • Weather-beaten Melody • Weather-related fatalities in the United States • Weather.com • When the Weather Is Fine • Whiteout (weather) • Winter weather • Wye weather station
Attrition (weathering) • Differential weathering • Enhanced weathering • Factors of polymer weathering • Honeycomb weathering • Mineral weathering • Space weathering • Spheroidal weathering • The Weathering Continent • Weathering nodule of ear • Weathering steel
météorologie et prévision du temps (fr)[termes liés]
(damp; dampness; moistness)[termes liés]
météorologie et prévision du temps (fr)[termes liés]
(damp; dampness; moistness)[termes liés]
atmospheric condition, conditions, weather, weather condition - meteorologist, weather expert, weather forecaster, weatherman, weatherperson - meteoric, meteorologic, meteorological, microphysical, weather[Dérivé]
météorologie et prévision du temps (fr)[termes liés]
météorologie et prévision du temps (fr)[termes liés]
crumble, decay, dilapidate[Hyper.]
(sailing; shipping; navigation)[termes liés]
angle, lean, slant, tilt, tip[Hyper.]
séduire, influencer, aider... (fr)[ClasseParExt...]
personne courageuse (fr)[Classe]
(fight; conflict; struggle; battle; action; combat; hassle; scuffle; tussle; rough-and-tumble; dogfight; scrimmage; clash), (dummy; dummy movement; feint maneuver; feint manoeuvre; sham maneuver; sham manoeuvre; feint)[Thème]
échapper à un danger (fr)[Classe]
(endanger; jeopardize; jeopardise; menace; threaten; imperil; expose to danger), (distress; danger; hazard; jeopardy; peril; risk; endangerment), (endanger; jeopardize; jeopardise; menace; threaten; imperil; expose to danger)[Thème]
weather (v. intr.)
passer un obstacle (fr)[Classe]
weather (v. tr.)
chose défavorable (fr)[ClasseParExt...]
action de (ou fait d'être) (fr)[Classe...]
abîmer par usure (fr)[Classe]
weathering (n.) [geology]
Weather is the state of the atmosphere, to the degree that it is hot or cold, wet or dry, calm or stormy, clear or cloudy. Most weather phenomena occur in the troposphere, just below the stratosphere. Weather refers, generally, to day-to-day temperature and precipitation activity, whereas climate is the term for the average atmospheric conditions over longer periods of time. When used without qualification, "weather" is understood to be the weather of Earth.
Weather is driven by density (temperature and moisture) differences between one place and another. These differences can occur due to the sun angle at any particular spot, which varies by latitude from the tropics. The strong temperature contrast between polar and tropical air gives rise to the jet stream. Weather systems in the mid-latitudes, such as extratropical cyclones, are caused by instabilities of the jet stream flow. Because the Earth's axis is tilted relative to its orbital plane, sunlight is incident at different angles at different times of the year. On Earth's surface, temperatures usually range ±40 °C (100 °F to −40 °F) annually. Over thousands of years, changes in Earth's orbit affect the amount and distribution of solar energy received by the Earth and influence long-term climate and global climate change.
Surface temperature differences in turn cause pressure differences. Higher altitudes are cooler than lower altitudes due to differences in compressional heating. Weather forecasting is the application of science and technology to predict the state of the atmosphere for a future time and a given location. The atmosphere is a chaotic system, so small changes to one part of the system can grow to have large effects on the system as a whole. Human attempts to control the weather have occurred throughout human history, and there is evidence that human activity such as agriculture and industry has inadvertently modified weather patterns.
Studying how the weather works on other planets has been helpful in understanding how weather works on Earth. A famous landmark in the Solar System, Jupiter's Great Red Spot, is an anticyclonic storm known to have existed for at least 300 years. However, weather is not limited to planetary bodies. A star's corona is constantly being lost to space, creating what is essentially a very thin atmosphere throughout the Solar System. The movement of mass ejected from the Sun is known as the solar wind.
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On Earth, common weather phenomena include wind, cloud, rain, snow, fog and dust storms. Less common events include natural disasters such as tornadoes, hurricanes, typhoons and ice storms. Almost all familiar weather phenomena occur in the troposphere (the lower part of the atmosphere). Weather does occur in the stratosphere and can affect weather lower down in the troposphere, but the exact mechanisms are poorly understood.
Weather occurs primarily due to density (temperature and moisture) differences between one place to another. These differences can occur due to the sun angle at any particular spot, which varies by latitude from the tropics. In other words, the farther from the tropics you lie, the lower the sun angle is, which causes those locations to be cooler due to the indirect sunlight. The strong temperature contrast between polar and tropical air gives rise to the jet stream. Weather systems in the mid-latitudes, such as extratropical cyclones, are caused by instabilities of the jet stream flow (see baroclinity). Weather systems in the tropics, such as monsoons or organized thunderstorm systems, are caused by different processes.
Because the Earth's axis is tilted relative to its orbital plane, sunlight is incident at different angles at different times of the year. In June the Northern Hemisphere is tilted towards the sun, so at any given Northern Hemisphere latitude sunlight falls more directly on that spot than in December (see Effect of sun angle on climate). This effect causes seasons. Over thousands to hundreds of thousands of years, changes in Earth's orbital parameters affect the amount and distribution of solar energy received by the Earth and influence long-term climate. (see Milankovitch cycles).
The uneven solar heating (the formation of zones of temperature and moisture gradients, or frontogenesis) can also be due to the weather itself in the form of cloudiness and precipitation. Higher altitudes are cooler than lower altitudes, which is explained by the lapse rate. On local scales, temperature differences can occur because different surfaces (such as oceans, forests, ice sheets, or man-made objects) have differing physical characteristics such as reflectivity, roughness, or moisture content.
Surface temperature differences in turn cause pressure differences. A hot surface heats the air above it and the air expands, lowering the air pressure and its density. The resulting horizontal pressure gradient accelerates the air from high to low pressure, creating wind, and Earth's rotation then causes curvature of the flow via the Coriolis effect. The simple systems thus formed can then display emergent behaviour to produce more complex systems and thus other weather phenomena. Large scale examples include the Hadley cell while a smaller scale example would be coastal breezes.
The atmosphere is a chaotic system, so small changes to one part of the system can grow to have large effects on the system as a whole. This makes it difficult to accurately predict weather more than a few days in advance, though weather forecasters are continually working to extend this limit through the scientific study of weather, meteorology. It is theoretically impossible to make useful day-to-day predictions more than about two weeks ahead, imposing an upper limit to potential for improved prediction skill.
Weather is one of the fundamental processes that shape the Earth. The process of weathering breaks down the rocks and soils into smaller fragments and then into their constituent substances. These are then free to take part in chemical reactions that can affect the surface further (such as acid rain) or are reformed into other rocks and soils. In this way, weather plays a major role in erosion of the surface.
Weather has played a large and sometimes direct part in human history. Aside from climatic changes that have caused the gradual drift of populations (for example the desertification of the Middle East, and the formation of land bridges during glacial periods), extreme weather events have caused smaller scale population movements and intruded directly in historical events. One such event is the saving of Japan from invasion by the Mongol fleet of Kublai Khan by the Kamikaze winds in 1281. French claims to Florida came to an end in 1565 when a hurricane destroyed the French fleet, allowing Spain to conquer Fort Caroline. More recently, Hurricane Katrina redistributed over one million people from the central Gulf coast elsewhere across the United States, becoming the largest diaspora in the history of the United States.
The Little Ice Age caused crop failures and famines in Europe. The 1690s saw the worst famine in France since the Middle Ages. Finland suffered a severe famine in 1696–1697, during which about one-third of the Finnish population died.
The human body is negatively affected by extremes in temperature, humidity, and wind.
Weather forecasting is the application of science and technology to predict the state of the atmosphere for a future time and a given location. Human beings have attempted to predict the weather informally for millennia, and formally since at least the nineteenth century. Weather forecasts are made by collecting quantitative data about the current state of the atmosphere and using scientific understanding of atmospheric processes to project how the atmosphere will evolve.
Once an all-human endeavor based mainly upon changes in barometric pressure, current weather conditions, and sky condition, forecast models are now used to determine future conditions. Human input is still required to pick the best possible forecast model to base the forecast upon, which involves pattern recognition skills, teleconnections, knowledge of model performance, and knowledge of model biases. The chaotic nature of the atmosphere, the massive computational power required to solve the equations that describe the atmosphere, error involved in measuring the initial conditions, and an incomplete understanding of atmospheric processes mean that forecasts become less accurate as the difference in current time and the time for which the forecast is being made (the range of the forecast) increases. The use of ensembles and model consensus helps to narrow the error and pick the most likely outcome.
There are a variety of end users to weather forecasts. Weather warnings are important forecasts because they are used to protect life and property. Forecasts based on temperature and precipitation are important to agriculture, and therefore to commodity traders within stock markets. Temperature forecasts are used by utility companies to estimate demand over coming days. On an everyday basis, people use weather forecasts to determine what to wear on a given day. Since outdoor activities are severely curtailed by heavy rain, snow and the wind chill, forecasts can be used to plan activities around these events, and to plan ahead and survive them.
The aspiration to control the weather is evident throughout human history: from ancient rituals intended to bring rain for crops to the U.S. Military Operation Popeye, an attempt to disrupt supply lines by lengthening the North Vietnamese monsoon. The most successful attempts at influencing weather involve cloud seeding; they include the fog- and low stratus dispersion techniques employed by major airports, techniques used to increase winter precipitation over mountains, and techniques to suppress hail. A recent example of weather control was China's preparation for the 2008 Summer Olympic Games. China shot 1,104 rain dispersal rockets from 21 sites in the city of Beijing in an effort to keep rain away from the opening ceremony of the games on 8 August 2008. Guo Hu, head of the Beijing Municipal Meteorological Bureau (BMB), confirmed the success of the operation with 100 millimeters falling in Baoding City of Hebei Province, to the southwest and Beijing's Fangshan District recording a rainfall of 25 millimeters.
Whereas there is inconclusive evidence for these techniques' efficacy, there is extensive evidence that human activity such as agriculture and industry results in inadvertent weather modification:
The effects of inadvertent weather modification may pose serious threats to many aspects of civilization, including ecosystems, natural resources, food and fiber production, economic development, and human health.
On Earth, temperatures usually range ±40 °C (100 °F to −40 °F) annually. The range of climates and latitudes across the planet can offer extremes of temperature outside this range. The coldest air temperature ever recorded on Earth is −89.2 °C (−128.6 °F), at Vostok Station, Antarctica on 21 July 1983. The hottest air temperature ever recorded was 57.7 °C (135.9 °F) at 'Aziziya, Libya, on 13 September 1922, but that reading is queried. The highest recorded average annual temperature was 34.4 °C (93.9 °F) at Dallol, Ethiopia. The coldest recorded average annual temperature was −55.1 °C (−67.2 °F) at Vostok Station, Antarctica. The coldest average annual temperature in a permanently inhabited location is at Eureka, Nunavut, in Canada, where the annual average temperature is −19.7 °C (−3.5 °F).
Studying how the weather works on other planets has been seen as helpful in understanding how it works on Earth. Weather on other planets follows many of the same physical principles as weather on Earth, but occurs on different scales and in atmospheres having different chemical composition. The Cassini–Huygens mission to Titan discovered clouds formed from methane or ethane which deposit rain composed of liquid methane and other organic compounds. Earth's atmosphere includes six latitudinal circulation zones, three in each hemisphere. In contrast, Jupiter's banded appearance shows many such zones, Titan has a single jet stream near the 50th parallel north latitude, and Venus has a single jet near the equator.
One of the most famous landmarks in the Solar System, Jupiter's Great Red Spot, is an anticyclonic storm known to have existed for at least 300 years. On other gas giants, the lack of a surface allows the wind to reach enormous speeds: gusts of up to 600 metres per second (about 2,100 km/h or 1,300 mph) have been measured on the planet Neptune. This has created a puzzle for planetary scientists. The weather is ultimately created by solar energy and the amount of energy received by Neptune is only about 1⁄900 of that received by Earth, yet the intensity of weather phenomena on Neptune is far greater than on Earth. The strongest planetary winds discovered so far are on the extrasolar planet HD 189733 b, which is thought to have easterly winds moving at more than 9,600 kilometres per hour (6,000 mph).
Weather is not limited to planetary bodies. Like all stars, the sun's corona is constantly being lost to space, creating what is essentially a very thin atmosphere throughout the Solar System. The movement of mass ejected from the Sun is known as the solar wind. Inconsistencies in this wind and larger events on the surface of the star, such as coronal mass ejections, form a system that has features analogous to conventional weather systems (such as pressure and wind) and is generally known as space weather. Coronal mass ejections have been tracked as far out in the solar system as Saturn. The activity of this system can affect planetary atmospheres and occasionally surfaces. The interaction of the solar wind with the terrestrial atmosphere can produce spectacular aurorae, and can play havoc with electrically sensitive systems such as electricity grids and radio signals.
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Weathering is the breaking down of rocks, soils and minerals as well as artificial materials through contact with the Earth's atmosphere, biota and waters. Weathering occurs in situ, or "with no movement", and thus should not be confused with erosion, which involves the movement of rocks and minerals by agents such as water, ice, snow, wind and gravity.
Two important classifications of weathering processes exist – physical and chemical weathering. Mechanical or physical weathering involves the breakdown of rocks and soils through direct contact with atmospheric conditions, such as heat, water, ice and pressure. The second classification, chemical weathering, involves the direct effect of atmospheric chemicals or biologically produced chemicals (also known as biological weathering) in the breakdown of rocks, soils and minerals.
The materials left over after the rock breaks down combined with organic material creates soil. The mineral content of the soil is determined by the parent material, thus a soil derived from a single rock type can often be deficient in one or more minerals for good fertility, while a soil weathered from a mix of rock types (as in glacial, aeolian or alluvial sediments) often makes more fertile soil. In addition many of Earth's landforms and landscapes are the result of weathering processes combined with erosion and re-deposition.
Physical weathering is the class of processes that causes the disintegration of rocks without chemical change. The primary process in physical weathering is abrasion (the process by which clasts and other particles are reduced in size). However, chemical and physical weathering often go hand in hand. Physical weathering can occur due to temperature, pressure, frost etc. For example, cracks exploited by physical weathering will increase the surface area exposed to chemical action. Furthermore, the chemical action of minerals in cracks can aid the disintegration process.
Thermal stress weathering (sometimes called insolation weathering) results from expansion or contraction of rock, caused by temperature changes. Thermal stress weathering comprises two main types, thermal shock and thermal fatigue. Thermal stress weathering is an important mechanism in deserts, where there is a large diurnal temperature range, hot in the day and cold at night. The repeated heating and cooling exerts stress on the outer layers of rocks, which can cause their outer layers to peel off in thin sheets. Although temperature changes are the principal driver, moisture can enhance thermal expansion in rock. Forest fires and range fires are also known to cause significant weathering of rocks and boulders exposed along the ground surface. Intense, localized heat can rapidly expand a boulder.
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Frost weathering, frost wedging, ice wedging or cryofracturing is the collective name for several processes where ice is present. These processes include frost shattering, frost-wedging and freeze-thaw weathering. This type of weathering is common in mountain areas where the temperature is around the freezing point of water. Certain frost-susceptible soils expand or heave upon freezing as a result of water migrating via capillary action to grow ice lenses near the freezing front. This same phenomenon occurs within pore spaces of rocks. The ice accumulations grow larger as they attract liquid water from the surrounding pores. The ice crystal growth weakens the rocks which, in time, break up. It is caused by the approximately 10% (9.87) expansion of ice when water freezes, which can place considerable stress on anything containing the water as it freezes.
Freeze induced weathering action occurs mainly in environments where there is a lot of moisture, and temperatures frequently fluctuate above and below freezing point, especially in alpine and periglacial areas. An example of rocks susceptible to frost action is chalk, which has many pore spaces for the growth of ice crystals. This process can be seen in Dartmoor where it results in the formation of tors. When water that has entered the joints freezes, the ice formed strains the walls of the joints and causes the joints to deepen and widen. When the ice thaws, water can flow further into the rock. Repeated freeze-thaw cycles weaken the rocks which, over time, break up along the joints into angular pieces. The angular rock fragments gather at the foot of the slope to form a talus slope (or scree slope). The splitting of rocks along the joints into blocks is called block disintegration. The blocks of rocks that are detached are of various shapes depending on rock structure.
In pressure release, also known as unloading, overlying materials (not necessarily rocks) are removed (by erosion, or other processes), which causes underlying rocks to expand and fracture parallel to the surface.
Intrusive igneous rocks (e.g. granite) are formed deep beneath the Earth's surface. They are under tremendous pressure because of the overlying rock material. When erosion removes the overlying rock material, these intrusive rocks are exposed and the pressure on them is released. The outer parts of the rocks then tend to expand. The expansion sets up stresses which cause fractures parallel to the rock surface to form. Over time, sheets of rock break away from the exposed rocks along the fractures, a process known as exfoliation. Exfoliation due to pressure release is also known as "sheeting".
Retreat of an overlying glacier can also lead to exfoliation due to pressure release.
Hydraulic action occurs when water (generally from powerful waves) rushes rapidly into cracks in the rock face, thus trapping a layer of air at the bottom of the crack, compressing it and weakening the rock. When the wave retreats, the trapped air is suddenly released with explosive force.
Salt crystallization, otherwise known as haloclasty, causes disintegration of rocks when saline (see salinity) solutions seep into cracks and joints in the rocks and evaporate, leaving salt crystals behind. These salt crystals expand as they are heated up, exerting pressure on the confining rock.
Salt crystallization may also take place when solutions decompose rocks (for example, limestone and chalk) to form salt solutions of sodium sulfate or sodium carbonate, of which the moisture evaporates to form their respective salt crystals.
It is normally associated with arid climates where strong heating causes strong evaporation and therefore salt crystallization. It is also common along coasts. An example of salt weathering can be seen in the honeycombed stones in sea wall. Honeycomb is a type of tafoni, a class of cavernous rock weathering structures, which likely develop in large part by chemical and physical salt weathering processes.
Living organisms may contribute to mechanical weathering (as well as chemical weathering, see 'biological' weathering below). Lichens and mosses grow on essentially bare rock surfaces and create a more humid chemical microenvironment. The attachment of these organisms to the rock surface enhances physical as well as chemical breakdown of the surface microlayer of the rock. On a larger scale, seedlings sprouting in a crevice and plant roots exert physical pressure as well as providing a pathway for water and chemical infiltration.
Chemical weathering changes the composition of rocks, often transforming them when water interacts with minerals to create various chemical reactions. Chemical weathering is a gradual and ongoing process as the mineralogy of the rock adjusts to the near surface environment. New or secondary minerals develop from the original minerals of the rock. In this the processes of oxidation and hydrolysis are most important.
The process of mountain block uplift is important in exposing new rock strata to the atmosphere and moisture, enabling important chemical weathering to occur; significant release occurs of Ca++ and other minerals into surface waters.
Rainfall is acidic because atmospheric carbon dioxide dissolves in the rainwater producing weak carbonic acid. In unpolluted environments, the rainfall pH is around 5.6. Acid rain occurs when gases such as sulfur dioxide and nitrogen oxides are present in the atmosphere. These oxides react in the rain water to produce stronger acids and can lower the pH to 4.5 or even 3.0. Sulfur dioxide, SO2, comes from volcanic eruptions or from fossil fuels, can become sulfuric acid within rainwater, which can cause solution weathering to the rocks on which it falls.
Some minerals, due to their natural solubility (e.g. evaporites), oxidation potential (iron-rich minerals, such as pyrite), or instability relative to surficial conditions (see Goldich dissolution series) will weather through dissolution naturally, even without acidic water.
One of the most well-known solution weathering processes is carbonation, the process in which atmospheric carbon dioxide leads to solution weathering. Carbonation occurs on rocks which contain calcium carbonate, such as limestone and chalk. This takes place when rain combines with carbon dioxide or an organic acid to form a weak carbonic acid which reacts with calcium carbonate (the limestone) and forms calcium bicarbonate. This process speeds up with a decrease in temperature, not because low temperatures generally drive reactions faster, but because colder water holds more dissolved carbon dioxide gas. Carbonation is therefore a large feature of glacial weathering.
The reactions as follows:
Carbonation on the surface of well-jointed limestone produces a dissected limestone pavement. This process is most effective along the joints, widening and deepening them.
Mineral hydration is a form of chemical weathering that involves the rigid attachment of H+ and OH- ions to the atoms and molecules of a mineral.
Hydrolysis is a chemical weathering process affecting silicate and carbonate minerals. In such reactions, pure water ionizes slightly and reacts with silicate minerals. An example reaction:
This reaction theoretically results in complete dissolution of the original mineral, if enough water is available to drive the reaction. In reality, pure water rarely acts as a H+ donor. Carbon dioxide, though, dissolves readily in water forming a weak acid and H+ donor.
This hydrolysis reaction is much more common. Carbonic acid is consumed by silicate weathering, resulting in more alkaline solutions because of the bicarbonate. This is an important reaction in controlling the amount of CO2 in the atmosphere and can affect climate.
Aluminosilicates when subjected to the hydrolysis reaction produce a secondary mineral rather than simply releasing cations.
Within the weathering environment chemical oxidation of a variety of metals occurs. The most commonly observed is the oxidation of Fe2+ (iron) and combination with oxygen and water to form Fe3+ hydroxides and oxides such as goethite, limonite, and hematite. This gives the affected rocks a reddish-brown coloration on the surface which crumbles easily and weakens the rock. This process is better known as 'rusting', though it is distinct from the rusting of metallic iron. Many other metallic ores and minerals oxidize and hydrate to produce colored deposits, such as chalcopyrites or CuFeS2 oxidizing to copper hydroxide and iron oxides.
A number of plants and animals may create chemical weathering through release of acidic compounds, i.e. moss on roofs is classed as weathering. Mineral weathering can also be initiated and/or accelerated by soil microorganisms. Lichens on rocks are thought to increase chemical weathering rates. For example, an experimental study on hornblende granite in New Jersey, USA, demonstrated a 3x - 4x increase in weathering rate under lichen covered surfaces compared to recently exposed bare rock surfaces.
The most common forms of biological weathering are the release of chelating compounds (i.e. organic acids, siderophores) and of acidifying molecules (i.e. protons, organic acids) by plants so as to break down aluminium and iron containing compounds in the soils beneath them. Decaying remains of dead plants in soil may form organic acids which, when dissolved in water, cause chemical weathering. Extreme release of chelating compounds can easily affect surrounding rocks and soils, and may lead to podsolisation of soils.
The symbiotic mycorrhizal fungi associated with tree root systems can release inorganic nutrients from minerals such as apatite or biotite and transfer these nutrients to the trees, thus contributing to tree nutrition. It was also recently evidenced that bacterial communities can impact mineral stability leading to the release of inorganic nutrients. To date a large range of bacterial strains or communities from diverse genera have been reported to be able to colonize mineral surfaces and/or to weather minerals, and for some of them a plant growth promoting effect was demonstrated. The demonstrated or hypothesised mechanisms used by bacteria to weather minerals include several oxidoreduction and dissolution reactions as well as the production of weathering agents, such as protons, organic acids and chelating molecules.
Buildings made of any stone, brick or concrete are susceptible to the same weathering agents as any exposed rock surface. Also statues, monuments and ornamental stonework can be badly damaged by natural weathering processes. This is accelerated in areas severely affected by acid rain.
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