Condenser (heat transfer)
In systems involving heat transfer, a condenser is a device or unit used to condense a substance from its gaseous to its liquid state, by cooling it. In so doing, the latent heat is given up by the substance and transferred to the surrounding environment. Condensers can be made according to numerous designs, come in many sizes ranging from rather small to large. For example, a refrigerator uses a condenser to get rid of heat extracted from the interior of the unit to the outside air. Condensers are used in air conditioning, industrial chemical processes such as distillation, steam power plants and other heat-exchange systems. Use of cooling water or surrounding air as the coolant is common in many condensers. A surface condenser is one in which condensing medium and vapors are physically separated and used when direct contact is not desired, it is a shell and tube heat exchanger installed at the outlet of every steam turbine in thermal power stations. The cooling water flows through the tube side and the steam enters the shell side where the condensation occurs on the outside of the heat transfer tubes.
The condensate drips down and collects at the bottom in a built-in pan called a hotwell. The shell side operates at a vacuum or partial vacuum, produced by the difference in specific volume between the steam and condensate. Conversely, the vapor can be fed through the tubes with the coolant water or air flowing around the outside. In chemistry, a condenser is the apparatus which cools hot vapors, causing them to condense into a liquid. See "Condenser" for laboratory-scale condensers, as opposed to industrial-scale condensers. Examples include the Liebig condenser, Graham condenser, Allihn condenser; this is not to be confused with a condensation reaction which links two fragments into a single molecule by an addition reaction and an elimination reaction. In laboratory distillation and rotary evaporators, several types of condensers are used; the Liebig condenser is a straight tube within a cooling water jacket, is the simplest form of condenser. The Graham condenser is a spiral tube within a water jacket, the Allihn condenser has a series of large and small constrictions on the inside tube, each increasing the surface area upon which the vapor constituents may condense.
Being more complex shapes to manufacture, these latter types are more expensive to purchase. These three types of condensers are laboratory glassware items since they are made of glass. Commercially available condensers are fitted with ground glass joints and come in standard lengths of 100, 200, 400 mm. Air-cooled condensers are unjacketed, while water-cooled condensers contain a jacket for the water. Larger condensers are used in industrial-scale distillation processes to cool distilled vapor into liquid distillate; the coolant flows through the tube side and distilled vapor through the shell side with distillate collecting at or flowing out the bottom. A condenser unit used in central air conditioning systems has a heat exchanger section to cool down and condense incoming refrigerant vapor into liquid, a compressor to raise the pressure of the refrigerant and move it along, a fan for blowing outside air through the heat exchanger section to cool the refrigerant inside. A typical configuration of such a condenser unit is as follows: The heat exchanger section wraps around the sides of the unit with the compressor inside.
In this heat exchanger section, the refrigerant goes through multiple tube passes, which are surrounded by heat transfer fins through which cooling air can circulate from outside to inside the unit. There is a motorized fan inside the condenser unit near the top, covered by some grating to keep any objects from accidentally falling inside on the fan; the fan is used to pull outside cooling air in through the heat exchanger section at the sides and blow it out the top through the grating. These condenser units are located on the outside of the building they are trying to cool, with tubing between the unit and building, one for vapor refrigerant entering and another for liquid refrigerant leaving the unit. Of course, an electric power supply is needed for the fan inside the unit. In a direct-contact condenser, hot vapor and cool liquid are introduced into a vessel and allowed to mix directly, rather than being separated by a barrier such as the wall of a heat exchanger tube; the vapor gives up its latent heat and condenses to a liquid, while the liquid absorbs this heat and undergoes a temperature rise.
The entering vapor and liquid contain a single condensable substance, such as a water spray being used to cool air and adjust its humidity. Other Types of Condensers There are three other condensers used in HVAC systems: Water-cooled Air-cooled EvaporativeApplications: Air cooled – If the condenser is located on the outside of the unit, the air cooled condenser can provide the easiest arrangement; these types of condensers are simple to install. Most common uses for this condenser are domestic refrigerators, upright freezers and in residential packaged air conditioning units. A great feature of the air cooled condenser is they are easy to clean. Since dirt can cause serious issues with the condensers performance, it is recommended that these be kept clear of dirt. Water cooled – Although a little more pricey to install, these condensers are the more efficient type. Used for swimming pools and condensers piped for city water flow, these condensers require regular service and maintenance, they require a cooling tower to conserve water.
To prevent corrosion and the fo
A snowflake is a single ice crystal that has achieved a sufficient size, may have amalgamated with others falls through the Earth's atmosphere as snow. Each flake nucleates around a dust particle in supersaturated air masses by attracting supercooled cloud water droplets, which freeze and accrete in crystal form. Complex shapes emerge as the flake moves through differing temperature and humidity zones in the atmosphere, such that individual snowflakes differ in detail from one another, but may be categorized in eight broad classifications and at least 80 individual variants; the main constituent shapes for ice crystals, from which combinations may occur, are needle, column and rime. Snow appears white in color despite being made of clear ice; this is due to diffuse reflection of the whole spectrum of light by the small crystal facets of the snowflakes. Snowflakes nucleate around mineral or organic particles in moisture-saturated, subfreezing air masses, they grow by net accretion to the incipient crystals in hexagonal formations.
The cohesive forces are electrostatic. In warmer clouds, an aerosol particle or "ice nucleus" must be present in the droplet to act as a nucleus; the particles that make ice nuclei are rare compared to nuclei upon which liquid cloud droplets form. Clays, desert dust, biological particles may be effective, although to what extent is unclear. Artificial nuclei include particles of silver iodide and dry ice, these are used to stimulate precipitation in cloud seeding. Experiments show that "homogeneous" nucleation of cloud droplets only occurs at temperatures lower than −35 °C. Once 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 grows by deposition of water molecules in the air onto the ice crystal surface where they are collected. Because water droplets are so much more numerous than the ice crystals due to their sheer abundance, the crystals are able to grow to hundreds of micrometers or millimeters in size at the expense of the water droplets.
This process is known as the Wegener–Bergeron–Findeisen process. The corresponding depletion of water vapor causes the droplets to evaporate, meaning that the ice crystals grow at the droplets' expense; these large crystals are an efficient source of precipitation, since they fall through the atmosphere due to their mass, may collide and stick together in clusters, or aggregates. These aggregates are the type of ice particle that falls to the ground. Guinness World Records lists the world's largest snowflakes as those of January 1887 at Fort Keogh, Montana. Although this report by a farmer is doubtful, aggregates of three or four inches width have been observed. Single crystals the size of a dime have been observed. Snowflakes encapsulated in rime form balls known as graupel. Although ice by itself is clear, snow appears white in color due to diffuse reflection of the whole spectrum of light by the scattering of light by the small crystal facets of the snowflakes of which it is comprised; the shape of the snowflake is determined broadly by the temperature and humidity at which it is formed.
At a temperature of around −2 °C, snowflakes can form in threefold symmetry — triangular snowflakes. The most common snow particles are visibly irregular, although near-perfect snowflakes may be more common in pictures because they are more visually appealing, it is unlikely that any two snowflakes are alike due to the estimated 1019 water molecules which make up a typical snowflake, which grow 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. Snowflakes that look identical, but may vary at the molecular level, have been grown under controlled conditions. Although snowflakes are never symmetrical, a non-aggregated snowflake grows so as to exhibit an approximation of six-fold radial symmetry; the symmetry gets started due to the hexagonal crystalline structure of ice. At that stage, the snowflake has the shape of a minute hexagon; the six "arms" of the snowflake, or dendrites grow independently from each of the corners of the hexagon, while either side of each arm grows independently.
The microenvironment in which the snowflake grows changes dynamically as the snowflake falls through the cloud and tiny changes in temperature and humidity affect the way in which water molecules attach to the snowflake. Since the micro-environment are nearly identical around the snowflake, each arm tends to grow in nearly the same way. However, being in the same micro-environment does not guarantee that each arm grow the same. Empirical studies suggest less than 0.1% of snowflakes exhibit the ideal six-fold symmetric shape. Twelve branched snowflakes are observed. Snowflakes form in a wide variety of intricate shapes, leading to the notion that "no two are alike". Although nearly-identical snowflakes have been made in laboratory, they are unlikely to be found in nature. Initial attempts to find identical snowflakes by photographing thousands of them with a microscope from 1885 onward by Wilson Alwyn Bentley found the wide variety of snowflakes we know about today. Ukichiro Nakaya developed a crystal morphology diagram, relating crystal shape to the temperature and moisture
Construction is the process of constructing a building or infrastructure. Construction differs from manufacturing in that manufacturing involves mass production of similar items without a designated purchaser, while construction takes place on location for a known client. Construction as an industry comprises six to nine percent of the gross domestic product of developed countries. Construction starts with planning and financing. Large-scale construction requires collaboration across multiple disciplines. A project manager manages the job, a construction manager, design engineer, construction engineer or architect supervises it; those involved with the design and execution must consider zoning requirements, environmental impact of the job, budgeting, construction-site safety and transportation of building materials, inconvenience to the public caused by construction delays and bidding. Large construction projects are sometimes referred to as megaprojects. Construction is a general term meaning the art and science to form objects, systems, or organizations, comes from Latin constructio and Old French construction.
To construct is the verb: the act of building, the noun construction: how a building was built, the nature of its structure. In general, there are three sectors of construction: buildings and industrial. Building construction is further divided into residential and non-residential. Infrastructure is called heavy civil or heavy engineering that includes large public works, bridges, railways, water or wastewater and utility distribution. Industrial construction includes refineries, process chemical, power generation and manufacturing plants. There are other ways to break the industry into sectors or markets. Engineering News-Record, a trade magazine for the construction industry, each year compiles and reports data about the size of design and construction companies. In 2014, ENR compiled the data in nine market segments divided as transportation, buildings, industrial, manufacturing, sewer/waste, hazardous waste and a tenth category for other projects. In their reporting, they used data on transportation, hazardous waste and water to rank firms as heavy contractors.
The Standard Industrial Classification and the newer North American Industry Classification System have a classification system for companies that perform or engage in construction. To recognize the differences of companies in this sector, it is divided into three subsectors: building construction and civil engineering construction, specialty trade contractors. There are categories for construction service firms and construction managers. Building construction is the process of adding structure to real property or construction of buildings; the majority of building construction jobs are small renovations, such as addition of a room, or renovation of a bathroom. The owner of the property acts as laborer and design team for the entire project. Although building construction projects consist of common elements such as design, financial and legal considerations, projects of varying sizes may reach undesirable end results, such as structural collapse, cost overruns, and/or litigation. For this reason, those with experience in the field make detailed plans and maintain careful oversight during the project to ensure a positive outcome.
Commercial building construction is procured or publicly utilizing various delivery methodologies, including cost estimating, hard bid, negotiated price, management contracting, construction management-at-risk, design & build and design-build bridging. Residential construction practices and resources must conform to local building authority regulations and codes of practice. Materials available in the area dictate the construction materials used. Cost of construction on a per square meter basis for houses can vary based on site conditions, local regulations, economies of scale and the availability of skilled tradesmen. Residential construction as well as other types of construction can generate waste such that planning is required. According to McKinsey research, productivity growth per worker in construction has lagged behind many other industries across different countries including in the United States and in European countries. In the United States, construction productivity per worker has declined by half since the 1960s.
The most popular method of residential construction in North America is wood-framed construction. Typical construction steps for a single-family or small multi-family house are: Obtain an engineered soil test of lot where construction is planned. From an engineer or company specializing in soil testing. Develop floor plans and obtain a materials list for estimations Obtain structural engineered plans for foundation and structure. To be completed by either a licensed engineer or architect. To include both a foundation and framing plan. Obtain lot survey Obtain government building approval if necessary If required obtain approval from HOA or ARC Clear the building site Survey to stake out for the foun
Desertification is a type of land degradation in which a dry area of land becomes a desert losing its bodies of water as well as vegetation and wildlife. It is caused by a variety of factors, such as through climate change and through the overexploitation of soil through human activity; when deserts appear automatically over the natural course of a planet's life cycle it can be called a natural phenomenon. Desertification is a significant global ecological and environmental problem with far reaching consequences on socio-economic and political conditions. Considerable controversy exists over the proper definition of the term "desertification" for which Helmut Geist has identified more than 100 formal definitions; the most accepted of these is that of the Princeton University Dictionary which defines it as "the process of fertile land transforming into desert as a result of deforestation, drought or improper/inappropriate agriculture". Desertification has been neatly defined in the text of the United Nations Convention to Combat Desertification as "land degradation in arid, semi-arid and dry sub-humid regions resulting from various factors, including climatic variations and human activities."Another major contribution to the controversy comes from the sub-grouping of types of desertification.
Spanning from the vague yet shortsighted view as the "man-made-desert" to the broader yet less focused type as the "Non-pattern-Desert". The earliest known discussion of the topic arose soon after the French colonization of West Africa, when the Comité d'Etudes commissioned a study on desséchement progressif to explore the prehistoric expansion of the Sahara Desert; the world's most noted deserts have been formed by natural processes interacting over long intervals of time. During most of these times, deserts have shrunk independent of human activities. Paleodeserts are large sand seas now inactive because they are stabilized by vegetation, some extending beyond the present margins of core deserts, such as the Sahara, the largest hot desert. Desertification has played a significant role in human history, contributing to the collapse of several large empires, such as Carthage and the Roman Empire, as well as causing displacement of local populations. Historical evidence shows that the serious and extensive land deterioration occurring several centuries ago in arid regions had three epicenters: the Mediterranean, the Mesopotamian Valley, the Loess Plateau of China, where population was dense.
Drylands occupy 40–41% of Earth’s land area and are home to more than 2 billion people. It has been estimated that some 10–20% of drylands are degraded, the total area affected by desertification being between 6 and 12 million square kilometres, that about 1–6% of the inhabitants of drylands live in desertified areas, that a billion people are under threat from further desertification; as of 1998, the then-current degree of southward expansion of the Sahara was not well known, due to a lack of recent, measurable expansion of the desert into the Sahel at the time. The impact of global warming and human activities are presented in the Sahel. In this area, the level of desertification is high compared to other areas in the world. All areas situated in the eastern part of Africa are characterized by a dry climate, hot temperatures, low rainfall. So, droughts are the rule in the Sahel region; some studies have shown that Africa has lost 650,000 km² of its productive agricultural land over the past 50 years.
The propagation of desertification in this area is considerable. Some statistics have shown that since 1900 the Sahara has expanded by 250 km to the south over a stretch of land from west to east 6,000 km long; the survey, done by the research institute for development, had demonstrated that this means dryness is spreading fast in the Sahelian countries. 70% of the arid area has deteriorated and water resources have disappeared, leading to soil degradation. The loss of topsoil means that plants cannot take root and can be uprooted by torrential water or strong winds; the United Nations Convention says that about six million Sahelian citizens would have to give up the desertified zones of sub-Saharan Africa for North Africa and Europe between 1997 and 2020. Another major area, being impacted by desertification is the Gobi Desert; the Gobi desert is the fastest moving desert on Earth. This has destroyed many villages in its path. Photos show that the Gobi Desert has expanded to the point the entire nation of Croatia could fit inside its area.
This is causing a major problem for the people of China. They will soon have to deal with the desert. Although the Gobi Desert itself is still a distance away from Beijing, reports from field studies state there are large sand dunes forming only 70 km outside the city; as the desertification takes place, the landscape may progress through different stages and continuously transform in appearance. On sloped terrain, desertification can create larger empty spaces over a large strip of land, a phenomenon known as "Brousse tigrée". A mathematical model of this phenomenon proposed by C. Klausmeier attributes this patterning to dynamics in plant-water interaction. One outcome of this observation suggests an optimal plan
Darkling beetle is the common name of the large family of beetles, Tenebrionidae. The number of species in the Tenebrionidae is estimated at more than 20,000 and the family is cosmopolitan in distribution. Tenebrio is the Latin generic name that Carl Linnaeus assigned to some flour beetles in his 10th edition of Systema Naturae 1758-59.. The word means "seeker of dark places". Numerous Tenebrionidae species do inhabit dark places, there are many species in genera such as Stenocara and Onymacris, which are active by day and inactive at night; the family covers a varied range of forms, such that classification presents great difficulties. The following list of subfamilies was accepted in 2005. Alleculinae Laporte, 1840 Cossyphodinae Wasmann, 1899 Diaperinae Latreille, 1802 Lagriinae Latreille, 1825 Nilioninae Lacordaire, 1859 Phrenapatinae Solier, 1834 Pimeliinae Latreille, 1802 Stenochiinae Kirby, 1837 Tenebrioninae Latreille, 1802 Zolodininae Watt, 1974Ongoing phylogenetic studies are showing that some taxonomic changes are needed.
For instance the tribal classification of tribe Pedinini has been altered. The misspelling "Terebrionidae" occurs enough to be overlooked; the error appears to have no particular significance, but to be the product of misreadings, mis-scans and mis-typings. The Tenebrionidae may be identified by a combination of features, including: Their 11-segmented antennae that may be filiform, moniliform or weakly clubbed First abdominal sternite is entire and not divided by the hind coxae Eyes notched by a frontal ridge The tarsi have four segments in the hind pair and five in the fore and mid legs, tarsal claws are simple Tenebrionid beetles occupy ecological niches in deserts and forests as plant scavengers. Most species are generalistic omnivores, feed on decaying leaves, rotting wood, fresh plant matter, dead insects, fungi as larvae and adults. Several genera, including Bolitotherus, are specialized fungivores. Many of the larger species are flightless, those that are capable, such as T. molitor, only do so when necessary, such as when dispersing or malnourished.
The larvae, known as mealworms or false wireworms, are fossorial armored and nocturnal. They may be an important resource for certain invertebrates and small mammals. However, the adults of many species have chemical defenses and are protected against predators. Adults of most species, except grain pests, have slow metabolisms, live long lives compared to other insects, ranging from six months to two years; some species live in intensely dry deserts such as the Namib, have evolved adaptions by which they collect droplets of fog that deposit on their elytra. As the droplets accumulate the water drains down the beetles' backs to their mouthparts, where they swallow it. Humans spread some species such that they have become cosmopolitan, such as Tribolium castaneum, the red flour beetle, spread through grain products; the larval stages of several species are cultured as feeder insects for captive insectivores or as laboratory subjects: Tenebrio molitor is used to feed terrestrial amniotes kept in terraria.
Tribolium castaneum is a laboratory animal useful as a model organism in studies of intragenomic conflict and population ecology. Zophobas morio, or superworm, is valued as a feed for captive reptiles. Alphitobius diaperinus, lesser mealworm Many tenebrionids are pests of cereal and flour silos and other storage facilities, including T. castaneum, other Tribolium species such as Tribolium confusum and Tribolium destructor, Gnathocerus cornutus. In southwestern North America, species of the genus Eleodes are well known as "pinacate beetles" or "desert stink beetles". Several genera, such as Stenocara and Onymacris, are of interest in ecological studies of arid conditions and their associated adaptations. Ulomoides dermestoides, known as "chinese weevil", "peanut beetle", "cancer beetle", or "asthma beetle", is eaten in Argentina where it is thought to be a treatment for cancer and other illnesses. Tenebrionidae.net- information and pictures about darkling beetles
Sequoia sempervirens is the sole living species of the genus Sequoia in the cypress family Cupressaceae. Common names include coastal redwood and California redwood, it is an evergreen, long-lived, monoecious tree living 1,200 -- more. This species includes the tallest living trees on Earth, reaching up to 379 feet in height and up to 29.2 feet in diameter at breast height. These trees are among the oldest living things on Earth. Before commercial logging and clearing began by the 1850s, this massive tree occurred in an estimated 2,100,000 acres along much of coastal California and the southwestern corner of coastal Oregon within the United States; the name sequoia sometimes refers to the subfamily Sequoioideae, which includes S. sempervirens along with Sequoiadendron and Metasequoia. Here, the term redwood on its own refers to the species covered in this article, not to the other two species. Scottish botanist David Don described the redwood as the evergreen taxodium in his colleague Aylmer Bourke Lambert's 1824 work A description of the genus Pinus.
Austrian botanist Stephan Endlicher erected the genus Sequoia in his 1847 work Synopsis coniferarum, giving the redwood its current binomial name of Sequoia sempervirens. Endlicher derived the name Sequoia from the Cherokee name of George Gist spelled Sequoyah, who developed the still-used Cherokee syllabary; the redwood is one of each in its own genus, in the subfamily Sequoioideae. Molecular studies have shown that the three are each other's closest relatives with the redwood and giant sequoia as each other's closest relatives; however and colleagues in 2010 queried the polyploid state of the redwood and speculate that it may have arisen as an ancient hybrid between ancestors of the giant sequoia and dawn redwood. Using two different single copy nuclear genes, LFY and NLY, to generate phylogenetic trees, they found that Sequoia was clustered with Metasequoia in the tree generated using the LFY gene, but with Sequoiadendron in the tree generated with the NLY gene. Further analysis supported the hypothesis that Sequoia was the result of a hybridization event involving Metasequoia and Sequoiadendron.
Thus and colleagues hypothesize that the inconsistent relationships among Metasequoia and Sequoiadendron could be a sign of reticulate evolution among the three genera. However, the long evolutionary history of the three genera make resolving the specifics of when and how Sequoia originated once and for all a difficult matter—especially since it in part depends on an incomplete fossil record; the coast redwood can reach 115 m tall with a trunk diameter of 9 m. It has a conical crown, with horizontal to drooping branches; the bark can be thick, up to 1-foot, quite soft and fibrous, with a bright red-brown color when freshly exposed, weathering darker. The root system is composed of wide-spreading lateral roots; the leaves are variable, being 15–25 mm long and flat on young trees and shaded shoots in the lower crown of old trees. On the other hand, they are scale-like, 5–10 mm long on shoots in full sun in the upper crown of older trees, with a full range of transition between the two extremes.
They have two blue-white stomatal bands below. Leaf arrangement is spiral, but the larger shade leaves are twisted at the base to lie in a flat plane for maximum light capture; the species is monoecious, with seed cones on the same plant. The seed cones are ovoid, 15–32 mm long, with 15–25 spirally arranged scales; each cone scale bears three to seven seeds, each seed 3–4 mm long and 0.5 mm broad, with two wings 1 mm wide. The seeds are open at maturity; the pollen cones are 4 -- 6 mm long. Its genetic makeup is unusual among conifers, being a hexaploid and allopolyploid. Both the mitochondrial and chloroplast genomes of the redwood are paternally inherited. Coast redwoods occupy a narrow strip of land 750 km in length and 5–47 mi in width along the Pacific coast of North America; the prevailing elevation range is 98–2,460 ft above sea level down to 0 and up to 3,000 ft. They grow in the mountains where precipitation from the incoming moisture off the ocean is greater; the tallest and oldest trees are found in deep valleys and gullies, where year-round streams can flow, fog drip is regular.
The trees above the fog layer, above about 2,296 ft, are shorter and smaller due to the drier and colder conditions. In addition, Douglas fir and tanoak crowd out redwoods at these elevations. Few redwoods grow close to the ocean, due to intense salt spray and wind. Coalescence of coastal fog accounts for a considerable part of the trees' water needs; the northern boundary of its range is marked by groves on the Chetco River on the western fringe of the Klamath Mountains, near the California-Oregon border. The largest populations are in Redwood National and State Parks (Del Norte and Humbo
Contrails are line-shaped clouds produced by aircraft engine exhaust or changes in air pressure at aircraft cruise altitudes several miles above the Earth's surface. Contrails are composed of water, in the form of ice crystals; the combination of water vapor in aircraft engine exhaust and the low ambient temperatures that exist at high altitudes allows the formation of the trails. Impurities in the engine exhaust from the fuel, including sulfur compounds provide some of the particles that can serve as sites for water droplet growth in the exhaust and, if water droplets form, they might freeze to form ice particles that compose a contrail, their formation can be triggered by changes in air pressure in wingtip vortices or in the air over the entire wing surface. Contrails, other clouds directly resulting from human activity, are collectively named homogenitus. Depending on the temperature and humidity at the altitude the contrails form, they may be visible for only a few seconds or minutes, or may persist for hours and spread to be several miles wide resembling natural cirrus or altocumulus clouds.
Persistent contrails are of particular interest to scientists because they increase the cloudiness of the atmosphere. The resulting cloud forms are formally described as homomutatus, may resemble cirrus, cirrocumulus, or cirrostratus, are sometimes called cirrus aviaticus. Persistent spreading contrails are suspected to have an effect on global climate; the main products of hydrocarbon fuel combustion are water vapor. At high altitudes this water vapor emerges into a cold environment, the local increase in water vapor can raise the relative humidity of the air past saturation point; the vapor condenses into tiny water droplets which freeze if the temperature is low enough. These millions of tiny water droplets and/or ice crystals form the contrails; the time taken for the vapor to cool enough to condense accounts for the contrail forming some distance behind the aircraft. At high altitudes, supercooled water vapor requires a trigger to encourage deposition or condensation; the exhaust particles in the aircraft's exhaust act as this trigger, causing the trapped vapor to condense rapidly.
Exhaust contrails form at high altitudes. They can form closer to the ground when the air is cold and moist. A 2013–2014 study jointly supported by NASA, the German aerospace center DLR, Canada's National Research Council NRC, determined that biofuels could reduce contrail generation; this reduction was explained by demonstrating that biofuels produce fewer soot particles, which are the nuclei around which the ice crystals form. The tests were performed by flying a DC-8 at cruising altitude with a sample-gathering aircraft flying in trail. In these samples, the contrail-producing soot particle count was reduced by 50 to 70 percent, using a 50% blend of conventional Jet A1 fuel and HEFA biofuel produced from camelina; as a wing generates lift, it causes a vortex to form at the wingtip, at the tip of the flap when deployed These wingtip vortices persist in the atmosphere long after the aircraft has passed. The reduction in pressure and temperature across each vortex can cause water to condense and make the cores of the wingtip vortices visible.
This effect is more common on humid days. Wingtip vortices can sometimes be seen behind the wing flaps of airliners during takeoff and landing, during landing of the Space Shuttle; the visible cores of wingtip vortices contrast with the other major type of contrails which are caused by the combustion of fuel. Contrails produced from jet engine exhaust are seen at high altitude, directly behind each engine. By contrast, the visible cores of wingtip vortices are seen only at low altitude where the aircraft is travelling after takeoff or before landing, where the ambient humidity is higher, they trail behind the wingtips and wing flaps rather than behind the engines. At high-thrust settings the fan blades at the intake of a turbofan engine reach transonic speeds, causing a sudden drop in air pressure; this creates the condensation fog, observed by air travelers during takeoff. The tips of rotating surfaces sometimes produce visible contrails. Contrails, by affecting the Earth's radiation balance, act as a radiative forcing.
Studies have found that contrails trap outgoing longwave radiation emitted by the Earth and atmosphere at a greater rate than they reflect incoming solar radiation. NASA conducted a great deal of detailed research on atmospheric and climatological effects of contrails, including effects on ozone, ice crystal formation, particle composition, during the Atmospheric Effects of Aviation Project. Global radiative forcing has been calculated from the reanalysis data, climatological models and radiative transfer codes, it is estimated to amount to 0.012 W/m² for 2005, with an uncertainty range of 0.005 to 0.026 W/m², with a low level of scientific understanding. Therefore, the overall net effect of contrails is positive, i.e. a warming effect. However, the effect varies daily and annually, overall the magnitude of the forcing is not well known: Globally, values range from 3.5 mW/m² to 17 mW/m². Other studies have determined that night flights are responsible for the warming effect: while accounting for only 25% of daily air traffic, they contribute 60 to 80% of contrail radiative forcing