Light

بسم الله الرحمن الرحيم

الْحَمْدُ لِلَّهِ رَبِّ الْعَالَمِينَ

"Reciting Salawath on our Prophet Muhammad (peace be upon him) is an activity that will be accepted by Allah, even if we don't have Ikhlas (piety)".

Light

Light is electromagnetic radiation of a wavelength that is visible to the human eye (about 400–700 nm). In a scientific context, the word light is sometimes used to refer to the entire electromagnetic spectrum. Light is composed of an elementary particle called a photon.

Perfect Example of Light

Three primary properties of light are:

• Intensity, or brightness;
• Frequency or wavelength and;
• Polarization or direction of the wave oscillation.

Light can exhibit properties of both waves and particles. This property is referred to as wave-particle duality. The study of light, known as optics, is an important research area in modern physics.

Speed of Light

The speed of light in a vacuum is exactly 299,792,458 m/s (about 186,282.397 miles per second). The speed of light depends upon the medium in which it is traveling, and the speed will be lower in a transparent medium. Although commonly called the "velocity of light", technically the word velocity is a vector quantity, having both magnitude and direction. Speed refers only to the magnitude of the velocity vector. This fixed definition of the speed of light is a result of the modern attempt, in physics, to define the basic unit of length in terms of the speed of light, rather than defining the speed of light in terms of a length.

Different physicists have attempted to measure the speed of light throughout history. Galileo attempted to measure the speed of light in the seventeenth century. A good early experiment to measure the speed of light was conducted by Ole Rømer, a Danish physicist, in 1676. Using a telescope, Ole observed the motions of Jupiter and one of its moons, Io. Noting discrepancies in the apparent period of Io's orbit, Rømer calculated that light takes about 18 minutes to traverse the diameter of Earth's orbit. Unfortunately, this was not a value that was known at that time. If Ole had known the diameter of the earth's orbit, he would have calculated a speed of 227,000,000 m/s.

Another, more accurate, measurement of the speed of light was performed in Europe by Hippolyte Fizeau in 1849. Fizeau directed a beam of light at a mirror several kilometers away. A rotating cog wheel was placed in the path of the light beam as it traveled from the source, to the mirror and then returned to its origin. Fizeau found that at a certain rate of rotation, the beam would pass through one gap in the wheel on the way out and the next gap on the way back. Knowing the distance to the mirror, the number of teeth on the wheel, and the rate of rotation, Fizeau were able to calculate the speed of light as 313,000,000 m/s.

Léon Foucault used an experiment which used rotating mirrors to obtain a value of 298,000,000 m/s in 1862. Albert A. Michelson conducted experiments on the speed of light from 1877 until his death in 1931. He refined Foucault's methods in 1926 using improved rotating mirrors to measure the time it took light to make a round trip from Mt. Wilson to Mt. San Antonio in California. The precise measurements yielded a speed of 299,796,000 m/s.

Refraction

Light in a vacuum propagates at a maximum finite speed, defined above, and denoted by the symbol c. While passing through any other transparent medium, the speed of light slows to some fraction of c. The reduction of the speed of light traveling in a transparent medium is indicated by the refractive index, n, which is defined as:

n = c/v

where v denotes the speed that light travels in the transparent medium.

Note, n = 1 in a vacuum and n > 1 in a transparent medium.

When a beam of light crosses the boundary between a vacuum and another medium, or between two different mediums, the wavelength of the light changes, but the frequency remains constant. If the beam of light is not orthogonal to the boundary, the change in wavelength results in a change in the direction of the beam. This change of direction is known as refraction.

The refraction quality of lenses is frequently used to manipulate light in order to change the apparent size of images. Magnifying glasses, spectacles, contact lenses, microscopes and refracting telescopes are all examples of this manipulation.

Optics

The study of light and the interaction of light and matter are termed optics. The observation and study of optical phenomena such as rainbows and the aurora borealis offer many clues as to the nature of light as well as much enjoyment.

Light sources

Mist illuminated by sunlight

A cloud illuminated by sunlight

There are many sources of light. The most common light sources are thermal: a body at a given temperature emits a characteristic spectrum of black-body radiation. Examples include sunlight (the radiation emitted by the chromosphere of the Sun at around 6,000 K peaks in the visible region of the electromagnetic spectrum), incandescent light bulbs (which emit only around 10% of their energy as visible light and the remainder as infrared), and glowing solid particles in flames. The peak of the blackbody spectrum is in the infrared for relatively cool objects like human beings. As the temperature increases, the peak shifts to shorter wavelengths, producing first a red glow, then a white one, and finally a blue color as the peak moves out of the visible part of the spectrum and into the ultraviolet. These colors can be seen when metal is heated to "red hot" or "white hot". The blue color is most commonly seen in a gas flame or a welder's torch.

Atoms emit and absorb light at characteristic energies. This produces "emission lines" in the spectrum of each atom. Emission can be spontaneous, as in light-emitting diodes, gas discharge lamps (such as neon lamps and neon signs, mercury-vapor lamps, etc.), and flames (light from the hot gas itself—so, for example, sodium in a gas flame emits characteristic yellow light). Emission can also be stimulated, as in a laser or a microwave maser.

Acceleration of a free charged particle, such as an electron, can produce visible radiation: cyclotron radiation, synchrotron radiation, and bremsstrahlung radiation are all examples of this. Particles moving through a medium faster than the speed of light in that medium can produce visible Cherenkov radiation.

Certain chemicals produce visible radiation by chemoluminescence. In living things, this process is called bioluminescence. For example, fireflies produce light by this means, and boats moving through water can disturb plankton which produces a glowing wake.

Certain substances produce light when they are illuminated by more energetic radiation, a process known as fluorescence. This is used in fluorescent lights. Some substances emit light slowly after excitation by more energetic radiation. This is known as phosphorescence.

Phosphorescent materials can also be excited by bombarding them with subatomic particles. Cathodoluminescence is one example of this. This mechanism is used in cathode ray tube televisions.

Certain other mechanisms can produce light:

• scintillation
• electroluminescence
• sonoluminescence
• triboluminescence
• Cherenkov radiation

When the concept of light is intended to include very-high-energy photons (gamma rays), additional generation mechanisms include:

• radioactive decay
• particle–antiparticle annihilation

Theories about light

Indian theories

In ancient India, the philosophical schools of Samkhya and Vaisheshika, from around the 6th–5th century BC, developed theories on light. According to the Samkhya School, light is one of the five fundamental "subtle" elements (tanmatra) out of which emerge the gross elements. The atomicity of these elements is not specifically mentioned and it appears that they were actually taken to be continuous.

On the other hand, the Vaisheshika School gives an atomic theory of the physical world on the non-atomic ground of ether, space and time. (See Indian atomism.) The basic atoms are those of earth (prthivı), water (apas), fire (tejas), and air (vayu) that should not be confused with the ordinary meaning of these terms. These atoms are taken to form binary molecules that combine further to form larger molecules. Motion is defined in terms of the movement of the physical atoms and it appears that it is taken to be non-instantaneous. Light rays are taken to be a stream of high velocity of tejas (fire) atoms. The particles of light can exhibit different characteristics depending on the speed and the arrangements of the tejas atoms. Around the first century BC, the Vishnu Purana correctly refers to sunlight as the "the seven rays of the sun".

Later in 499, Aryabhata, who proposed a heliocentric solar system of gravitation in his Aryabhatiya, wrote that the planets and the Moon do not have their own light but reflect the light of the Sun.

The Indian Buddhists, such as Dignāga in the 5th century and Dharmakirti in the 7th century, developed a type of atomism that is a philosophy about reality being composed of atomic entities that are momentary flashes of light or energy. They viewed light as being an atomic entity equivalent to energy, similar to the modern concept of photons, though they also viewed all matter as being composed of these light/energy particles.

Greek and Hellenistic theories

In the fifth century BC, Empedocles postulated that everything was composed of four elements; fire, air, earth and water. He believed that Aphrodite made the human eye out of the four elements and that she lit the fire in the eye which shone out from the eye making sight possible. If this were true, then one could see during the night just as well as during the day, so Empedocles postulated an interaction between rays from the eyes and rays from a source such as the sun.

In about 300 BC, Euclid wrote Optica, in which he studied the properties of light. Euclid postulated that light travelled in straight lines and he described the laws of reflection and studied them mathematically. He questioned that sight is the result of a beam from the eye, for he asks how one sees the stars immediately, if one closes one's eyes, then opens them at night. Of course if the beam from the eye travels infinitely fast this is not a problem.

In 55 BC, Lucretius, a Roman who carried on the ideas of earlier Greek atomists, wrote:

"The light and heat of the sun; these are composed of minute atoms which, when they are shoved off, lose no time in shooting right across the interspace of air in the direction imparted by the shove." - On the nature of the Universe

Despite being similar to later particle theories, Lucretius's views were not generally accepted and light was still theorized as emanating from the eye.

Ptolemy (c. 2nd century) wrote about the refraction of light, and developed a theory of vision that objects are seen by rays of light emanating from the eyes.

Optical theory

The Muslim scientist Ibn al-Haytham (c. 965-1040), known as Alhacen in the West, in his Book of Optics, developed a broad theory that explained vision, using geometry and anatomy, which stated that each point on an illuminated area or object radiates light rays in every direction, but that only one ray from each point, which strikes the eye perpendicularly, can be seen. The other rays strike at different angles and are not seen. He described the pinhole camera and invented the camera obscura, which produces an inverted image, and used it as an example to support his argument.[1] This contradicted Ptolemy's theory of vision that objects are seen by rays of light emanating from the eyes. Alhacen held light rays to be streams of minute particles that traveled at a finite speed. He improved Ptolemy's theory of the refraction of light, and went on to discover the laws of refraction.

He also carried out the first experiments on the dispersion of light into its constituent colors. His major work Kitab al-Manazir was translated into Latin in the Middle Ages, as well his book dealing with the colors of sunset. He dealt at length with the theory of various physical phenomena like shadows, eclipses, the rainbow. He also attempted to explain binocular vision, and gave a correct explanation of the apparent increase in size of the sun and the moon when near the horizon. Because of his extensive research on optics, Al-Haytham is considered the father of modern optics.

Al-Haytham also correctly argued that we see objects because the sun's rays of light, which he believed to be streams of tiny particles traveling in straight lines, are reflected from objects into our eyes. He understood that light must travel at a large but finite velocity, and that refraction is caused by the velocity being different in different substances. He also studied spherical and parabolic mirrors, and understood how refraction by a lens will allow images to be focused and magnification to take place. He understood mathematically why a spherical mirror produces aberration.

The 'plenum'

René Descartes (1596-1650) held that light was a disturbance of the plenum, the continuous substance of which the universe was composed. In 1637 he published a theory of the refraction of light that assumed, incorrectly, that light traveled faster in a denser medium than in a less dense medium. Descartes arrived at this conclusion by analogy with the behavior of sound waves. Although Descartes was incorrect about the relative speeds, he was correct in assuming that light behaved like a wave and in concluding that refraction could be explained by the speed of light in different media. As a result, Descartes' theory is often regarded as the forerunner of the wave theory of light.

Particle theory

Pierre Gassendi (1592-1655), an atomist, proposed a particle theory of light which was published posthumously in the 1660s. Isaac Newton studied Gassendi's work at an early age, and preferred his view to Descartes' theory of the plenum. He stated in his Hypothesis of Light of 1675 that light was composed of corpuscles (particles of matter) which were emitted in all directions from a source. One of Newton's arguments against the wave nature of light was that waves were known to bend around obstacles, while light traveled only in straight lines. He did, however, explain the phenomenon of the diffraction of light (which had been observed by Francesco Grimaldi) by allowing that a light particle could create a localized wave in the aether.

Newton's theory could be used to predict the reflection of light, but could only explain refraction by incorrectly assuming that light accelerated upon entering a denser medium because the gravitational pull was greater. Newton published the final version of his theory in his Opticks of 1704. His reputation helped the particle theory of light to hold sway during the 18th century.

Wave theory

In the 1660s, Robert Hooke published a wave theory of light. Christiaan Huygens worked out his own wave theory of light in 1678, and published it in his Treatise on light in 1690. He proposed that light was emitted in all directions as a series of waves in a medium called the Luminiferous ether. As waves are not affected by gravity, it was assumed that they slowed down upon entering a denser medium.

The wave theory predicted that light waves could interfere with each other like sound waves (as noted around 1800 by Thomas Young), and that light could be polarized. Young showed by means of a diffraction experiment that light behaved as waves. He also proposed that different colors were caused by different wavelengths of light, and explained color vision in terms of three-colored receptors in the eye.

Another supporter of the wave theory was Leonhard Euler. He argued in Nova theoria lucis et colorum (1746) that diffraction could more easily be explained by a wave theory.

Later, Augustin-Jean Fresnel independently worked out his own wave theory of light, and presented it to the Académie des Sciences in 1817. Simeon Denis Poisson added to Fresnel's mathematical work to produce a convincing argument in favour of the wave theory, helping to overturn Newton's corpuscular theory.

The weakness of the wave theory was that light waves, like sound waves, would need a medium for transmission. A hypothetical substance called the luminiferous aether was proposed, but its existence was cast into strong doubt in the late nineteenth century by the Michelson-Morley experiment.

Newton's corpuscular theory implied that light would travel faster in a denser medium, while the wave theory of Huygens and others implied the opposite. At that time, the speed of light could not be measured accurately enough to decide which theory was correct. The first to make a sufficiently accurate measurement was Léon Foucault, in 1850. His result supported the wave theory, and the classical particle theory was finally abandoned.

Electromagnetic theory

In 1845, Michael Faraday discovered that the angle of polarization of a beam of light as it passed through a polarizing material could be altered by a magnetic field, an effect now known as Faraday rotation. This was the first evidence that light was related to electromagnetism. Faraday proposed in 1847 that light was a high-frequency electromagnetic vibration, which could propagate even in the absence of a medium such as the ether.

Faraday's work inspired James Clerk Maxwell to study electromagnetic radiation and light. Maxwell discovered that self-propagating electromagnetic waves would travel through space at a constant speed, which happened to be equal to the previously measured speed of light. From this, Maxwell concluded that light was a form of electromagnetic radiation: he first stated this result in 1862 in On Physical Lines of Force. In 1873, he published A Treatise on Electricity and Magnetism, which contained a full mathematical description of the behavior of electric and magnetic fields, still known as Maxwell's equations. Soon after, Heinrich Hertz confirmed Maxwell's theory experimentally by generating and detecting radio waves in the laboratory, and demonstrating that these waves behaved exactly like visible light, exhibiting properties such as reflection, refraction, diffraction, and interference. Maxwell's theory and Hertz's experiments led directly to the development of modern radio, radar, television, electromagnetic imaging, and wireless communications.

The special theory of relativity

The wave theory was wildly successful in explaining nearly all optical and electromagnetic phenomena, and was a great triumph of nineteenth century physics. By the late nineteenth century, however, a handful of experimental anomalies remained that could not be explained by or were in direct conflict with the wave theory. One of these anomalies involved a controversy over the speed of light. The constant speed of light predicted by Maxwell's equations and confirmed by the Michelson-Morley experiment contradicted the mechanical laws of motion that had been unchallenged since the time of Galileo, which stated that all speeds were relative to the speed of the observer. In 1905, Albert Einstein resolved this paradox by revising the Galilean model of space and time to account for the constancy of the speed of light. Einstein formulated his ideas in his special theory of relativity, which radically altered humankind's understanding of space and time. Einstein also demonstrated a previously unknown fundamental equivalence between energy and mass with his famous equation

where E is energy, m is mass, and c is the speed of light.

Particle theory revisited

Another experimental anomaly was the photoelectric effect, by which light striking a metal surface ejected electron from the surface, causing an electric current to flow across an applied voltage. Experimental measurements demonstrated that the energy of individual ejected electrons was proportional to the frequency, rather than the intensity, of the light. Furthermore, below a certain minimum frequency, which depended on the particular metal, no current would flow regardless of the intensity. These observations clearly contradicted the wave theory, and for years physicists tried in vain to find an explanation. In 1905, Einstein solved this puzzle as well, this time by resurrecting the particle theory of light to explain the observed effect. Because of the preponderance of evidence in favor of the wave theory, however, Einstein's ideas were met initially by great skepticism among established physicists. But eventually Einstein's explanation of the photoelectric effect would triumph, and it ultimately formed the basis for wave–particle duality and much of quantum mechanics.

Quantum theory

A third anomaly that arose in the late 19th century involved a contradiction between the wave theory of light and measurements of the electromagnetic spectrum emitted by thermal radiators, or so-called black bodies. Physicists struggled with this problem, which later became known as the ultraviolet catastrophe, unsuccessfully for many years. In 1900, Max Planck developed a new theory of black-body radiation that explained the observed spectrum correctly. Planck's theory was based on the idea that black bodies emit light (and other electromagnetic radiation) only as discrete bundles or packets of energy. These packets were called quanta, and the particle of light was given the name photon, to correspond with other particles being described around this time, such as the electron and proton. A photon has an energy, E, proportional to its frequency, f, by

where h is Planck's constant, λ is the wavelength and c is the speed of light. Likewise, the momentum p of a photon is also proportional to its frequency and inversely proportional to its wavelength:

As it originally stood, this theory did not explain the simultaneous wave- and particle-like natures of light, though Planck would later work on theories that did. In 1918, Planck received the Nobel Prize in Physics for his part in the founding of quantum theory.

Wave–particle duality

The modern theory that explains the nature of light includes the notion of wave–particle duality, described by Albert Einstein in the early 1900s, based on his study of the photoelectric effect and Planck's results. Einstein asserted that the energy of a photon is proportional to its frequency. More generally, the theory states that everything has both a particle nature and a wave nature, and various experiments can be done to bring out one or the other. The particle nature is more easily discerned if an object has a large mass, so it took until a bold proposition by Louis de Broglie in 1924 to realise that electrons also exhibited wave–particle duality. The wave nature of electrons was experimentally demonstrated by Davission and Germer in 1927. Einstein received the Nobel Prize in 1921 for his work with the wave–particle duality on photons (especially explaining the photoelectric effect thereby), and de Broglie followed in 1929 for his extension to other particles.

Quantum electrodynamics

The quantum mechanical theory of light and electromagnetic radiation continued to evolve through the 1920's and 1930's, and culminated with the development during the 1940's of the theory of quantum electrodynamics, or QED. This so-called quantum field theory is among the most comprehensive and experimentally successful theories ever formulated to explain a set of natural phenomena. QED was developed primarily by physicists Richard Feynman, Freeman Dyson, Julian Schwinger, and Shin-Ichiro Tomonaga. Feynman, Schwinger, and Tomonaga shared the 1965 Nobel Prize in Physics for their contributions.

Light pressure

Light pushes on objects in its way, just as the wind would do. This pressure is most easily explainable in particle theory: photons hit and transfer their momentum. Light pressure can cause asteroids to spin faster, acting on their irregular shapes as on the vanes of a windmill. The possibility to make solar sails that would accelerate spaceships in space is also under investigation.

Although the motion of the Crookes radiometer was originally attributed to light pressure, this interpretation is incorrect; the characteristic Crookes rotation is the result of a partial vacuum. This should not be confused with the Nichols radiometer, in which the motion is directly caused by light pressure.

Spirituality

The sensory perception of light plays a central role in spirituality (vision, enlightenment, darshan, Tabor Light), and the presence of light as opposed to its absence (darkness) is a common Western metaphor of good and evil, knowledge and ignorance, and similar concepts.

Fog

بسم الله الرحمن الرحيم

الْحَمْدُ لِلَّهِ رَبِّ الْعَالَمِينَ

"Reciting Salawath on our Prophet Muhammad (peace be upon him) is an activity that will be accepted by Allah, even if we don't have Ikhlas (piety)".

Fog

Fog is a cloud in contact with the ground. Fog differs from other clouds only in that fog touches the surface of the Earth. The same cloud that is not fog on lower ground may be fog where it contacts higher ground such as hilltops or mountain ridges. Fog is distinct from mist only in its density. Fog is defined as cloud which reduces visibility to less than 1 km, whereas mist is that which reduces visibility to less than 2 km.

The foggiest place in the world is the Grand Banks off the island of Newfoundland, Canada. Fog is frequent there as the Grand Banks is the meeting place of the cold Labrador Current from the north and the much warmer Gulf Stream from the south. The foggiest land areas in the world are Point Reyes, California, and Argentina, Newfoundland and Labrador, both with over 200 foggy days a year.

Characteristics

Fog forms when the difference (Δ) between temperature and dew point are (5 °F) 3 °C, or less.

Fog forms when water vapours in the air, at the surface, begin to condense into liquid water. Fog normally occurs at a relative humidity of 100%. This can be achieved by either adding moisture to the air or dropping the ambient air temperature. Fog can form at lower humidities, and fog can sometimes not form with relative humidity at 100%. A reading of 100% relative humidity does not mean that the air can not hold any more moisture, but the air will then become supersaturated. Fog formation does require all of the elements that normal cloud formation requires with the most important being condensation nuclei. When the air is saturated, additional moisture tends to condense rather than staying in the air as vapor. Condensation nuclei must be present in the form of dust, aeresols, pollutants, etc., for the water to condense upon. When there are exceptional amounts of condensation nuclei present, especially hydroscopic (water seeking such as salt, see below) then the water vapor may condense below 100% relative humidity.

Fog can form suddenly, and can dissipate just as rapidly, depending what side of the dew point the temperature is on. This phenomenon is known as Flash Fog and is the inspiration for an antiburglary device that stops burglars by filling the room with artificial fog.

Another type of formation also common is sea fog (also knows as salt fog or salty Fog). This is due to the peculiar effect of salt. Clouds of all types require minute hygroscopic particles upon which water vapor can condense. Over the ocean surface, the most common particles are salt from salt spray produced by breaking waves. Except in areas of storminess, the most common areas of breaking waves are located near coastlines; hence the greatest densities of airborne salt particles are there. Condensation on salt particles has been observed to occur at humidities as low as 70%, thus fog can occur even in relatively dry air in suitable locations such as the California coast. Typically, such lower humidity fog is preceded by a transparent mistiness along the coastline as condensation competes with evaporation, a phenomenon that is typically noticeable by beachgoers in the afternoon.

Fog occasionally produces precipitation in the form of drizzle. Drizzle occurs when the humidity of fog attains 100% and the minute cloud droplets begin to coalesce into larger droplets. This can occur when the fog layer is lifted and cooled sufficiently, or when it is forcibly compressed from above. Drizzle becomes freezing drizzle when the temperature at the surface drops below the freezing point.

The thickness of fog is largely determined by the altitude of the inversion boundary, which in coastal or oceanic locales is also the top of the marine layer, above which the airmass is warmer and drier. The inversion boundary varies its altitude primarily in response to the weight of the air above it which is measured in terms of atmospheric pressure. The marine layer and any fogbank it may contain will be "squashed" when the pressure is high, and conversely, may expand upwards when the pressure above it is lowering.

Fog as a visibility hazard

Fog reduces visibility. Although most sea vessels can penetrate fog using radar, road vehicles have to travel slowly and use low-beam headlights. Localized fog is especially dangerous, as drivers can be caught by surprise.

At airports, some attempts have been made to develop methods (such as using heating or spraying salt particles) to aid fog dispersal. These methods enjoy some success at temperatures below freezing.

Accidents

Accident in a Level Crossing

Fog contributes to accidents, particularly with modes of transportation. Ships, trains, cars and planes cannot see each other and collide. Notable examples of accidents due to fog include the July 28, 1945 crash of a B-25 Mitchell into the Empire State Building, and the July 25, 1956 collision of the ocean liners the SS Andrea Doria and SS Stockholm.

Types

Radiation fog

Radiation fog is formed by the cooling of land after sunset by thermal radiation in calm conditions with clear sky. The cool ground produces condensation in the nearby air by heat conduction. In perfect calm the fog layer can be less than a meter deep but turbulence can promote a thicker layer. Radiation fogs occur at night, and usually do not last long after sunrise. Radiation fog is common in autumn and early winter. Examples of this phenomenon include the Tule fog. For clarity, Radiation fog is not radioactive.

Ground fog

Ground fog is fog that obscures less than 60% of the sky and does not extend to the base of any overhead clouds. However, the term is sometimes used to refer to radiation fog.

Advection fog

Advection fog occurs when moist air passes over a cool surface by advection (wind) and is cooled. It is common as a warm front passes over an area with significant snowpack. It's most common at sea when tropical air encounters cooler waters, or in areas of upwelling, such as along the California coast. The advection of fog along the California coastline is propelled onto land by one of several processes. A cold front can push the marine layer coastward, an occurrence most typical in the spring or late fall. During the summer months, a low pressure trough produced by intense heating inland creates a strong pressure gradient, drawing in the dense marine layer. Also during the summer, strong high pressure aloft over the desert southwest, usually in connection with the summer monsoon, produces a south to southeasterly flow which can drive the offshore marine layer up the coastline, a phenomenon known as a "southerly surge", typically following a coastal heat spell. However, if the monsoonal flow is sufficiently turbulent, it might instead break up the marine layer and any fog it may contain.

Steam fog

Steam fog, also called evaporation fog, is the most localized form and is created by cold air passing over much warmer water or moist land. It often causes freezing fog, or sometimes hoar frost.

Precipitation fog

Precipitation fog (or frontal fog) forms as precipitation falls into drier air below the cloud, the liquid droplets evaporate into water vapor. The water vapor cools and at the dew point it condenses and fog forms.

Upslope fog

Upslope fog forms when winds blow air up a slope (called orographic lift), adiabatical cooling it as it rises, and causing the moisture in it to condense. This often causes freezing fog on mountaintops, where the cloud ceiling would not otherwise be low enough.

Valley fog

Valley fog forms in mountain valleys, often during winter. It is the result of a temperature inversion caused by heavier cold air settling into in a valley, with warmer air passing over the mountains above. It is essentially radiation fog confined by local topography, and can last for several days in calm conditions. In California's Central Valley, Valley fog is often referred to as Tule fog.

Ice fog

Ice fog is any kind of fog where the droplets have frozen into extremely tiny crystals of ice in midair. Generally this requires temperatures at or below −35 °C (−30 °F), making it common only in and near the Arctic and Antarctic regions. It is most often seen in urban areas where it is created by the freezing of water vapor present in automobile exhaust and combustion -products from heating and power generation. Urban ice fog can become extremely dense and will persist day and night until the temperature rises. Extremely small amounts of ice fog falling from the sky form a type of precipitation called ice crystals, often reported in Barrow, Alaska. Ice fog often leads to the visual phenomenon of light pillars.

Freezing fog

Freezing fog occurs when liquid fog droplets freeze to surfaces, forming white rime ice. This is very common on mountain tops which are exposed to low clouds. It is equivalent to freezing rain, and essentially the same as the ice that forms inside a freezer which is not of the "frostless" or "frost-free" type. In some areas such as in the State of Oregon, the term "freezing fog" refers to fog where water vapor is super-cooled filling the air with small ice crystals similar to very light snow. It seems to make the fog "tangible", as if one could "grab a handful".

Artificial fog

Artificial Fog used in the Tamil Movie Sachien

Artificial fog is artificially generated fog that is usually created by vaporizing a water and glycol-based or glycerin-based fluid. The fluid is injected into a heated block, and evaporates quickly. The resulting pressure forces the vapor out of the exit. Upon coming into contact with cool outside air the vapor forms a fog—see fog machine.

Garua fog

Garua fog is a type of fog which occurs at the western coast of Chile. The normal fog produced by the sea travels inland, but suddenly meets an area of hot air. This causes the water particles of fog to shrink by evaporation, producing a transparent mist. Garua fog is nearly invisible, yet it still forces drivers to use windshield wipers.

Hail fog

Hail fog sometimes occurs in the vicinity of significant hail accumulations due to increased temperature and increased moisture leading to saturation in a shallow layer near the surface.

Rain

بسم الله الرحمن الرحيم

الْحَمْدُ لِلَّهِ رَبِّ الْعَالَمِينَ

"Reciting Salawath on our Prophet Muhammad (peace be upon him) is an activity that will be accepted by Allah, even if we don't have Ikhlas (piety)".

Rain is a type of precipitation, a product of the condensation of atmospheric water vapour that is deposited on the earth's surface. It forms when separate drops of water fall to the Earth's surface from clouds. Not all rain reaches the surface; some evaporates while falling through dry air. When none of it reaches the ground, it is called virga, a phenomenon often seen in hot, dry desert regions.

How rain is formed

Rain

Rain plays a role in the hydrologic cycle in which moisture from the oceans evaporates, condenses into drops, precipitates (falls) from the sky, and eventually returns to the ocean via rivers and streams to repeat the cycle again. The water vapor from plant respiration also contributes to the moisture in the atmosphere.

A major scientific explanation of how rain forms and falls is called the Bergeron process. More recent research points to the influence of Cloud condensation nuclei released as the result of biological processes.

Differing conditions for rainfall

Based on the reason for precipitation, rain is classified into:

• Orographic rain
• Convective rain
• Frontal or cyclonic rain

Orographic rain (relief rain)

View of monsoon rain in Kerala, South India

Orographic rain (or relief rain) is caused when the warm moisture-laden wind blowing in to the land from the sea encounters a natural barrier such as mountains. This forces the air to rise. With gain in altitude, the air expands dynamically due to a decrease in air pressure. Due to this the wind experiences a decrease in temperature (by adiabatic cooling), which results in the increase of the relative humidity. This causes condensation of water vapor into water droplets to form clouds. The relative humidity continues to increase until the dew point reaches the level of condensation, causing air to be saturated. This height where the condensation occurs is called the level of condensation. When the cloud droplets become too heavy to be suspended, rain falls.

As the wind descends on the leeward side of the mountain range, it becomes compressed and warms; which results in the decrease of the relative humidity of the wind, which is already dry after precipitating its moisture on the windward side of the mountain. Hence the leeward side of the mountains does not receive any rain from these winds and it's called the rain shadow region of the mountains.

The Indian Ocean monsoon is a good example of orographic rain. About 80% of the rain that occurs in India is of this category.

Convective rain

Convective rain mainly occurs in the equatorial climatic regions and tropical climatic regions where it is very hot during the day. The rate of evaporation of moisture from the water bodies and respiration from the dense vegetation is very high. The evaporated moisture along with its hot surrounding air begins to ascend. With gain in altitude, the air expands dynamically due to a decrease in air pressure. Due to this the wind experiences a decrease in temperature (per adiabatic cooling), which results in the increase of the relative humidity. This causes condensation of water vapor into water droplets to form unstable towering cumulonimbus clouds. When the cloud droplets become too heavy to be suspended, rain falls.

Frontal rain

Frontal rain is caused by cyclonic activity and it occurs along the fronts of the cyclone. It is formed when two masses of air of different temperature, humidity and density meet, e.g., meetings of moisture laden warm tropical wind with a polar air mass. A layer separating them is called the front. This front has two parts — the warm front and the cold front. At the warm front, the warm lighter air rises gently over the heavier cold air. As the warm air rises, it cools, and the moisture present in it condenses to form clouds — altostratus clouds. This rain falls steadily for a few hours to a few days.

At the cold front, the cold air forces the warm air to rise rapidly causing its moisture to condense quickly, which results in the formation of cumulonimbus clouds. The rainfall from these clouds is usually heavy and of short duration.

A view of rain falling on a street of Kolkata, India

Human influence

The fine particulate matter produced by car exhaust and other human sources of pollution form cloud condensation nuclei, leads to the production of clouds and increases the likelihood of rain. As commuters and commercial traffic cause pollution to build up over the course of the week, the likelihood of rain increases: it peaks by Saturday, after five days of weekday pollution has been built up. In heavily populated areas that are near the coast, such as the United States' Eastern Seaboard, the effect can be dramatic: there is a 22% higher chance of rain on Saturdays than on Mondays.

Classifying the amount of rain

When classified according to amount of precipitation, rain can be divided into:

• Very light rain — when the precipitation rate is <>
• Light rain — when the precipitation rate is between 0.25 mm/hour - 1.0 mm/hour
• Moderate rain — when the precipitation rate is between 1.0 mm/hour - 4.0 mm/hour
• Heavy rain — when the precipitation rate is between 4.0 mm/hour - 16.0 mm/hour
• Very heavy rain — when the precipitation rate is between 16.0 mm/hour - 50 mm/hour
• Extreme rain — when the precipitation rate is > 50.0 mm/hour

Properties

Falling raindrops are often depicted in cartoons as "teardrop-shaped" — round at the bottom and narrowing towards the top — but this is incorrect. Only drops of water dripping from some sources are tear-shaped at the moment of formation. Small raindrops are nearly spherical. Larger ones become increasingly flattened on the bottom, like hamburger buns; very large ones are shaped like parachutes. The shape of raindrops was studied by Philipp Lenard in 1898. He found that small raindrops (less than about 2 mm diameter) are approximately spherical. As they get larger (to about 5 mm diameter) they become more doughnut shaped. Beyond about 5 mm they become unstable and fragment. On average, raindrops are 1 to 2 mm in diameter. The biggest raindrops on Earth were recorded over Brazil and the Marshall Islands in 2004 — some of them were as large as 10 mm. The large size is explained by condensation on large smoke particles or by collisions between drops in small regions with particularly high content of liquid water.

Rain falling

Raindrops impact at their terminal velocity, which is greater for larger drops. At sea level and without wind, 0.5 mm drizzle impacts at about 2 m/s, while large 5 mm drops impact at around 9 m/s. The sound of raindrops hitting water is caused by bubbles of air oscillating underwater.

Generally, rain has a pH slightly under 6. This is because atmospheric carbon dioxide dissolves in the droplet to form minute quantities of carbonic acid, which then partially dissociates, lowering the pH. In some desert areas, airborne dust contains enough calcium carbonate to counter the natural acidity of precipitation, and rainfall can be neutral or even alkaline. Rain below pH 5.6 is considered acid rain.

Measuring rainfall

Rainfall is typically measured using a rain gauge. It is expressed as the depth of water that collects on a flat surface, and is routinely measured with an accuracy up to 0.1 mm or 0.01 in. Rain gauges are usually placed at a uniform height above the ground, which may vary depending on the country. There are two types of gauges. Storage rain gauges are used to make daily or monthly measurements. Recording rain gauges measure the intensity of rainfall using a tipping bucket which will only tip when a certain volume of water is in it. An electrical switch can be used to record the tips.

Effect on agriculture

Precipitation, especially rain, has a dramatic effect on agriculture. All plants need at least some water to survive; therefore rain (being the most effective means of watering) is important to agriculture. While a regular rain pattern is usually vital to healthy plants, too much or too little rainfall can be harmful, even devastating to crops. Drought can kill crops in massive numbers, while overly wet weather can cause disease and harmful fungus. Plants need varying amounts of rainfall to survive. For example, cacti need small amounts of water while tropical plants may need up to hundreds of inches of rain to survive.

Agriculture of all nations at least to some extent is dependent on rain. Indian agriculture, for example, (which accounts for 25 percent of the GDP and employs 70 percent of the nation's population) is heavily dependent on the rains, especially crops like cotton, rice, oilseeds and coarse grains. A delay of a few days in the arrival of the monsoon can, and does, badly affect the economy, as evidenced in the numerous droughts in India in the 90s.

Culture

Cultural attitudes towards rain differ across the world. In the largely temperate Europe, rain metaphorically has a sad and negative connotation — reflected in children's rhymes like Rain Rain Go Away — in contrast to the bright and happy sun. Though the traditional notion of rain in the Western World is negative, rain can also bring joy, as some consider it to be soothing or enjoy the aesthetic appeal of it. In dry places, such as parts of Africa, Australia, India, and the Middle East, rain is greeted with euphoria. (In Botswana, the Setswana word for rain, "pula," is used as the name of the national currency, in recognition of the economic importance of rain in this desert country.)

Several cultures have developed means of dealing with rain and have developed numerous protection devices such as umbrellas and raincoats, and diversion devices such as gutters and storm drains that lead rains to sewers. Many people also prefer to stay inside on rainy days, especially in tropical climates where rain is usually accompanied by thunderstorms or rain is extremely heavy (monsoon). Rain may be harvested, though rainwater is rarely pure (as acid rain occurs naturally), or used as greywater. Excessive rain, particularly after a dry period that has hardened the soil so that it cannot absorb water, can cause floods.

Many people find the scent during and immediately after rain especially pleasant or distinctive. The source of this scent is petrichor, an oil produced by plants, then absorbed by rocks and soil, and later released into the air during rainfall. Light or heavy rain is sometimes seen as romantic. Rain can be depressing to some people due to bleak clouds.

Rain around the world

Europe

A country noted for its raininess is the United Kingdom. The reputation is partly deserved because of the frequency of rain driven into the country by the south-western trade winds following the warm Gulf Stream currents. Areas along the western coasts (including those in Ireland) can receive between 1016 mm (40 inches, at sea-level) and 2540 mm (100 inches, on the mountains) of rain per year. However, what is less well known is that the eastern and southern half of the country is much drier, with the south east having a lower rainfall average than Jerusalem and Beirut at between 450 and 600 mm per year.

Meanwhile, Bergen in Norway is one of the more famous European rain-cities with its yearly precipitation of 2250 mm (88 inches) on average.

North America

One city that is known for rain is Seattle, Washington. Rain is common in the winter, but mostly the climate is cloudy with little rain. Seattle's average rainfall is 942 mm (37.1 inches) per year, less than New York City with 1173 mm (46.2 inches), but has 201 cloudy days per year (compared to 152 in New York). However, it should be noted that Seattle lies in the rain shadow of the nearby Olympic Mountains, with some locations on the windward sides of the mountains receiving close to 5080 mm (200 inches) per year.

Vancouver, British Columbia could be considered the world's capital of rain, despite having some snow during special periods, receiving as much as 40 mm at one time. Almost every day in the winter the Greater Vancouver Area is pummeled by rain.

Australia

Melbourne has a similar reputation to Vancouver's. In the popular imagination it is thought of as being much rainier than Sydney; however, Sydney receives an average of 1094 mm (43.1 inches) of rain per year compared to Melbourne's 544 mm (21.4 inches). Sydney, meanwhile, experiences 53 fewer overcast days per year than Melbourne.

Although Australia is the world's driest continent, Mt Bellenden Ker in the north-east of the country records an average of 8000 mm (315 inches) per year, with over 12000 mm (472 inches) of rain recorded in the year 2000.

Asia

Cherrapunji, situated on the southern slopes of the Eastern Himalaya in Shillong, India is one of the wettest places on Earth.

on the way to Cherrapunji

Mythology

The Ancient Greeks believed that rain was a sign of the gods anger towards them. They thought that it symbolized drowning and frustration as it often disturbed what they were doing.

Rainy Clouds

A Little Boy & Gal in a Rainy Day

A Little gal with her pet while it’s raining outside :)