Sprites!

Sprites are a fleeting, ethereal and a relatively unknown aspect of lightning storms.
 
Since the 1960s, and probably before then, pilots have been seeing but seldom reporting what have become known as sprites and elves above the clouds. Sprites are electrically-charged lightning funnels which shoot up from the top of a cloud as much as 60 miles into the atmosphere. These charges are vivid red and usually occur in clusters of three or more but are only visible for nanoseconds. They are sometimes preceded by lower altitude red flashes known as elves, and can have striking blue tendrils which are easily mistaken for blue jets. While they are a similar visual phenomenon, blue jets are less powerful than the sprites and travel neither as quickly nor as far.

 

Because 'everyone knows' lightning goes to ground, pilots were naturally reluctant to report this phenomenon in case they found themselves grounded for hallucinating. As a result, serious research was delayed until the last 15 years or so.

While sprites are more common during positively-charged lightning storms, this is not due to any preference on the part of the sprite, but rather due to the greater internal energy of a positively charged storm. It was not until 1999 that the first sprites of a negatively-charged storm were recorded.

 

During a powerful storm it is possible to see red sprites, elves and blue  jets, but the exact atmospheric conditions which create such a show are uncertain.
 
As sprites are relatively new to the science world there is still a lot more to learn about them.

It is only with the advent of high speed photography that the existence of these light shows could be confirmed, and even with that they were first photographed by accident in 1989. Amazingly, there have since been more than 10,000 confirmed sightings. They are also known to create a very low-frequency thunder which was only recently captured with the use of specialist listening equipment.

 

As their energy is spread more thinly than the traditional thunderbolt due to the cone like dispersal from cloud to atmosphere, they are thought to be relatively weak. Sprites are cold plasma phenomena that lack the hot channel temperatures of tropospheric lightning, so they are more akin to fluorescent tube discharges than to lightning discharges.

The effects of sprites are currently being investigated by various agencies including NASA who seriously addressed them as a possible cause for the tragic loss of the space shuttle Columbia, which was, incidentally, on a mission to record data about the very same sprite phenomenon.
 
The link below shows footage from the ISS and shows a red sprite over East Asia at around 0:06.
http://upload.wikimedia.org/wikipedia/commons/f/f8/Red_Sprite.ogv

Sprite Halos
Sprites are sometimes preceded, by about 1 millisecond, by a sprite halo, a pancake-shaped region of weak, transient optical emissions approximately 50 kilometres (31 mi) across and 10 kilometres (6.2 mi) thick. The halo is centred at about 70 kilometres (43 mi) altitude above the initiating lightning strike. These halos are thought to be produced by the same physical process that produces sprites, but for which the ionization is too weak to cross the threshold required for streamer formation.

Recent research carried out at the University of Houston in 2002 indicates that some normal (negative) lightning discharges produce a sprite halo, and that every lightning bolt between cloud and ground attempts to produce a sprite or a sprite halo.

For more information visit:-
http://en.wikipedia.org/wiki/Sprite_(lightning)
http://h2g2.com/approved_entry/A13492398
http://apod.nasa.gov/apod/ap990616.html

What is a Barometer?

A barometer is a scientific instrument used in meteorology to measure atmospheric pressure. Pressure tendency can forecast short term changes in the weather. Numerous measurements of air pressure are used within surface weather analysis to help find surface troughs, high pressure systems, and frontal boundaries.

 

Evangelista Torricelli is universally credited with inventing the barometer in 1643.

Mercury barometers

A mercury barometer has a glass tube with a height of at least 84 cm, closed at one end, with an open mercury-filled reservoir at the base. The weight of the mercury creates a vacuum in the top of the tube. Mercury in the tube adjusts until the weight of the mercury column balances the atmospheric force exerted on the reservoir. High atmospheric pressure places more force on the reservoir, forcing mercury higher in the column. Low pressure allows the mercury to drop to a lower level in the column by lowering the force placed on the reservoir. Since higher temperature at the instrument will reduce the density of the mercury, the scale for reading the height of the mercury is adjusted to compensate for this effect.

In 1646, Blaise Pascal along with Pierre Petit, had repeated and perfected Torricelli's experiment and went even further to test the mechanical theory. If, as suspected by mechanical philosophers like Torricelli and Pascal, air had lateral weight, the weight of the air would be less at higher altitudes. Therefore, Pascal wrote to his brother-in-law, Florin Perier, who lived near a mountain called the Puy de Dome, asking him to perform a crucial experiment. Perier was to take a barometer up the Puy de Dome and make measurements along the way of the height of the column of mercury. He was then to compare it to measurements taken at the foot of the mountain to see if those measurements taken higher up were in fact smaller. In September 1648, Perier carefully and meticulously carried out the experiment, and found that Pascal's predictions had been correct. The mercury barometer stood lower the higher one went

Aneroid Barometers

An aneroid barometer, invented in 1843 by French scientist Lucien Vidie uses a small, flexible metal box called an aneroid cell (capsule), which is made from an alloy of beryllium and copper. The evacuated capsule (or usually more capsules) is prevented from collapsing by a strong spring. Small changes in external air pressure cause the cell to expand or contract. This expansion and contraction drives mechanical levers such that the tiny movements of the capsule are amplified and displayed on the face of the aneroid barometer. Many models include a manually set needle which is used to mark the current measurement so a change can be seen. In addition, the mechanism is made deliberately "stiff" so that tapping the barometer reveals whether the pressure is rising or falling as the pointer moves.

 

Changes in atmospheric pressure are one of the most commonly used ways to forecast changes in the weather because weather patterns are carried around in regions of high and low pressure. Weather maps use lines of equal pressure called isobars to indicate areas of equal pressure.

A slowly rising atmospheric pressure, over a week or two, typically indicates settled weather that will last a long time. A sudden drop in atmospheric pressure over a few hours often forecasts an approaching storm, which will not last long, with heavy rain and strong winds.

By carefully watching the pressure on a barometer, you can forecast local weather using these simple guidelines:

·         Decreasing barometric pressure indicates storms, rain and windy weather.

·         Rising barometric pressure indicates good, dry, and colder weather.

·         Slow, regular and moderate falls in pressure suggest a low pressure area is passing in a nearby region. Marked changes in the weather where you are located are unlikely.

·         Small rapid decreases in pressure indicate a nearby change in weather. They are usually followed by brief spells of wind and showers.

·         A quick drop in pressure over a short time indicates a storm is likely in 5 to 6 hours.

·         Large, slow and sustained decreasing pressure forecasts a long period of poor weather. The weather will be more pronounced if the pressure started rising before it began to drop.

·         A rapid rise in pressure, during fair weather and average, or above average pressure, indicates a low pressure cell is approaching. The pressure will soon decrease forecasting poorer weather.

·         Quickly rising pressure, when the pressure is low, indicates a short period of fair weather is likely.

·         A large, slow and sustained rise in pressure forecasts a long period of good weather is on its way.

For more information:-

http://en.wikipedia.org/wiki/Barometer

http://weather.about.com/od/weatherinstruments/a/barometers.htm

 

Hello Sunshine!!

Space Instrument Adds Big Piece to the Solar Corona Puzzle

The Sun is a fascinating object and is not fully understood.  It's a complex entity and a recent scientific instrument has helped to unlock some of its secrets.

The Sun's visible surface, or photosphere, is 10,000 degrees Fahrenheit. As you move outward from it, you pass through a tenuous layer of hot, ionized gas or plasma called the corona. The corona is familiar to anyone who has seen a total solar eclipse, since it glimmers ghostly white around the hidden Sun.

But how can the solar atmosphere get hotter, rather than colder, the farther you go from the Sun's surface? This mystery has puzzled solar astronomers for decades. A suborbital rocket mission that launched in July 2012 has just provided a major piece of the puzzle.

The High-resolution Coronal Imager, or Hi-C, revealed one of the mechanisms that pumps energy into the corona, heating it to temperatures up to 7 million degrees F. The secret is a complex process known as magnetic reconnection.

"This is the first time we've had images at high enough resolution to directly observe magnetic reconnection," explained Smithsonian astronomer Leon Golub (Harvard-Smithsonian Center for Astrophysics). "We can see details in the corona five times finer than any other instrument."

"Our team developed an exceptional instrument capable of revolutionary image resolution of the solar atmosphere. Due to the level of activity, we were able to clearly focus on an active sunspot, thereby obtaining some remarkable images," said heliophysicist Jonathan Cirtain (Marshall Space Flight Center).

Magnetic braids and loops

The Sun's activity, including solar flares and plasma eruptions, is powered by magnetic fields. Most people are familiar with the simple bar magnet, and how you can sprinkle iron filings around one to see its field looping from one end to the other. The Sun is much more complicated.

The Sun's surface is like a collection of thousand-mile-long magnets scattered around after bubbling up from inside the Sun. Magnetic fields poke out of one spot and loop around to another spot. Plasma flows along those fields, outlining them with glowing threads.

The images from Hi-C showed interweaved magnetic fields that were braided just like hair. When those braids relax and straighten, they release energy. Hi-C witnessed one such event during its flight.

It also detected an area where magnetic field lines crossed in an X, then straightened out as the fields reconnected. Minutes later, that spot erupted with a mini solar flare.

Hi-C showed that the Sun is dynamic, with magnetic fields constantly warping, twisting, and colliding in bursts of energy. Added together, those energy bursts can boost the temperature of the corona to 7 million degrees F when the Sun is particularly active.

Selecting the target

The telescope aboard Hi-C provided a resolution of 0.2 arcseconds - about the size of a dime seen from 10 miles away. That allowed astronomers to tease out details just 100 miles in size. (For comparison, the Sun is 865,000 miles in diameter.)

Hi-C photographed the Sun in ultraviolet light at a wavelength of 19.3 nanometers - 25 times shorter than wavelengths of visible light. That wavelength is blocked by Earth's atmosphere, so to observe it astronomers had to get above the atmosphere. The rocket's suborbital flight allowed Hi-C to collect data for just over 5 minutes before returning to Earth.

Hi-C could only view a portion of the Sun, so the team had to point it carefully. And since the Sun changes hourly, they had to select their target at the last minute - the day of the launch. They chose a region that promised to be particularly active.

"We looked at one of the largest and most complicated active regions I've ever seen on the Sun," said Golub. "We hoped that we would see something really new, and we weren't disappointed."

Next steps

Golub said that data from Hi-C continues to be analyzed for more insights. Researchers are hunting areas where other energy release processes were occurring.

In the future, the scientists hope to launch a satellite that could observe the Sun continuously at the same level of sharp detail.

"We learned so much in just five minutes. Imagine what we could learn by watching the Sun 24/7 with this telescope," said Golub.

This research is being published in the journal Nature in a paper co-authored by Cirtain, Golub, A. Winebarger (Marshall), B. De Pontieu (Lockheed Martin), K. Kobayashi (University of Alabama - Huntsville), R. Moore (Marshall), R. Walsh (University of Central Lancashire), K. Korreck, M. Weber and P. McCauley (CfA), A. Title (Lockheed Martin), S. Kuzin (Lebedev Physical Institute), and C. DeForest (Southwest Research Institute).

Headquartered in Cambridge, Mass., the Harvard-Smithsonian Center for Astrophysics (CfA) is a joint collaboration between the Smithsonian Astrophysical Observatory and the Harvard College Observatory. CfA scientists, organized into six research divisions, study the origin, evolution and ultimate fate of the universe.

The Science of Snowflakes...

Snowflakes

Snow crystals form when tiny supercooled cloud droplets (about 10 μm in diameter) freeze. These droplets are able to remain liquid at temperatures lower than −18 °C (−0 °F), because to freeze, a few molecules in the droplet need to get together by chance to form an arrangement similar to that in an ice lattice, then the droplet freezes around this "nucleus." Experiments show that this "homogeneous" nucleation of cloud droplets only occurs at temperatures lower than −35 °C (−31 °F). In warmer clouds an aerosol particle or "ice nucleus" must be present in (or in contact with) the droplet to act as a nucleus. The particles that make ice nuclei are very rare compared to nuclei upon which liquid cloud droplets form, however it is not understood what makes them efficient. Clays, desert dust and biological particles may be effective, although to what extent is unclear. Artificial nuclei include particles of silver iodide and dry ice, and these are used to stimulate precipitation in cloud seeding.

Symmetry

A snowflake often exhibits six-fold radial symmetry. The initial symmetry can occur because the crystalline structure of ice is six-fold. The six "arms" of the snowflake, or dendrites, then grow independently, and each side of each arm grows independently. Most snowflakes are not completely symmetric. The micro-environment 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 (and its changes) are very nearly identical around the snowflake, each arm can grow in nearly the same way. However, being in the same micro-environment does not guarantee that each arm grows the same; indeed, for some crystal forms it does not because the underlying crystal growth mechanism also affects how fast each surface region of a crystal grow.

Uniqueness

Snowflakes form in a wide variety of intricate shapes, leading to the popular expression that "no two are alike". Although statistically possible, it is very unlikely for any two snowflakes to appear exactly alike. 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.

http://en.wikipedia.org/wiki/Snowflake

http://www.its.caltech.edu/~atomic/snowcrystals/faqs/faqs.htm

http://snowflakebentley.com/

Why does salt melt ice?

­If you live in a place that has lots of snow and ice in the winter, then you have probably seen the highway department spreading salt on the road to melt the ice.­ You may have also used salt on ice when making home-made ice cream. Salt lowers the freezing/melting point of water, so in both cases the idea is to take advantage of the lower melting point.

 Ice forms when the­ temperature of water reaches 32 degrees Fahrenheit (0 degrees Celsius). When you add salt, that temperature drops: A 10-percent salt solution freezes at 20 F (-6 C), and a 20-percent solution freezes at 2 F (-16 C).

On a roadway, this means that if you sprinkle salt on the ice, you can melt it. The salt dissolves into the liquid water in the ice and lowers its freezing point.

If you ever watch salt melting ice, you can see the dissolving process happen -- the ice immediately around the grain of salt melts, and the melting spreads out from that point. If the temperature of the roadway is lower than 15 F or so, then the salt really won't have any effect -- the solid salt cannot get into the structure of the solid water to start the dissolving process. In that case, spreading sand over the top of the ice to provide traction is a better option.

When you are making ice cream, the temperature around the ice cream mixture needs to be lower than 32 F if you want the mixture to freeze. Salt mixed with ice creates a brine that has a temperature lower than 32 F. When you add salt to the ice water, you lower the melting temperature of the ice down to 0 F or so. The brine is so cold that it easily freezes the ice cream mixture.

Two things happen when ice and water are placed in contact:

• Molecules on the surface of the ice escape into the water (melting), and
• molecules of water are captured on the surface of the ice (freezing).

When the rate of freezing is the same as the rate of melting, the amount of ice and the amount of water won't change on average (although there are short-term fluctuations at the surface of the ice). The ice and water are said to be in dynamic equilibrium with each other. The balance between freezing and melting can be maintained at 0°C, the melting point of water, unless conditions change in a way that favours one of the processes over the other.

The balance between freezing and melting processes can easily be upset. If the ice/water mixture is cooled, the molecules move slower. The slower-moving molecules are more easily captured by the ice, and freezing occurs at a greater rate than melting.

Conversely, heating the mixture makes the molecules move faster on average, and melting is favoured.  Adding salt to the system will also disrupt the equilibrium. Consider replacing some of the water molecules with molecules of some other substance. The foreign molecules dissolve in the water, but do not pack easily into the array of molecules in the solid. This leads to fewer water molecules on the liquid side because the some of the water has been replaced by salt. The total number of waters captured by the ice per second goes down, so the rate of freezing goes down. The rate of melting is unchanged by the presence of the foreign material, so melting occurs faster than freezing. That's why salt melts ice.

For more information see
http://science.howstuffworks.com/
http://antoine.frostburg.edu/chem/senese/101/solutions/faq/why-salt-melts-ice.shtml

Thunderbolts & Lightning...very very frightening?

Thunderstorms – An Introduction

Continuing on our weather theme from last week, we're looking at Thunder & Lightning today.

A thunderstorm can be described as one or more sudden electrical discharges, manifested by a flash of light (lightning) and a sharp or rumbling sound (thunder). Thunderstorms are associated with convective clouds and are most often, but not necessarily, accompanied by precipitation at the ground.

 

Cumulonimbus clouds (Latin: cumulus – heap; nimbus – rainy cloud)

 

A cumulonimbus ‘cloud factory’ (© J. Corey).

Not all cumulonimbus clouds bring thunderstorms; some just bring heavy showers or hail. On average, an individual cumulonimbus cloud takes only one hour to take shape, grow and dissipate. It produces less than 30 minutes of thunder and lightning. If a thunderstorm lasts longer than this, it is probably because there is more than one cumulonimbus present.

Electrical charges within a cumulonimbus cloud

Lightning is a large electrical spark caused by electrons moving from one place to another

 

Lightning seen over Iowa (© M. Clark).

Electrons are fundamental sub-atomic particles that carry a negative electric charge. They are so small they cannot be seen, but when lightning flashes they are moving so fast that the air around them glows. The actual streak of lightning is the path the electrons follow when they move.

Water droplets form inside a storm cloud. They are propelled towards the top of the cloud by strong internal winds (updraughts), where they turn to ice. Some of the pieces of ice grow large into hail, but others remain very small. As the pieces of hail get larger, they fall back through the cloud, bumping into smaller ice particles that are still being forced upwards. When the ice particles collide, some electrons are transferred to the hail. The electrons give the hail a negative charge, while the ice particles that have lost electrons gain a positive charge.

The winds continue to carry the ice particles upwards, giving the top of the cloud a positive charge. Some of the hail has now grown so heavy that the winds can no longer propel them upwards and so collect in the lower part of the cloud, giving it a negative charge. As well as being attracted to the positive atoms in the top of the cloud, the atoms are attracted to positive atoms in other clouds and on the ground. If the attraction is strong enough, the electrons will move towards the positive atoms. The path they make in doing so is the flash of lightning.

As negative charges collect at the base of the cloud, they repel the electrons near the ground’s surface. This leaves the ground and the objects on it with a positive charge. As the attraction between the cloud and the ground grows stronger, electrons shoot down from the cloud. The electrons move in a path that spreads in different directions — like a river delta. Each step is approximately 50 metres long and the branching path is called a stepped leader. Further electrons follow, making new branches. The average speed at which the stepped leader cuts through the air is about 270,000 miles per hour.

Types of lightning

 

There are several types of lightning, these are:

• Ball lightning — a rare form of lightning in which a persistent and moving luminous white or coloured sphere is seen.

• Rocket lightning — a very rare and unexplained form of lightning in which the speed of propagation of the lightning stroke is slow enough to be perceptible to the eye.

• Pearl-necklace lightning — a rare form of lightning, also termed ‘chain lightning’ or ‘beaded lightning’, in which variations of brightness along the discharge path give rise to a momentary appearance similar to pearls on a string.

• Ribbon lightning — ordinary cloud-to-ground lightning that appears to be spread horizontally into a ribbon of parallel luminous streaks when a very strong wind is blowing at right angles to the observer’s line of sight.

• Forked lightning — lightning in which many luminous branches from the main discharge channel are visible.

• Sheet lightning — the popular name applied to a ‘cloud discharge’ form of lightning in which the emitted light appears diffuse and there is an apparent absence of a main channel because of the obscuring effect of the cloud.

• Streak lightning — lightning discharge which has a distinct main channel, often tortuous and branching, the discharge may be from cloud to ground or from cloud to air.

Forked lightning (© M.J.O. Dutton).

 

Thunder

The word ‘thunder’ is derived from ‘Thor’, the Norse god of thunder. He was supposed to be a red-bearded man of tremendous strength; his greatest attribute being the ability to forge thunderbolts. The word Thursday is also derived from his name.

 

Thunder is the sharp or rumbling sound that accompanies lightning. It is caused by the intense heating and expansion of the air along the path of the lightning. The rumble of thunder is caused by the noise passing through layers of the atmosphere at different temperatures. Thunder lasts longer than lightning because of the time it takes for the sound to travel from different parts of the flash.

 

How far away is a thunderstorm?

This can roughly be estimated by measuring the interval between the lightning flash and the start of the thunder. If you count the time in seconds and then divide by three, you will have the approximate distance in kilometres. Thunder is rarely heard at a distance of more than 20 km.

 

Are thunderstorms dangerous?

Most people are frightened by the crackles and rumbles of thunder rather than the flash of lightning. However, thunder cannot hurt anybody, and the risk of being struck by lightning is far less than that of being killed in a car crash. Ninety per cent of lightning discharges go from cloud to cloud or between parts of the same cloud, never actually reaching the Earth. Most of the discharges that do strike the ground cause little or no damage or harm. Lightning takes the shortest and quickest route to the ground, usually via a high object standing alone.

 

How common are thunderstorms?

Thunderstorms occur throughout the world, even in polar regions, with the greatest frequency in tropical rainforest areas, where they may occur nearly daily. The most thundery part of the earth is the island of Java where the annual frequency of thunderstorms is about 220 days per year. In temperate regions, they are most frequent in spring and summer, although they can occur in cold fronts at any time of year. Thunderstorms are rare in polar regions due to the cold climate and stable air masses that are generally in place, but they do occur from time to time, mainly in the summer months. In recent years, thunderstorms have taken on the role of a curiosity. Every spring, storm chasers head to the Great Plains of the United States and the Canadian Prairies to explore the visual and scientific aspects of storms and tornadoes.

 

In the United Kingdom thunder is a variable element, the highest and lowest annual totals of thunderstorm days at many individual stations ranges from less than 5 in a quiet year to 20 or more in an active one. One consequence of this is that published maps showing the average frequency of days of thunder differ considerably in detail according to the period of records used. They agree, however, in showing that the average annual frequency is less than 5 days in western coastal districts and over most of central and northern Scotland, and 15 to 20 days over the east Midlands and parts of southeast England. There is relatively little seasonal variation on the western seaboard but elsewhere summer is the most thundery season.

 

For more information on this subject visit www.metoffice.gov.uk

What is a Rainbow?

Double rainbow and supernumerary rainbows on the inside of the primary arc. The shadow of the photographer's head on the bottom marks the centre of the rainbow circle (antisolar point).

A rainbow is an optical and meteorological phenomenon that is caused by reflection of light in water droplets in the Earth's atmosphere, resulting in a spectrum of light appearing in the sky. It takes the form of a multicoloured arc.

Rainbows caused by sunlight always appear in the section of sky directly opposite the sun.

In a "primary rainbow", the arc shows red on the outer part and violet on the inner side. This rainbow is caused by light being refracted while entering a droplet of water, then reflected inside on the back of the droplet and refracted again when leaving it.

In a double rainbow, a second arc is seen outside the primary arc, and has the order of its colours reversed, red facing toward the other one, in both rainbows. This second rainbow is caused by light reflecting twice inside water droplets.

Overview

The rainbow is not located at a specific distance, but comes from any water droplets viewed from a certain angle relative to the Sun's rays. Thus, a rainbow is not an object, and cannot be physically approached. Indeed, it is impossible for an observer to manoeuvre to see any rainbow from water droplets at any angle other than the customary one of 42 degrees from the direction opposite the Sun. Even if an observer sees another observer who seems "under" or "at the end" of a rainbow, the second observer will see a different rainbow further off-yet, at the same angle as seen by the first observer. A rainbow spans a continuous spectrum of colours. Any distinct bands perceived are an artefact of human colour vision, and no banding of any type is seen in a black-and-white photo of a rainbow, only a smooth gradation of intensity to a maximum, then fading towards the other side. For colours seen by a normal human eye, the most commonly cited and remembered sequence is Newton's sevenfold red, orange, yellow, green, blue, indigo and violet.

Rainbows can be caused by many forms of airborne water. These include not only rain, but also mist, spray, and airborne dew.

Rainbows can form in mist, such as that of a waterfall

Rainbow with a faint reflected rainbow in the lake

Rainbows may form in the spray created by waves (called spray bows)

Rainbow after sunlight bursts through after an intense shower in Maraetai, New Zealand

Circular rainbow seen while skydiving over Rochelle, Illinois

Visibility

Rainbows can be observed whenever there are water drops in the air and sunlight shining from behind at a low altitude angle. The most spectacular rainbow displays happen when half the sky is still dark with raining clouds and the observer is at a spot with clear sky in the direction of the sun. The result is a luminous rainbow that contrasts with the darkened background.

The rainbow effect is also commonly seen near waterfalls or fountains. In addition, the effect can be artificially created by dispersing water droplets into the air during a sunny day. Rarely, a moonbow, lunar rainbow or nighttime rainbow, can be seen on strongly moonlit nights. As human visual perception for colour is poor in low light, moonbows are often perceived to be white. It is difficult to photograph the complete semicircle of a rainbow in one frame, as this would require an angle of view of 84°. For a 35 mm camera, a lens with a focal length of 19 mm or less wide-angle lens would be required. Now that powerful software for stitching several images into a panorama is available, images of the entire arc and even secondary arcs can be created fairly easily from a series of overlapping frames. From an aeroplane, one has the opportunity to see the whole circle of the rainbow, with the plane's shadow in the centre.

Number of colours in spectrum or rainbow

A spectrum obtained using a glass prism and a point source, is a continuum of wavelengths without bands. The number of colours that the human eye is able to distinguish in a spectrum is in the order of 100. Accordingly, the Munsell colour system (a 20th century system for numerically describing colours, based on equal steps for human visual perception) distinguishes 100 hues. However, the human brain tends to divide them into a small number of primary colours. The apparent discreteness of primary colours is an artefact of the human brain. Newton originally (1672) divided the spectrum into five primary colours: red, yellow, green, blue and violet. Later he included orange and indigo, giving seven primary colours by analogy to the number of notes in a musical scale. The Munsell colour system removed orange and indigo again, and returned to five primary colours. The exact number of primary colours for humans is a somewhat arbitrary choice.

Explanation

Light rays enter a raindrop from one direction (typically a straight line from the Sun), reflect off the back of the raindrop, and fan out as they leave the raindrop. The light leaving the rainbow is spread over a wide angle, with a maximum intensity at 40.89–42°

White light separates into different colours on entering the raindrop due to dispersion, causing red light to be refracted less than blue light.

Variations

Multiple rainbows

Secondary rainbows are caused by a double reflection of sunlight inside the raindrops, and appear at an angle of 50–53°. As a result of the second reflection, the colours of a secondary rainbow are inverted compared to the primary bow, with blue on the outside and red on the inside. The secondary rainbow is fainter than the primary because more light escapes from two reflections compared to one and because the rainbow itself is spread over a greater area of the sky. The dark area of unlit sky lying between the primary and secondary bows is called Alexander's band, after Alexander of Aphrodisias who first described it.

A double rainbow features reversed colours in the outer (secondary) bow, with the dark Alexander's band between the bows.

Monochrome rainbow

Occasionally a shower may happen at sunrise or sunset, where the shorter wavelengths like blue and green have been scattered and essentially removed from the spectrum. Further scattering may occur due to the rain, and the result can be the rare and dramatic monochrome rainbow.

Fogbow

Fogbows form in the same way as rainbows, but they are formed by much smaller cloud and fog droplets which diffract light extensively. They are almost white with faint reds on the outside and blues inside. The colours are dim because the bow in each colour is very broad and the colours overlap. Fogbows are commonly seen over water when air in contact with the cooler water is chilled, but they can be found anywhere if the fog is thin enough for the sun to shine through and the sun is fairly bright. They are very large—almost as big as a rainbow and much broader. They sometimes appear with a glory at the bow's centre

For more information on Rainbows visit one of the following:-

http://en.wikipedia.org/wiki/Rainbow

http://www.sciencedaily.com/releases/2011/12/111206151526.htm

http://eo.ucar.edu/rainbows/

http://www.ouramazingplanet.com/1059-rainbows-formation-explainer.html