The parhelic circle is a white horizontal band, in line with the sun, passing through the site of the parhelion. They can be very bright, red coloured closest to the sun or may just be visible as a smudge in the sky. They are produced by the refraction or reflection of sunlight by ice crystals suspended in the atmosphere. Parhelion are one of the most commonly spotted halo phenomena. Confusingly not bearing any resemblance to a dog, it can often be seen on the side of a halo that circles the sun. On days when high cirrostratus clouds are visible in the sky, whether that be ahead of an approaching frontal system or on a benign, bright day, it’s possible you may see a bizarrely named sun dog in the sky. Michal Krzysztofowicz, Halley Research Station, Antarctica © Alan Tough via Royal Meteorological Society / Natural History Museum Some forms made up of small, supercooled water droplets and nitric acid are known to play a role in the Antarctic ozone hole by providing a surface for chemical reactions that produce an ozone destroying form of chlorine to occur. High-altitude clouds that form in the stratosphere above polar regions in the extreme cold of winter. Nitrogen molecules produce blue, violet and red light. Different gases will emit a different coloured light, with oxygen molecules at Aurora about 60 miles above the Earth’s surface responsible for green light and at 200 miles up, red. When those molecules come back together again, they’ve got more energy than they need and so release the excess as coloured light. Why the colour? When the particles enter our atmosphere, they smash into gas molecules there, breaking them apart. Occasionally, there are an increased number of particles - a coronal mass ejection - and the force is enough that, when they collide with the magnetosphere, they break through, heading down along magnetic field lines, triggering the aurora.Īuroras are a riot of colour, most commonly seen as a pale green or pink light, but they can also be violet, blue, yellow and occasionally orange or white. Earth is surrounded by a magnetic field, called the magnetosphere, which protects it from the solar wind, essentially stopping the atmosphere from being swept away into space. This wind moves around the solar system, bumping into planets, including Earth, on its way. It does still all come back to the sun, as a ball of plasma releasing a constant stream of electrically charged particles called the solar wind. © Vitaliy Novikov via Royal Meteorological Society / Natural History Museum Freezing fog is fog that forms when the air temperature is less than 0˚C (32˚F), often depositing rime on surfaces.Īurora. The band between the two arcs, known as Alexander’s band (after Alexander of Aphrodisias, who described it in AD200) is dark, as the light is being bent away from the observer.įog is a suspension of water droplets or ice crystals in the surface layer of the atmosphere, occasionally accompanied by smoke particles, which reduces visibility to less than half a mile. They are formed when light is reflected twice within a raindrop, resulting in a second arc with the colour order reversed. The second bow appears about 10˚ above the primary bow, but fainter and nearly twice as wide. There may still be no pot of gold, but a double rainbow can be doubly rewarding and they’re not that uncommon if you know to look for them. If you’re lucky, there are times you can see two for the price of one. From a plane, the top of a mountain or with a hose in your garden, it is possible to see a full circle. When the sun is higher in the sky, we may see a shallow arc close to the horizon, but being nearer sunrise or sunset gives the best chance of seeing a full tall semi-circle. How much we see of the arc depends on where we are and how high the sun is in the sky. Every observer will see a slightly different angle of refraction, so each rainbow is unique to that person. If the angle is right, some of that light will be reflected off the internal surface of the raindrop, exiting the drop and refracting again, so that ultimately we see the rainbow with the red band on the outside. This is known as refraction - each colour with its different wavelength will bend at slightly different angles so that the light splits into its different component colours. Sunlight is made up of a spectrum of different colours that will slow down and change direction when entering water. © Cammie Czuchnicki via Royal Meteorological Society / Natural History Museum Country Life's Top 100 architects, builders, designers and gardenersĭouble Rainbow.
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