Two Extremely Rare Cloud Formations Spotted in UK Skies

February 5, 2016 | Joanne Kennell

Polar stratospheric clouds
Photo credit: Mathiasm/Wikipedia (CC BY-SA 3.0)

Don't be fooled — these aren't the northern lights.

Recently, some very colorful cloud formations were seen across the UK sky, sparking false reports of the aurora borealis.

People in the northern regions of the world, for example Scandinavia and Canada, are usually the only ones that get to see these rainbow-colored clouds, the first of the two rare cloud formations that appeared in British skies recently.  Over the past few days, people across the UK and Ireland have been lucky enough to witness this rare phenomenon, known as nacreous or polar stratospheric clouds.

As you can imagine, people were very surprised to see these bright clouds in the sky on the evening of February 1.  In fact, nacreous clouds are so unusual that AuroraWatch UK, a service that monitors the likelihood or auroral sightings, received several reports that these displays were auroras borealis — also known as the northern lights.

However, the aurora monitoring service announced that conditions were not right for the northern lights to appear over the country that evening.

SEE ALSO: Mystery of Northern Light Bursts Finally Solved

Nacreous clouds tend to form in the winter, high in the polar stratosphere — a layer of our atmosphere around 15,000 to 25,000 meters (49,000 to 82,000 feet) in altitude.  The stratosphere is generally very dry, so cloud formation in this region is very rare, however recent storms in the area may have driven moisture high into the atmosphere.

Nacreous clouds will also only form when temperatures in the stratosphere are below -78 degrees Celsius (-108 degrees Fahrenheit) — a temperature that turns moisture into the air into supercooled liquid (below zero but not frozen) or ice crystals.  It is a temperature that usually only occurs in the winter at high latitudes.

When spotted, nacreous clouds are often seen at either dawn or dusk when the sun is between 1 and 6 degrees below the horizon.  The light is diffracted by the ice crystals in the clouds, a process known as cloud iridescence, which produces the stunning rainbow effect.  

Circumhorizontal arcs, also nicknamed fire rainbows — even though there is no fire involved, and technically they are not rainbows — were also spotted in the UK.  However, the only way a fire rainbow can form is if the sun is positioned higher than 58 degrees above the horizon, and light is refracted as it passes through high-level cirrus clouds (made of hexagonal ice) at an altitude of more than 6,000 meters (20,000 feet).

Circumhorizontal arcs
 Circumhorizontal arcs. photo credit: Attila Magyar/Flickr (CC BY-SA 2.0)

This phenomenon is also very rare for the UK, since the sun is not very high above the horizon for most of the year.  “For example, in London, the Sun is only high enough for 140 hours between mid-May and late July. While in Los Angeles, the Sun is higher than 58 degrees for 670 hours between late March and late September,” explained Amusing Planet.  

Since both these rare phenomena were also spotted in the UK this month, you may be wondering if the two are somehow related?  Not really — although the both require high and cold clouds, the filtering of sunlight and the location of the sun on the horizon are quite different.

SEE ALSO: New Cloud to Be Added to the International Cloud Atlas

It is very easy to confuse cloud iridescence and circumhorizontal arcs, but there are ways to distinguish the two.  First, circumhorizontal arcs always have a fixed location in the sky relative to the sun or moon, while iridescence can occur in different positions.  

Second, the color bands in a circumhorizontal arc always run horizontally with red on top, while the iridescence of nacreous clouds is random and follows the contours of the clouds.  

Lastly, the colors of a circumhorizontal arc are pure and spectral, more so than in a rainbow, while the colors in iridescence appear more washed-out and blended.

Weather forecasters predict that more sightings of these nacreous clouds may be possible in the UK until Saturday.  After that, the polar vortex, which is responsible for the cold conditions currently in the stratosphere above the UK, with move northwards back to its usual position.

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