OPOD - Nacreous Clouds

OPOD - Nacreous Clouds: A Spectacular Atmospheric Phenomenon

Nacreous clouds, also known as polar stratospheric clouds (PSCs), are a mesmerizing atmospheric phenomenon that occurs at extremely high altitudes in the stratosphere, ranging from 9 to 16 miles (15-25 km) above the Earth's surface. These ethereal clouds are characterized by their vibrant and electric colors, which glow eerily when illuminated by sunlight before sunrise or after sunset. While they are a rare occurrence, nacreous clouds have been observed in various locations, with Sweden being a particularly favored spot for witnessing their breathtaking beauty.

Unveiling the Beauty of Nacreous Clouds

The mesmerizing display of nacreous clouds is a result of the interaction between sunlight and tiny ice crystals suspended in the stratosphere. These ice crystals diffract sunlight, creating iridescent hues that seem to twist, stretch, and curl majestically in the sky. The surreal colors of these clouds are unlike anything you would see in ordinary iridescent clouds. Their formation requires an exceptionally cold stratosphere, with temperatures dropping below -85 degrees Celsius. Consequently, nacreous clouds predominantly appear during winter and are associated with very windy weather conditions.

The Role of Tropospheric Disturbances

One intriguing aspect of nacreous cloud formation is their association with tropospheric disturbances. These disturbances are believed to play a crucial role in lofting necessary water vapor into the lower stratosphere, creating the ideal conditions for the development of these stunning clouds. Additionally, nacreous clouds are often observed downwind of mountains, suggesting that the topography of an area can influence their occurrence.

Nacreous Cloud Spotting: When and Where to Look

If you're eager to witness the awe-inspiring beauty of nacreous clouds, timing and location are key. These elusive clouds are most commonly observed in the Northern Hemisphere between late December and February. While Norway, Sweden, Iceland, and Finland are considered prime locations for nacreous cloud sightings, they can occasionally be seen further south, although this is quite rare.

To distinguish nacreous clouds from ordinary iridescent clouds, keep the following characteristics in mind:

  • Nacreous clouds glow in the sky up to an hour before sunrise and after sunset, allowing for an extended period of observation.
  • They are slow-moving, often appearing as filmy wave-like structures that gradually change shape over several minutes.
  • The colors of nacreous clouds are exceptionally vibrant and intense, making lower iridescent clouds pale in comparison.

A Lasting Impression

Once you've had the privilege of witnessing the ethereal beauty of nacreous clouds, it's an experience that will remain etched in your memory. The enchanting colors and graceful movements of these high-altitude clouds create a captivating spectacle that is truly unforgettable.

In conclusion, nacreous clouds are a remarkable atmospheric phenomenon that offers a glimpse into the awe-inspiring wonders of our planet. Their rarity, stunning colors, and unique characteristics make them a sought-after sight for sky enthusiasts and photographers alike. So, keep an eye on the sky during the winter months in the Northern Hemisphere, and you may be fortunate enough to witness the breathtaking display of nacreous clouds firsthand.

Nacreous Clouds, Sweden

Sweden saw spectacular nacreous clouds on 12th and 13th January. Richard Blaine took these images of the rare stratospheric clouds from Åre 340 miles NW of Stockholm. Above: Nacreous clouds blaze 5 minutes after sunset on 13th. Dark ragged lower tropospheric clouds frame the picture's base. ©Richard Blaine, shown with permission.

Nacreous clouds are very high, 9-16 miles (15 – 25 km) in the stratosphere. They catch sunlight long before ground level sunrise and after sunset to glow eerily with unbelievably bright electric colours. They twist, stretch and curl majestically as lower dark tropospheric clouds hurry beneath.

Their tiny ice crystals diffract sunlight to give the iridescent hues. They need an unusually cold stratosphere (less than -85 Celsius) and are therefore rare winter occurrences. They tend to form in very windy weather and downwind of mountains. The resulting tropospheric disturbances possibly loft necessary water vapour into the lower stratosphere. They are a sub-class of polar stratospheric clouds, PSCs. The top image perhaps shows other less iridescent PSC types dotting the sky.

Search before sunrise or after sunset late December to February in the Northern Hemisphere. Norway, Sweden, Iceland and Finland are favoured locations but they are very occasionally seen further south.

How to distinguish them from “ordinary” iridescent clouds? Nacreous clouds glow in the sky up to an hour before sunrise and after sunset. They are slow moving. They are filmy wave clouds that over minutes twist, stretch and change. They are very brightly coloured and lower iridescent clouds are pallid ghosts by comparison. Once seen you will never forget!

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Reference Atmospheric Optics

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  • "OPOD - Nacreous Clouds". Atmospheric Optics. Accessed on March 28, 2024. https://atoptics.co.uk/blog/opod-nacreous-clouds-2/.

  • "OPOD - Nacreous Clouds". Atmospheric Optics, https://atoptics.co.uk/blog/opod-nacreous-clouds-2/. Accessed 28 March, 2024

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