Mother of Pearl clouds, also known as nacreous clouds, are luminous and iridescent clouds that appear at high altitudes, usually when the sun is a few degrees below the horizon.
Understanding Mother of Pearl Clouds
These clouds get their unique appearance from the way they interact with sunlight. Here’s a breakdown:
Key Characteristics
- High Altitude: These clouds form in the stratosphere, typically at altitudes between 15 to 25 kilometers (9 to 16 miles) above the Earth's surface.
- Iridescent Colors: They display a characteristic pearly, shimmering appearance, exhibiting vibrant colors like pink, blue, and green, which shift as the viewer's perspective changes.
- Sunlight Interaction: The iridescent effect is a result of diffraction and interference of sunlight by the tiny ice crystals that make up the cloud. This happens more effectively when the sun is just below the horizon.
- Synonym: Mother of Pearl clouds are also called nacreous clouds, both terms referencing their pearl-like appearance.
Formation
Mother of Pearl clouds form under specific atmospheric conditions:
- Very Cold Temperatures: Stratospheric temperatures need to be extremely cold, typically below -80°C (-112°F), for water vapor to condense and form ice crystals.
- Polar Vortex: These conditions are more common in polar regions, especially during the winter, where the strong polar vortex can isolate extremely cold air.
- Mountain Waves: They are often associated with mountain waves, which cause air to rise and cool, leading to the formation of the cloud.
Why They Are Seen Near Sunset or Sunrise
- The clouds need to be illuminated by sunlight when the sun is below the horizon. This is what creates the spectacular iridescent display. If the sun is too high, the effect is much less pronounced.
Summary
| Feature | Description |
|---|---|
| Altitude | 15-25 kilometers (9-16 miles) |
| Appearance | Luminous, iridescent, pearly |
| Color | Pink, blue, green |
| Formation | Requires extremely cold temperatures and usually a polar vortex. |
| Viewing Time | When the sun is a few degrees below the horizon. |
| Synonym | Nacreous cloud |
In essence, Mother of Pearl clouds, or nacreous clouds, are high-altitude, iridescent clouds that display a beautiful, shimmering appearance when illuminated by the sun just below the horizon. The definition provided in the reference states they are "a luminous iridescent cloud at a high altitude that may be seen when the sun is a few degrees below the horizon", with the synonym being nacreous cloud.
