Noctilucent clouds or night shining clouds are wispy clouds in the upper atmosphere. They form in the mesophere, making them the highest clouds in the sky. Here is an overview of noctilucent cloud appearance, where and when to see them, and how they form.
- Noctilucent clouds or night shining clouds are visible at twilight, before sunrise and after sunset.
- They usually appear luminous blue or white, but can be other colors.
- They are the highest clouds, forming in the mesophere between 76 to 85 km (249,000 to 279,000 ft).
- Noctilucent clouds are visible in summer, from about 15 days before the summer solstice to two months after the solstice.
- Usually, the clouds become visible between ±50° and ±70° latitude.
- Noctilucent clouds strongly reflect radar.
What Do Noctilucent Clouds Look Like?
Noctilucent clouds typically are luminous blue, silver, or white. The blue color comes from ozone in the atmosphere absorbing sunlight. Sometimes they are yellow, orange, red, or iridescent. Rocket vapor, in particular, produces colors besides blue or white. This is because the exhaust releases water vapor particles with a very uniform size. They are wispy, fragmented clouds, resembling cirrus clouds. The clouds take a variety of shapes, including whirls, waves, bands, and streaks.
How to See Noctilucent Clouds
First, look for the clouds at the right time of year. In the Northern Hemisphere, noctilucent clouds appear between May and August. In the Southern Hemisphere, look for the clouds between November and February. While the clouds form in both hemispheres, they tend to be fainter and less common in the Southern Hemisphere.
Noctilucent clouds are most visible between 50° to 65° latitude, but you have a decent chance of seeing them anywhere from 40° North or South Latitude up to 70°. While the clouds are present over the polar regions, it does not get dark enough in the summer to see them. Spacecraft can view the polar clouds, which are called Polar Mesospheric Clouds or PMC.
So, go outside and look toward the East before sunrise or to the West after sunset. The clouds are visible when the Sun is below the horizon, about 90 minutes to 2 hours before sunrise or after sunset. The clouds you see at this time are noctilucent clouds.
Noctilucent clouds are not visible all the time. However, rocket launches, meteors, volcanoes, and other events make them not-uncommon.
How Noctilucent Clouds Form
Noctilucent clouds consist of tiny water ice crystals in the mesosphere between 76 to 85 km (249,000 to 279,000 ft). The ice forms as water vapor freezes or it directly deposits from water vapor as ice crystals onto dust or smoke particles. There are multiple sources for the dust, including volcanic eruptions, large fires, meteors and micrometeors, and rocket exhaust. For example, the space shuttle, SpaceX launches, and Chelyabinsk bolide all produced noctilucent clouds.
The mesosphere is very dry and very thin. So, ice crystals only form in this layer when the temperature drops below −120 °C (−184 °F). Counterintuitively, this occurs in the summer rather than the winter. In the summer, seasonal vertical winds drive moisture upward, where adiabatic cooling drops its temperature significantly. Additionally, some scientists believe gaps in the tropopause allow moisture into the mesosphere. The reaction between methane and hydroxyl radicals in the stratosphere may also produce water.
Radar Reflectivity
One property of noctilucent clouds that distinguish them from ordinary clouds is there extremely high radar reflectivity in the 50 MHz to 1.3 GHz frequency range. Scientists do not agree on the reason for this behavior. Some argue a thin coating of sodium and iron metal coats the ice crystals. Others believe homogeneous nucleation of water onto smoke particles accounts electron depletions in the region.
History
The first record of noctilucent clouds comes from 1885, following the 1883 Krakatoa volcanic eruption. Either the clouds appeared as a result of the eruption or else people simply noticed them because the volcano produced glorious sunsets, so more people were looking at the sky. A German named Otto Jesse coined the term “noctilucent cloud” in 1887. Photographic records indicate the clouds likely were not in the sky prior to the volcanic eruption. Observations by the Berlin Observatory used triangulation to determine the height of the clouds.
In 2001, the HALOE instrument on the Upper Atmosphere Research Satellite determined the clouds consist of water ice. That same year, Sweden’s Odin satellite produced global maps of noctilucent clouds and measured their spectra. The 2007 AIM (Aeronomy of Ice in the Mesosphere) satellite observed the shapes of noctilucent clouds and found them similar to tropospheric clouds.
In 2006, the Mars Express mission discovered carbon dioxide crystal clouds in the Martian atmosphere. These clouds form 100 km (330,000 ft) above the surface of Mars. Like noctilucent clouds on Earth, the Martian clouds only become visible when the Sun drops below the horizon.
References
- Donahue, T. M.; Guenther, B.; Blamont, J. E. (1972). “Free Access Noctilucent Clouds in Daytime: Circumpolar Particulate Layers Near the Summer Mesopause”. Journal of the Atmospheric Sciences. 29 (6): 1205–1209. doi: 10.1175/1520-0469(1972)029<1205:NCIDCP>2.0.CO;2
- Murray, B.J.; Jensen, E.J. (2000). “Homogeneous nucleation of amorphous solid water particles in the upper mesosphere”. Journal of Atmospheric and Solar-Terrestrial Physics. 72 (1): 51–61. doi: 10.1016/j.jastp.2009.10.007
- Naval Research Laboratories (2003). “ Study Finds Space Shuttle Exhaust Creates Night-Shining Clouds“.
- Rapp, M.; Lubken, F.J. (2009). “Comment on ‘Ice iron/sodium film as cause for high noctilucent cloud radar reflectivity’ by P. M. Bellan”. Geophys. Res. Lett. 114 (D11): D11204. doi: 10.1029/2008JD011323
- World Meteorological Organization, ed. (2017). “ Type I Veils“. International Cloud Atlas.