The Coolest Clouds You've (Probably) Never Seen

Most people have seen clouds. Most people who have seen clouds have also noticed that no two are alike. The fluffy white ones are almost mandatory on a sunny summer day, the grey ones signal snow might be on the way, and the dark, low clouds suggest rain or a thunderstorm in the near future. For millennia people have used clouds to forecast immediate changes in local weather, strategies that have kept people safe from lightning strikes, flash floods, hypothermia, among other perils of dangerous weather. Though today there are satellites and radar used to track large weather patterns, clouds are still excellent indicators of weather.

Generally clouds can only be seen during the day, unless there is a particularly bright moon that reflects the sun's light to the sky at night. Other times clouds are only visually detected because the stars cannot be seen through them. What's being is seen is more of the absence of light blocked by the clouds rather than the clouds themselves.

So when bright clouds are seen at night, what the heck does that mean about the weather?

Noctilucent clouds. Matthias Süßen/Wikimedia Commons

 

 

These are called noctilucent, or "night-shining" clouds. The physical structure of these clouds are collections of small water ice crystals surrounding tiny particles of dust that are suspended in the high atmosphere. In fact, noctilucent clouds are the highest clouds of all! While most clouds hang out in the troposphere, the bottom-most layer of the Earth's atmosphere that goes to 5-6 miles above the ground, noctilucent clouds exist around 50 miles above the surface, in a layer called the mesosphere, just below the famous aurora borealis light waves.

Structure of Earth's atmosphere:
https://scied.ucar.edu/atmosphere-layers

These clouds are typically only visible at high latitudes in both the northern and southern hemispheres between 50 and 70 degrees. For reference in North America, that means going up north into Canada, at least as far north as Vancouver. These clouds are only visible during twilight hours during the North American summers. After the sun has set below the horizon, its rays linger for a little while, yielding the lovely colors associated with sunsets. For a few moments the Earth blocks the part of the sun's rays that illuminate the lower atmosphere, allowing the sun's rays to illuminate only the highest parts of the atmosphere, where the noctilucent clouds are. This only lasts until the Earth actually gets in the way of the sun and blocks all light, creating full night. To view noctilucent clouds takes luck and a lot of patience to wait for the right moment, and a camera never hurts!

These clouds are a relatively recent weather phenomenon, though they've probably existed for all time. There is no written mention of these clouds being observed until 1885, when Otto James of Germany began to study these clouds extensively. It was he who coined the name "noctilucent" in 1887, and his notes are the first to mention them at all. He had already been studying the changes in sunsets since the historic eruption of the Krakatoa volcano in 1883, and it was perhaps accidental that he noticed these unusual clouds at all. Subsequent observations of these clouds by James and his colleagues at the Berlin Observatory until 1896 determined their heights and their general visibility patterns. Not much science was done on these clouds after James' death in 1901 until a satellite, NASA's OGO-6, observed them for the first time in 1972.

Geometry of the sky needed for viewing noctilucent clouds.
Credit: NASA

Even today, these clouds are "not fully understood," which is just scientist speak for "we're really not sure what conditions make these clouds, what they mean, or how they vary with different latitudes, altitudes, dust concentration, and ice crystal size." This is really exciting news! It means there's a lot left to discover for anyone interested in studying them, and it's practically a new branch of meteorology. And there are always new mysteries around the corner to learn about.

For example, what has really confused everyone from expert meteorologists to anyone who goes starwatching is why these clouds have recently been observed as far south as northern New Mexico (36 degrees N. latitude). Noctilucent clouds also appear to occur more frequently and are brighter when seen, at least over the past decade. Atmospheric scientists point to increased levels of carbon dioxide and methane as potential causes for the increase in noctilucent clouds. These gases rise into the upper atmosphere and interact with other gases, creating water vapor that can form additional high-altitude clouds. Stronger, more frequent storms on Earth can also supply more water vapor to the high atmosphere, further driving noctilucent cloud formation. These scientists are actively testing their hypotheses now to understand if these clouds are signals of a major shift in Earth's atmosphere.

Until we know more, there is not much to do but enjoy their beauty on a quiet summer night.

Credit: https://scied.ucar.edu/imagecontent/noctilucent-clouds

Sources:
https://en.wikipedia.org/wiki/Noctilucent_cloud
https://www.skyandtelescope.com/observing/noctilucent-clouds-3/
https://www.sciencenews.org/article/night-shining-noctilucent-clouds-have-crept-south-summer
https://scied.ucar.edu/