DISCUSSION: Earth's atmosphere can be divided into four layers based on how temperature changes with height. In the lowest layer of the atmosphere (troposphere), temperature generally decreases with height. Above that is the stratosphere where temperature increases with height. Then above the stratosphere is the mesosphere and thermosphere where temperature decreases and increases with height, respectively. In order for clouds to form, there must be water vapor in the air. Since the source of water vapor is the surface, most of the water in the atmosphere and associated clouds occur in the troposphere. However, on rare occasions, the small amounts of vapor in higher layers of the atmosphere can lead to cloud development. In particular, noctilucent clouds are thin clouds that occur in the mesosphere (~50 miles above the ground). When these clouds form, they tend to occur near the poles where the extremely low temperatures help them to develop. In addition, these clouds are extremely thin due to the extremely low amounts of vapor in the mesosphere. Hence, they can't be seen when the sun is high in the sky. Instead they are illuminated when the sun is low in the sky or below the horizon, which occurs a lot in polar regions.
While noctilucent clouds almost exclusively appear in polar regions, certain conditions may allow them to form at lower latitudes. For example, the picture above shows a noctilucent cloud near San Francisco, CA. In this case, as a meteor burned up high in Earth's atmosphere, it created the right conditions for a noctilucent cloud to form at an unusually low latitude. Rocket launches can also help to generate these clouds in places where you wouldn't normally expect them to form. The picture below from spaceweather.com shows such a noctilucent cloud formation over Orlando, FL as a result of a rocket launch from Cape Canaveral.
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© 2019 Meteorologist Dr. Ken Leppert II