Noctilucent (or night shining) clouds are being reported farther south than ever before, with reports from southern California, New Mexico, Oklahoma, and southern Ohio. Below is a shortened version of the excellent article by Roy W. Spencer, Ph. D. explaining the phenomenon and its prevalence during solar minimums (for Spencer’s article in full, click here).
NLCs form in the upper mesosphere at an altitude of about 85 km (around 50 miles), which is above 99.999% of the atmosphere. It is the same altitude where meteors and thunderstorm sprites occur, but somewhat below the altitude of the aurora.
They are visible about 1-2 hours after sunset or before sunrise, when other clouds are in darkness but these are still illuminated by the sun due to their high altitude. It is believed that water vapor from oxidation of methane condenses on meteor dust, leading to the ice cloud formation.
The conditions for NLC formation require extremely cold temperatures, as low as -150 deg. F.
I’ve been looking for online sources of near real time satellite data which might be used to monitor them, but there isn’t much out there. The AIM instrument on the TIMED satellite provides daily images, but for some reason the clouds the satellite views seem to be only from the latitude of central Canada northward.
NLCs have been increasing in recent decades.
They are always more prevalent during solar minimum conditions (which we are now experiencing), when there is less solar energy heating the extreme upper atmosphere.
But there is also a long-term trend upward:
Anecdotally, the most dramatic increase has been at mid-latitudes where no previous reports of NLC sightings exist. In the last couple days, NLCs have been observed as far south as Joshua Tree, CA (34 deg. N) and Albuquerque, New Mexico (35 deg. N).
One of the interesting things about the mesosphere where NLCs form is that the summer hemisphere is colder, and the winter hemisphere warmer.
The reason for the reversal is the Brewer-Dobson circulation, which causes upwelling (and thus adiabatic cooling) in the summer mesosphere. That rising air forces subsidence (sinking air, and thus adiabatic warming) in the winter hemisphere.
This hemispheric difference is clearly seen in upper mesospheric temperatures from channel 20 on the DMSP SSMIS instrument. These data sets are not widely available, and this pair of images, one year apart, was provided to me by Steve Swadley at Naval Research Laboratory – Monterey.
SSMIS 60 GHz imagery of upper mesospheric temperatures one year apart shows cooler temperatures in 2019 than 2018, presumably leading to more frequent noctilucent cloud sightings.
The SSMIS imagery can’t be directly related to NLC sightings because it is a vertical average for a fairly deep layer in the upper mesosphere, while NLCs form in only the very coldest layer in the upper part of that deeper layer, at the “mesopause”. But I would wager they are correlated.
By ROY W. SPENCER, Ph. D. — http://www.drroyspencer.com
Roy W. Spencer (Ph.D. Meteorology) is a weather and climate researcher at the University of Alabama in Huntsville.
The cold times are returning in line with historically low solar activity.
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Grand Solar Minimum + Pole Shift