NLC seasons getting longer, clouds moving farther south


New data provided by NASA's AIM (Aeronomy of Ice in the Mesosphere) satellite mission shows that the first noctilucent clouds (NLCs) of summer have been trending earlier since 2007, when the spacecraft was launched. Each year, NLCs first appear in the Arctic and then spin outward to lower latitudes but they were recently seen appearing as far south as Los Angeles and Las Vegas, U.S. setting records for low-latitude sightings.

"The season appears to be starting earlier, which is making it longer by about 5 days," said Cora Randall of the University of Colorado’s Laboratory for Atmospheric and Space Physics (LASP).

Randall created the following graph, showing data from AIM's CIPS sensor since 2007. Each blue box shows the day of the year when the sensor detected the first NLC of the northern hemisphere's summer:

Image credit: NASA AIM/CIPS, Cora Randall

AIM data also show that seasons do not end later — they still peak mid-June to July and end in August.

In recent years, the clouds have started appearing farther south toward the equator, breaking records for low-latitude sightings, and in 2019 and 2020 we had major outbreaks over North America and Europe.

Interestingly, last year's southern hemisphere's season had an unusually late start. 

"The southern hemisphere stratosphere is very unusual this year," Randall said at the end of 2020. "The ozone hole is exceptionally large, until recently zonal winds have been blowing in the wrong direction, and overall the stratosphere is much more 'winter-like' than it should be in December." [Tag: noctilucent clouds]

First NLCs of the 2021 season in the northern hemisphere appeared over Poland, Czech Republic, Canada, and Scotland, among other countries, on May 25 and 26. In only four days since the clouds were first seen, their coverage of the Arctic has multiplied 10 times.

Dr. Tony Phillips of the SpaceWeather.com said the reason for their early start this year may be extra water in the mesosphere as satellite data show that 2021 is one of the wettest years since 2007. "NLCs have more water to work with, thus, it started early in the season and spread quickly."

According to Lynn Harvey (LASP), temperatures at the mesosphere have been breaking 14-year records at the beginning of June last year, causing NLCs to spill across the Arctic and descend to middle latitudes.

The plots she made at the time showed that 2020 was shaping up to be a very cold and wet year:


Image credit: NASA AIM

The AIM mission was launched on April 25, 2007, and has been extended by NASA through the end of FY20. During this time the instruments monitor noctilucent clouds to better understand their variability and possible connection to climate change.

The primary goal of the mission is to help scientists understand whether the clouds' ephemeral nature, and their variation over time, is related to Earth's changing climate – and to investigate why they form in the first place.

By measuring the thermal, chemical and other properties of the environment in which the mysterious clouds form, the AIM mission will provide researchers with a foundation for the study of long-term variations in the mesosphere and its relationship to global climate change.

In addition to measuring environmental conditions, the AIM mission will collect data on cloud abundance, how the clouds are distributed, and the size of particles within them.

Featured image credit: NASA AIM/CIPS


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  1. The significance of this rare event is clear: it signals the beginning of an era in which NLCs (also known as Polar Mesospheric Clouds) are visible to a much greater population than previously due to a fluctuation of a naturally variable phenomenon. NLCs are currently being studied intensively as indicators of physical changes in the upper atmosphere because their occurrence, frequency and latitudinal coverage appear to be increasing, as a result of increasing water vapor and decreasing temperatures. This is because we are entering a deep solar minimum, the period of the Sun’s 11-year cycle when it is least active. That means the ultraviolet radiation from the Sun that usually destroys the water modules which form these clouds is less intense, so more of them can form. This phenomenon along the record cold breaking temperatures during April and May, shows that Earth is cooling. NOAA and NASA appear to agree, if you read between the lines, with NOAA saying we’re entering a ‘full-blown’ Grand Solar Minimum in the late-2020s, and NASA seeing this upcoming solar cycle (25) as “the weakest of the past 200 years”, with the agency correlating previous solar shutdowns to prolonged periods of global cooling. Calculations show that a change of 50% in mesospheric H20 could have a tenfold effect on the brightness of ice clouds (Jensen, 1989; Thomas, 1995). Thus, one would expect NLC to become both brighter and more numerous during solar cycle minimum. This notion was supposed by observations of a solar-cycle dependence of the occurrence rate of bright Polar Mesospheric Clouds (Thomas et al., 1991).

    1. You’re still arguing from basis of outdated models that are already way off target of currently observed solar activity… We’ve already entered the next cycle last year, and it will very likely be the strongest since a long time. Activity is stronger and earlier, and the sunspot count is higher than on previous cycles. Both NOAA and Chinese Academy of Sciences have recalled previous predictions of a decrease in activity, after noticing a sharp acceleration.

      I would argue it’s much more likely that increase in NLCs is largely due to moisture being introduced into the mesosphere by spacecraft launches, and decomposition of methane from agricultural emissions.

      1. You gotta be kidding. These outdated “models” refer to the relationship between variations of mesospheric H20 and its effects on the brightness of ice clouds, not solar activity. Your claim that current solar activity is stronger and the sunspot count is higher than on previous cycle is baseless. Spaceweather.com shows us that sunspots are low. Europe experienced a bone-chiller of a spring. To call it a “spring” actually does a disservice to the severity and prolonged nature of the cold. I like to think of March-through-May this year as an extension of winter due to low solar activity. Finally, moisture in the upper atmosphere cannot be originated by decomposition of methane as you claim, because it is broken down in a process called hydroxyl oxidation into CO2, entering a carbon cycle which sees the gas absorbed by plants and soils as part of the sink effect. The COLD TIMES are returning, the mid-latitudes are REFREEZING, in line with the great conjunction, historically low solar activity, cloud-nucleating cosmic rays, and a meridional jet stream flow.

        1. I would know, I live in Europe. But I also know that Earth is more than just Europe. A stationary Rossby wave that chilled down Europe for entire May doesn’t mean anything for global temperatures. At the same time, there was an opposite hot anomaly in western Siberia, caused by the opposite phase of that same wave.
          Sunspots are “low” because we’re currently at the minimum of 11yr cycle, but are also much higher than the norm for this phase of the cycle.

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