The archive, assembled by Jānis Kauliņš of the Institute of Astronomy at the University of Latvia and described in the journal History of Geo- and Space Sciences, documents systematic visual and photographic observations of mesospheric clouds carried out between 1957 and 1983. The campaign began during the International Geophysical Year of 1957, a landmark global effort to coordinate Earth science observations, and continued uninterrupted for more than a quarter of a century under the auspices of the Latvian branch of the All-Union Astronomical and Geodesic Society, later renamed the Latvian Astronomical Society. Because the observations followed a uniform, internationally recognized methodology throughout, they constitute a methodically consistent data series of unusual duration, something that exists for only a handful of locations on the planet.

The scientific stakes are considerable. Until the 1980s, very few instruments could reach or study the mesosphere, the atmospheric layer between roughly 50 and 85 kilometers, leaving ground-based cloud observations as one of the only practical windows into its state and dynamics. In recent decades, interest in noctilucent clouds has surged dramatically. The clouds have been appearing at latitudes where they were never seen before, and several studies have debated whether their increasing brightness, frequency, and shifting heights are connected to climate change. Rising greenhouse gases cool the mesosphere even as they warm the lower atmosphere, and a colder mesopause favors the formation of the ice crystals that make these clouds shine. Some researchers report a clear downward trend in the average height of the clouds’ lower boundary that has accelerated since the 1960s, while others analyzing cloud statistics and physical parameters find no significant long-term trends, arguing that solar-cycle effects can mask underlying changes. Long, homogeneous historical series like the Latvian one are precisely what is needed to settle such disputes.

The Latvian observations were conducted from four sites: Riga, Sigulda, Baldone, and Lielauce. The most enduring of these was Sigulda, where a dedicated observation pavilion, nicknamed the Cloud Hut, was built in 1956 on land granted to the astronomical society. Its opening faced north, the direction where noctilucent clouds appear against the dark twilight sky, and it housed an impressive array of equipment: aerial photo cameras mounted on a masonry pillar with a deep, stable foundation, a wide-angle theodolite, a geodetic theodolite, binoculars, marine and aviation chronometers, and a radio receiver for capturing precise time signals. Observations there ran from 1957 until 1983, managed throughout by Matīss Dīriķis, a scientific associate of the University of Latvia’s Astronomical Observatory. In Riga, observations began on the roof platform of a six-story residential building before moving to a tower of Riga Palace in 1958 and later to the grounds of the university observatory.

The observation logs themselves are fascinating documents of scientific practice under constraint. Observers recorded the visibility and typology of the clouds at the beginning of every quarter of an hour, with times noted to the minute, alongside brightness ratings, morphological classifications, twilight-segment visibility graded from completely clear to completely obscured, and total and low cloudiness on a zero-to-ten scale. Air temperature and pressure were added at Sigulda from 1964 onward. Timekeeping was a serious business: a marine chronometer made by J. Bruce and Sons around 1900 kept time at Sigulda, later joined by a similar Thomas Mercer instrument, with corrections checked against radio time signals. By 1981, a household quartz clock had entered service, and in 1983 an amateur-built quartz clock constructed by electronic engineer Valdis Gedrovics was used. One curious quirk of the archive is that observers were required to write only in pencil, a rule introduced during the International Geophysical Year that persisted for decades.

The photographic collection is the archive’s crown jewel. It contains 2,106 large-format negatives, most with frames of 180 by 130 millimeters and 84 at 180 by 240 millimeters, all remarkably well preserved. The workhorse camera was the AFA-IM, a semi-military aerial camera developed in 1938 for the Soviet armed forces, fitted with a four-lens anastigmatic Industar-51 lens of 210-millimeter focal length. The large frame size was no accident: it allowed photogrammetric processing, meaning cloud heights could be measured by triangulation from simultaneous exposures taken at different sites. Fifty-three pairs of synchronously captured photographs from Sigulda with Riga or Lielauce survive, and these could yield 53 independent measurements of cloud height for the 1961 to 1964 window, a dataset that could directly test whether the lower boundary of noctilucent clouds has been sinking as some climate models suggest. Exposure times were calibrated against the depth of the Sun below the horizon using an experimentally derived table, eliminating the subjective factor from brightness estimates.

Digitizing the archive was a painstaking exercise in historical rescue. Some logs were lost in the 1990s when the University of Latvia museum changed location and custodians several times, and all photo negatives taken after 1967 have not survived. Kauliņš and volunteer students converted the surviving journals into structured spreadsheets, with one file per year containing four sheets: an explanatory front page, a summary of observation nights and observers, the full night-by-night record of cloud detections and meteorological parameters, and a catalog of photographs. Early entries from 1957 to about 1960 were written in Russian and had to be translated, and the Cyrillic alphabet occasionally tripped up Latvian-speaking observers, producing ambiguities between visually similar letters that correspond to different twilight-segment grades. The digitized logs and appendices are now openly accessible through the University of Latvia’s e-resources repository.

The statistical picture emerging from the logs shows the mid-1960s and 1970s as periods of particularly intense observing activity, with hundreds of nights documented, while 1957 yielded only a single dubious sighting. The records also capture unexpected environmental detail: early Riga observers frequently noted smoke from factory chimneys drifting from the industrial district of the Port of Riga as a factor disturbing their observations, an incidental record of mid-century urban air pollution that, according to the author, has no modern equivalent in the city. Such contextual notes, preserved alongside the quantitative data, give the archive a richness that pure instrumental records often lack.

What happens next could matter for climate science. The Institute of Astronomy at the University of Latvia plans to scan the full negative archive with a professional high-performance scanner, the same class of equipment used to digitize the Schmidt telescope glass-plate library at Baldone Observatory, using a custom positioning device built at the university’s satellite laser ranging station. Beyond height measurements, analyses of cloud movement and morphology could illuminate the dynamics of the upper atmosphere, its relationship with the solar cycle, and its long-term changes. A preliminary statistical evaluation even hints at an apparent inverse relationship between cloud observations and solar activity as measured by the Wolf sunspot number, though the author cautions that recent observations cast doubt on this assumption and that further processing is needed. With so few comparably long noctilucent cloud series in existence, this rescued Latvian archive stands to become a benchmark for understanding how the edge of space has changed over six decades.

Subject of Research: Historical ground-based observations of mesospheric (noctilucent) clouds in Latvia from 1957 to 1983 and their digitization for climate and mesosphere research

Article Title: Observations of mesospheric clouds in Latvia in 1957–1983

Article References: Kauliņš, J. (2026). Observations of mesospheric clouds in Latvia in 1957–1983. History of Geo- and Space Sciences, 17(2), 113-122. https://doi.org/10.5194/hgss-17-113-2026

Image Credits: AI Generated

DOI: 10.5194/hgss-17-113-2026

Keywords: noctilucent clouds, mesosphere, Latvia, International Geophysical Year, climate change, photogrammetry, observation archives, aerial cameras, solar cycle, citizen science, upper atmosphere, data digitization