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

The team installed pollen traps at two marked hives per apiary, pooling material into composite two-week samples across seven sampling periods each season, from early May to mid-August in 2025 and mid-May to late August in 2024. Samples were frozen at minus 18 degrees Celsius until analysis, and accredited laboratories in Berlin used validated multi-residue methods based on liquid and gas chromatography coupled with tandem mass spectrometry, with dedicated LC-MS/MS screening for the highly polar herbicide glyphosate and its breakdown product AMPA. Botanical composition was reconstructed under light microscopy, with more than 500 pollen grains identified per sample, allowing the researchers to link each residue detection to the plants the bees had actually visited.

In total, 15 active substances were detected in 2024 and 19 in 2025. Fungicides dominated the detection profile in both years, accounting for 51 percent of detections in 2024 and 54 percent in 2025, followed by herbicides and, trailing behind, insecticides. The most frequently recorded compounds included boscalid and azoxystrobin among the fungicides, glyphosate and clopyralid among the herbicides, and the neonicotinoid-class insecticide acetamiprid. Maximum concentrations were striking: glyphosate reached 840 micrograms per kilogram in 2024 and 327 in 2025, boscalid peaked at 610 micrograms per kilogram in 2024, and the herbicide fluazifop-P-butyl hit 660 and 1800 micrograms per kilogram across the two years, far exceeding the European Union recommended maximum residue level of 50 micrograms per kilogram for bee products.

The fluazifop-P-butyl findings stand out as among the most consequential for beekeepers and regulators. This grass-specific herbicide is not typically considered a bee-toxicity headline grabber, yet its concentrations in Latvian pollen exceeded most values reported elsewhere in Europe, though they remained below the extreme 6831 micrograms per kilogram documented in southern Germany by earlier researchers. Acetamiprid tells a different regulatory story. The European Union’s acceptable concentration in bee products was 50 micrograms per kilogram in 2024 but rose to 300 micrograms per kilogram in 2025, and individual Latvian samples exceeded the limits applicable at the time of sampling in both years, with maxima of 350 and 200 micrograms per kilogram respectively.

Perhaps the most unexpected detection was terbutryn, a herbicide banned for agricultural use in the EU, found once in 2025 exclusively at the urban-influenced Jelgava apiary. Because it appeared in no earlier sampling year, the researchers rule out current farming as the source. Terbutryn is known to leach from biocide-treated facade coatings and paints, and it is used for aquatic algae control; a wastewater treatment plant within the foraging radius of the Jelgava hives may represent a plausible contamination pathway. The finding is a reminder that pollinator pesticide exposure is not confined to fields, and that urban infrastructure can quietly contribute to the chemical loads carried back to the hive.

Seasonal patterns proved decisive. Residue concentrations and risk values rose mainly between mid-May and early July, coinciding with the flowering of oilseed rape and legumes and the associated pesticide applications. In 2024, the highest total residue load occurred in the second sampling period, when pollen from the crucifer family dominated the samples and bees were foraging intensively on winter oilseed rape. Elevated levels followed in early June, tied to legume crops such as field beans and peas, and again in late July when flowering weeds of the goosefoot family provided a late-summer pollen source. In 2025, detections were spread more evenly across the season, with peaks in late May and early June linked to crucifers and rose-family bloom such as apple, plum and pear, and renewed elevations in late July associated with summer oilseed rape.

To translate chemical concentrations into biological risk, the team applied the Pollen Hazard Quotient, a screening metric that divides the detected concentration in pollen by the acute oral or contact LD50 for honey bees, assuming a maximum daily intake of up to 9.5 milligrams of bee bread per nurse bee. Values below 50 are considered low risk, between 50 and 500 moderate, and above 500 high. Median PHQ values stayed safely under 50 in every sampling period of both years, indicating generally low colony-level toxicological pressure. But individual samples told a sharper story: several 2025 samples crossed the threshold, and one sample from Vecauce parish in the fifth period of 2025 reached a staggering PHQ of 1173.91, driven almost entirely by cypermethrin, a pyrethroid insecticide whose very low contact LD50 means even a moderate concentration translates into a high hazard score.

That cypermethrin result crystallises a central lesson of the study: toxicity, not abundance, determines risk. Frequently detected fungicides and herbicides contributed little to cumulative PHQ because their LD50 values are comparatively high, while compounds such as acetamiprid, flupyradifurone and cypermethrin punched far above their concentration weight. The researchers caution that PHQ is a screening tool with real limitations. It rests on acute endpoints only, ignoring chronic exposure, sublethal effects and the potential interactions within the pesticide mixtures that bees inevitably encounter simultaneously. Real-world exposure risk may therefore be underestimated, particularly when colonies face repeated pulses throughout a brood-rearing season.

What the study did not find may matter just as much. The team hypothesised that apiaries surrounded by more agricultural land, more nectar-producing crops, or crops under higher pesticide-use intensity would show higher cumulative PHQ values. Graphical comparisons across the six sites and two years found no consistent pattern. Platone parish, with the highest proportion of agricultural land, did not post the highest risk values, while some sites with little farmland still recorded measurable exposure. High proportions of crucifer, legume or rose pollen did not reliably track the mapped crop areas around each apiary, suggesting that bees forage selectively, that systemic pesticides move residues away from treated fields, and that broad landscape categories are simply too coarse to capture field-level application timing. Exposure, the authors conclude, is shaped by temporal dynamics, local management and bee behaviour rather than landscape composition alone.

The study builds on earlier Latvian work published in 2024 and offers the Baltic region some of its first long-term pollen monitoring data, filling a gap where oilseed rape covers 145,100 hectares and legume areas have grown 84 percent since 2020. For beekeepers, the practical message is that danger is concentrated in identifiable windows, chiefly mid-May through early July, when crop flowering and spraying overlap with intensive foraging and brood rearing. For regulators and researchers, the findings argue for monitoring designs that capture short-term exposure peaks and application timing rather than averaging the season into invisibility. Honey bee-collected pollen, the authors note, proved a suitable and temporally resolved tool for exactly this purpose, identifying precisely when and where agricultural environments turn most hazardous for their most industrious visitors.

Subject of Research: Pesticide residues and seasonal exposure risk in honey bee-collected pollen across Latvian agricultural landscapes

Article Title: Pesticide exposure risks and seasonal variation in honey bee-collected pollen across agricultural landscapes in Latvia

Article References: Pesticide exposure risks and seasonal variation in honey bee-collected pollen across agricultural landscapes in Latvia. (n.d.). https://doi.org/10.1007/s11356-026-38238-1

Image Credits: AI Generated

DOI: 10.1007/s11356-026-38238-1

Keywords: honey bees, pesticide residues, pollen, pollinator health, glyphosate, cypermethrin, Pollen Hazard Quotient, agricultural landscapes, fungicides, Latvia, ecotoxicology, bioindicators