Significance: The annual appearance of massive mayfly swarms is a source of public fascination and spectacular natural phenomenon that plays a key role in regional food webs. Alarming reports of insect declines motivate efforts to uncover long-term and large-scale invertebrate population trends. Monitoring aquatic insect abundance across ecosystems continues to be logistically infeasible, leaving the vulnerability of these communities to intensifying anthropogenic impacts unknown. We apply radar remote sensing to quantify aquatic insect abundance at scales that have been previously impossible, revealing persistent declines in biomass flux from aquatic to terrestrial habitats. As ecological indicators, these losses may signal deterioration in water quality and, if current population trends continue, could cascade to widespread disappearance from some of North America’s largest waterways.

Nocturnal avian migration flyways remain an elusive concept, as we have largely lacked methods to map their full extent. We used the network of European weather radars to investigate nocturnal bird movements at the scale of the European flyway. We mapped the main migration directions and showed the intensity of movement across part of Europe by extracting biological information from 70 weather radar stations from northern Scandinavia to Portugal, during the autumn migration season of 2016. On average, over the 20 nights and all sites, 389 birds passed per 1 km transect per hour. The night with highest migration intensity showed an average of 1621 birds km ^–1 h ^–1 passing the radar stations, but there was considerable geographical and temporal variation in migration intensity. The highest intensity of migration was seen in central France. The overall migration directions showed strong southwest components. Migration dynamics were strongly related to synoptic wind conditions. A wind‐related mass migration event occurred immediately after a change in wind conditions, but quickly diminished even when supporting winds continued to prevail. This first continental‐scale study using the European network of weather radars demonstrates the wealth of information available and its potential for investigating large‐scale bird movements, with consequences for ecosystem function, nutrient transfer, human and livestock health, and civil and military aviation.

Environmental supersaturation is a key parameter in the formation of fog and clouds, yet it cannot be measured directly and must be inferred. Calculating the ambient supersaturation in fog requires knowledge of the aerosol hygroscopicity as well as particle size distribution, and relatively few values have been reported in the literature. Here we use κ‐Köhler theory to derive aerosol activation properties based on particle hygroscopicity and dry particle size distributions, and then estimate the effective peak supersaturation during eight cases of radiative fog at the Southern Great Plains site in rural north‐central Oklahoma, USA. The mean hygroscopicity parameter κ of particles likely to act as cloud condensation nuclei varied from 0.14 to 0.43. Ambient effective peak supersaturation during the fog episodes was between 0.01%–0.07%, with most values below 0.04%. The minimum 50% dry activation diameter generally ranged between 300–400 nm with little activation of particles with diameters below 300 nm.