Data from: Weather conditions determine attenuation and speed of sound: environmental limitations for monitoring and analysing bat echolocation

Echolocating bats are regularly studied to investigate auditory-guided
behaviours and as important bioindicators. Bioacoustic monitoring methods
based on echolocation calls are increasingly used for risk assessment and
to ultimately inform conservation strategies for bats. As echolocation
calls transmit through the air at the speed of sound, they undergo changes
due to atmospheric and geometric attenuation. Both the speed of sound and
atmospheric attenuation, however, are variable and determined by weather
conditions, particularly temperature and relative humidity. Changing
weather conditions thus cause variation in analysed call parameters,
limiting our ability to detect and correctly analyse bat calls. Here, I
use real-world weather data to exemplify the effect of varying weather
conditions on the acoustic properties of air. I then present atmospheric
attenuation and speed of sound for the global range of weather conditions
and bat call frequencies to show their relative effects. Atmospheric
attenuation is a non-linear function of call frequency, temperature,
relative humidity and atmospheric pressure. While atmospheric attenuation
is strongly positively correlated with call frequency, it is also
significantly influenced by temperature and relative humidity in a complex
non-linear fashion. Variable weather conditions thus result in variable
and unknown effects on the recorded call, affecting estimates of call
frequency and intensity, particularly for high frequencies.
Weather-induced variation in speed of sound reaches up to about ±3%, but
is generally much smaller and only relevant for acoustic localisation
methods of bats. The frequency- and weather-dependent variation in
atmospheric attenuation has a three-fold effect on bioacoustic monitoring
of bats: it limits our capability (1) to monitor bats equally across time,
space, and species, (2) to correctly measure frequency parameters of bat
echolocation calls, particularly for high-frequencies, and (3) to
correctly identify bat species in species-rich assemblies or for sympatric
species with similar call designs.