Near‐zero humidities on Ben Nevis, Scotland, revealed by pioneering 19th‐century observers and modern volunteers

The weather on Ben Nevis—the highest mountain in the British Isles at 1345 m above mean sea level—sometimes shows episodes of remarkably low relative humidity (RH) with few precedents anywhere else in the British Isles. We are able to quantify this for the first time using a high‐quality series of hourly dry‐ and wet‐bulb observations, made on the summit. These observations were made between 1883 and 1904, but have only just become available to modern science, thanks to thousands of volunteers who worked to rescue this unique and exemplary data set from published volumes. Careful examination and analysis of the original observations using modern psychrometric theory revealed several occasions where we are confident that the summit RH fell close to zero as a result of anticyclonic subsidence. Three case histories are examined in some detail. The 19th‐century Ben Nevis humidity records are also compared with contemporary automatic weather station data from two high‐altitude Scottish mountain sites.