 In situ observations of conditions away from earth’s surface are much less plentiful than surface observations. Radiosondes, which are balloon-borne instrument packages that ascend through the atmosphere measuring pressure, temperature, humidity and other variables, are now launched twice daily at hundreds of sites around the world, but they do not provide even, global coverage (Ingleby et al. 2016). Satellite-borne instruments can provide global coverage—and are of significant value in the study of weather and climate—but they do not directly measure vertical profiles of key climatological variables like temperature; approximate values of these variables must be inferred in a complex way from radiance measurements. A recently-established system of several thousand ocean floats, known as Argo, takes local profiles of temperature and salinity in the top 2000 meters of the ocean roughly every 10 days (Riser et al. 2016). None of these data sources existed a century ago, though observations of conditions away from earth’s surface were sometimes made (e.g., using thermometers attached to kites).