Concentrations of ice nuclei detected with refrigerated devices vary with observational techniques and may be influenced by small differences in the thermal compatibility of otherwise similar instruments. To provide improved homogeneity in the observations obtained for this investigation, all mechanically identical equipment was calibrated against a “reference” instrument. The median concentration levels of the standardized measurements at 15 observational sites spanned approximately 2 orders of magnitude at −20°C. Data obtained at the 11,150-ft. elevation of Mauna Loa Observatory, Hawaii, clearly revealed low values compared with all other locations. It may be inferred from this evidence that terrestrial aerosols contribute dominantly to the ice nuclei budget of the lower atmosphere, at least as detected by the expansion method under conditions surveyed in the United States.