The lag coefficient of a thermometer of cylindrical shape is shown to be approximately proportional to the one‐half power of its diameter and inversely proportional to the one‐half power of the product of ventilation speed and density of the environmental air. This leads to estimated lag coefficients on the order of twenty‐five seconds for radiosonde thermistors near their useful ceiling which in turn implies that radiation errors of one to several degrees may be expected in radiosonde temperature measurements at high altitudes.