This paper explores the low-humidity problem that has plagued the radiosonde hygristor for nearly 30 years and that makes the hygristor appear to become insensitive at relative humidities (RH) below about 20% RH. The problem led the National Weather Service (NWS) in 1973 to begin a practice of terminating the radiosonde humidity measurement at 20% RH and to begin reporting any humidities evaluated at less than 20% using a 30°C dewpoint depression in the coded radiosonde message. This practice remains in effect today and has resulted in a permanent, 20-year gap in the radiosonde humidity archive for the United States. This study examines a number of factors that can potentially affect the accuracy of NWS radiosonde low-humidity data, including 1) characteristics of the hygristor response curve, 2) characteristics of the NWS analog radiosonde and methods used to transmit and record the radiosonde signal, and 3) reduction techniques used to convert the radiosonde signal into RH. It is shown that the... Abstract This paper explores the low-humidity problem that has plagued the radiosonde hygristor for nearly 30 years and that makes the hygristor appear to become insensitive at relative humidities (RH) below about 20% RH. The problem led the National Weather Service (NWS) in 1973 to begin a practice of terminating the radiosonde humidity measurement at 20% RH and to begin reporting any humidities evaluated at less than 20% using a 30°C dewpoint depression in the coded radiosonde message. This practice remains in effect today and has resulted in a permanent, 20-year gap in the radiosonde humidity archive for the United States. This study examines a number of factors that can potentially affect the accuracy of NWS radiosonde low-humidity data, including 1) characteristics of the hygristor response curve, 2) characteristics of the NWS analog radiosonde and methods used to transmit and record the radiosonde signal, and 3) reduction techniques used to convert the radiosonde signal into RH. It is shown that the...