Influence of Radiation on the Temperature Sensor Mounted on the Swiss Radiosonde

Abstract

The Swiss radiosonde (SRS400) measures the air temperature with a very thin copper?constantan thermocouple. The influence of the visible and infrared radiation, as well as the dependency of the air pressure on the measured temperature, is analyzed. After a brief review of the heat transfer by convection, diffusion, and radiation, two independent ways of estimating the difference of temperature between the sensor of the sonde and its environment are presented: 1) laboratory experiments followed by 2) a statistical analysis of aerological soundings. Good agreement between theory, laboratory experiments, and statistical analyses (based on day?night differences) was found. The overall influence of radiation amounts to about 0.8 K at 100 hPa (1.8 K at 10 hPa). At high altitude (low pressure), the heat transfer by diffusion equals the one by convection. Therefore, the diffusion term should not be neglected, as it is often reasonable for the larger sensors or at atmospheric pressure close to ground. As a result of applying the experiments in the laboratory to Eq. (1), the influence of longwave radiation is negligible compared to other influences. Based on the results herein, a second-degree polynomial fit was calculated for correcting the bias caused by radiation on the measured temperatures. This correction is operationally applied with success to the daytime soundings performed with the SRS400.