RADIOSONDE TEMPERATURE, HUMIDITY, AND PRESSURE RESPONSE AT LOW TEMPERATURES Stephen R. Hudson, Michael S. Town, Von P. Walden, and Stephen G. Warren 24 June 2003

Introduction Vaisala RS80 and AIR 4A and 5A radiosondes were tested during summer and winter at South Pole. Their response to sudden, large changes in temperature and humidity was characterized. Some data were collected with the RS80s to characterize their response to smaller, more gradual changes in temperature, humidity and pressure.

Large, Sudden Changes Moved sondes from inside to outside and recorded response of reported temperature (T), humidity (RH) and pressure (P). Usually building was heated, with an indoor T of –5° to +25°C. A couple tests were done with an unheated building, with indoor T between –43° and –56°C. Outside T ranged from -24° to –71°C. Range of T differences was 11 to 94 K.

RS80 Temperature Response

Minimum2.6 s Median5.4 s Mean5.9 s Maximum10.9 s Reported temperature responded with a simple exponential decay. Exponential-decay time constants ranged from 2.6 to 10.9 seconds, with a mean of 5.9 seconds. There was no apparent difference in the character or time of the response between summer and winter.

RS80 Humidity Response

When moved outside, reported RH initially decreased, then increased with an exponential decay towards the outside value. The exponential decay began between 5 and 120 seconds after being moved outside. E-folding time constants were between 13 and 420 seconds. Response was significantly slower in winter (T<-40°C) than in summer (T near –25°C). Response in unheated winter cases (  T = 17 K) was similar to winter cases using a heated building. SummerWinterUnheated winter # of Tests5242 Outside T (°C)-24 to to t 1 (sec)5-30 (30)5-120 (33)60-90  (sec) (15) (146)50-120

RS80 Pressure Response

When moved outside, the sonde initially reported increasing P, followed by a slow exponential decay back to the correct P. Reported P increased by 0.4 to 10.0 mb over a period of 175 to 450 seconds. E-folding time constants were between 230 and 600 seconds. Magnitude of maximum error increased with increasing thermal shock. SummerWinterUnheated Winter # of Tests1242 Outside T (°C) to t 1 (sec) to 450 (300)275 to 400  P (mb) to 10.0 (6.0)0.4 to 1.6  (sec) N/A230 to 600 (380)N/A, ~1500 to recover

Small, Gradual Changes After equilibrating to conditions outside, sonde was raised and lowered on a 22 m tower at speeds of 0.4 to 1.0 ms -1. At the top of the tower, the temperature was 3 to 5 K warmer, the relative humidity was 3 to 5% higher, and the pressure was about 2 mb lower than at the surface.

Tower Tests Pressure responded to within the noise level by the time descent was completed. Change (about 2.2 mb) is approximately correct, according to hypsometric eqn. Temperature responded fully within 8 to 15 seconds of completion of descent. Relative Humidity took 15 to 20 seconds after descent to fully equilibrate. The sonde was able to correctly report the supersaturation with respect to ice.

Conclusions Radiosondes should be stored and prepared at ambient temperatures If sondes must be prepared inside, they should be given at least 30 minutes to equilibrate to cold environments before launching Problems can arise even from small temperature differences, so unheated buildings should be avoided unless well ventilated

Conclusions (continued) Tests on the tower indicate that the sondes are capable of providing better data in cold conditions when given time to equilibrate to shock of being moved outside Further work should be done in more controlled environments, and with proper ventilation Radiosonde instrumentation still remains sluggish at low temperatures