Humidity Observations with the 1972 U.S. Radiosonde Instrument.
As noted in various publications (Morrissey and Brousaides, 1970) considerable difficulty has been experienced with humidity measurements from previous U.S. radiosonde instruments because of ventilation problems and radiation errors in sunlight. Artificial decreases of specific humidity were observed regularly from ascents after sunrise.
Accordingly, a new radiosonde was produced by the VIZ Instrument Company of Philadelphia, Pa., for the National Weather Service (Ostapoff et al., 1970). At the begin· ning of our second observational radiosonde project in Venezuela (VIMHEX II) the characteristics of the humidity sensor of this instrument were tested. On five days radio· sondes were launched about 0400, 0500, 0900, and 1000 local time in order to observe the change of specific humidity at different levels across sunrise in comparison with the hourto-hour variations during nighttime or daytime. There was only little morning cloudiness on those days.
The mixing ratios for all of these 20 soundings from the surface to 700 mb are reproduced in Table 1. It is readily seen that there is no systematic change in specific humidity belween day and night soundings and that the instrument design has overcome the previous defects successfully. No further tests were considered necessary.
It may be added that from the viewpoint of the tropical meteorologist the new sonde also has the advantage that it permits a more detailed analysis of the low troposphere due to an increase of contact points. Further, the response of the instrument to decreases of humidity is very rapid since the ventilation of the hygristor has been improved. Even within 10 mb, sharp boundaries of dry layers can be seen very clearly.
References Morrissey, J. F., and F. J. Brousaides, 1970: Temperature induced errors in the ML-476 humidity data. J. Appl. Meteor., 9, 805.
Ostapoff, Feodor, Willard W. Shinners, and Ernst Augstein, 1970: Some tests on the radiosonde humidity error. NOAA Tech. Rept. ERL 195·AOML 4. 888
Riehl, H., and A. K. Betts, 1972: Humidity Observations with the 1972 U.S. Radiosonde Instrument. Bull. Amer. Meteor. Soc., 53, pp. 887-888