MEASUREMENTS OF HUMIDITY AND TEMPERATURE IN THE MARINE-ENVIRONMENT DURING THE HEXOS MAIN EXPERIMENT

Accurate measurement of fluctuations in temperature and humidity are n eeded for determination of the surface evaporation rate and the air-se a sensible heat flux using either the eddy correlation or inertial dis sipation method for flux calculations. These measurements are difficul t to make over the ocean, and are subject to large errors when sensors are exposed to marine air containing spray droplets. All currently av ailable commercial measurement devices for atmospheric humidity requir e frequent maintenance. Included in the objectives of the Humidity Exc hange over the Sea program were testing and comparison of sensors used for measuring both the fluctuating and mean humidity in the marine at mosphere at high wind speeds and development of techniques for the pro tection of these sensors against contamination by oceanic aerosols. Th ese sensors and droplet removal techniques are described and compariso ns between measurements from several different systems are discussed i n this paper. To accomplish these goals, participating groups devised and tested three methods of removing sea spray from the sample airstre am. The best performance was given by a rotating screen device, the '' spray flinger.'' Several high-frequency temperature and humidity instr uments, based on different physical principles, were used in the colla borative field experiment. Temperature and humidity fluctuations were measured with sufficient accuracy inside the spray removal devices usi ng Lyman-alpha hygrometers and a fast thermocouple psychrometer. Compa rison of several types of psychrometers (using electric thermometers) and a Rotronic MP-100 humidity sensor for measuring the mean humidity illustrated the hysteresis of the Rotronic MP-100 device after periods of high relative humidity. Confidence in the readings of the electron ic psychrometer was established by in situ calibration with repeated a nd careful readings of ordinary hand-held Assman psychrometers (based on mercury thermometers). Electronic psychrometers employing platinum resistance thermometers perform very well.