To develop improved techniques for predicting advection fog, fog droplet spectra data were collected during several advection fog episodes at Otis AFB. The data were collected with two PMS forward scatter probes (FSSP-100) and two optical array probes (OAP-200X). These two probes cover the ranges from 0.5- to 47-micrometers and 20- to 300-micrometer diameter, respectively. The probes were mounted at 5 m and 30 m above the ground. Drop size data were obtained in nine fogs over a total period of 70 h. Other data that were collected continuously during the fog episodes include extinction coefficient, temperature, dew point, wind direction, and speed at the 5-, 15-, 30-, 45-, and 60-m levels. The droplet data show three basic types of spectra, all of which have a high droplet count below 2.5-micrometer diameter. Some spectra show as many as three modes while others show no modes within the size range of the instruments. The shape of the droplet spectra and the number concentration vary considerably from one fog to another and during a fog episode. The total droplet concentration appears to be a function, primarily, of the past trajectory of the air mass; lower counts being observed in air masses with long over water trajectories. The drop-size data were used to parameterize the extinction coefficient and the mean droplet terminal velocity in terms of liquid water content and droplet concentration for use in numerical prediction models. The new parameterizations will result in the prediction of lower liquid-water contents and higher visibilities than with previous parameterization used in fog prediction models.

This topical volume of the Journal of Pure and Applied Geophysics utilizes new information not previously accessible for fog related research. It focuses on surface and remote sensing observations of fog, various numerical model applications using new parameterizations, fog climatology, and new statistical methods. The results presented in this special issue come from research efforts in North America and Europe.

In November 1981 a first symposium with the topics of "Acid Deposition of Atmospheric Pollutants" was organised in OberursellTaunus to introduce the problems and first results of research-activities on wet and dry deposition of pollutants and on acid precipitation. In the meantime the hazard to forest and vegetation became more dramatic and research-projects to investigate the input of pollutants to forest-ecosystems have been initiated by several interdisciplinary groups. The rapidly increasing interest in the problems of forest decay and the many open questions with respect to the diagnosis of the forest-damage were the background for the organisation of a second symposium which was held in November 1985 at the same location in Oberursel/Taunus. It was mainly concerned with new techniques of sampling and analyzing pollutants in forest areas. Besides deposition, one important pathway of pollutants in orographic terrain is the interception of fog-droplets by vegetation. Special emphasis was laid on the chemical composition of fog. The symposium successfully assembled scientists from the field of atmospheric research with those studying the effect of pollutants on trees and vegetation in order to reduce the many open questions in connection with forest desease. The proceedings presented in this volume are a substantial contribution to the understanding of deposition and interception of pollutants in forest-areas. Thanks to the authors the volume contains a lot of new research ·results and presents therefore a true picture of our present knowledge.