The effects of fog on the atmospheric electrical field close to the surface

For almost a decade, ground-based measurements of the electric field (Ez) have been conducted continuously at Tel-Aviv University's Wise astronomical observatory, located in the Negev desert highland in southern Israel. The data enabled identifying the characteristics of Ez in fair weather, during dust storms, lightning activity and the passage of different cloud types overhead. We present new results of observations of the variability of the atmospheric electric field during several foggy days along with meteorological data of wind speed and relative humidity. The results show a substantial increase of the electric field (up to 400-650 V m^-1) compared with the mean fair-weather values at the site (180-190 V m^-1) during times of high values of relative humidity (>95%) and low wind speed (<3 m s^-1). This increase is a consequence of the reduction in the conductivity at low levels due to the attachment of ions to fog droplets. We suggest that closely monitoring the electric field when there is a forecast for the occurrence of fog can offer a precise indication when fog begins and ends.