Evaluating stratiform cloud base charge remotely

Stratiform clouds acquire charge at their upper and lower horizontal boundaries due to vertical current flow in the global electric circuit. Cloud charge is expected to influence microphysical processes, but understanding is restricted by the infrequent in situ measurements available. For stratiform cloud bases below 1 km in altitude, the cloud base charge modifies the surface electric field beneath, allowing a new method of remote determination. Combining continuous cloud height data during 2015–2016 from a laser ceilometer with electric field mill data, cloud base charge is derived using a horizontal charged disk model. The median daily cloud base charge density found was −0.86 nC m −2 from 43 days' data. This is consistent with a uniformly charged region ~40 m thick at the cloud base, now confirming that negative cloud base charge is a common feature of terrestrial layer clouds. This technique can also be applied to planetary atmospheres and volcanic plumes. The idea that clouds in the atmosphere can charge electrically has been appreciated since the time of Benjamin Franklin, but it is less widely recognized that it is not just thunderclouds which contain electric charge. For example, water droplets in simple layer clouds, that are abundant and often responsible for an overcast day, carry electric charges. The droplet charging arises at the upper and lower edges of the layer cloud. This occurs because the small droplets at the edges draw charge from the air outside the cloud. Understanding how strongly layer clouds charge is important in evaluating electrical effects on the development of such clouds, for example, how thick the cloud becomes and whether it generates rain. Previously, cloud charge measurement has required direct measurements within the cloud using weather balloons or aircraft. This work has monitored the lower cloud charge continuously using instruments placed at the surface beneath. From measurements made over 2 years, the cloud base charge is negative in the majority of cases. This confirms that charging of layer clouds is not a random process but instead arises from fundamental aspects of the atmosphere's structure. Cloud base charge is sensed remotely using its effect on the surface electric field About 75% of layer clouds observed have appreciably negatively charged cloud bases