Investigating vertical distribution of charge in fog through tethered balloon measurements and modelling

Fog, a reduction in visibility caused by water droplets suspended in the atmosphere, is a weather phenomenon which is linked to atmospheric electrical changes. Measurements of the potential gradient (PG) in particular have been shown to be useful for predicting fog, which has important applications for the aviation industry. The underlying theory behind these changes in PG during and before fog events is still an area of active research. Previously, in many studies of fog and atmospheric electricity, it has been assumed that fog droplets are neutral, for simplicity. However, it is well known that many clouds contain significant layers of space charge, and it is likely that fog droplets may also be charged. In this work, the distribution of charge in fog is studied using both numerical modelling and real-world measurements.

Numerical investigations use an earth-electrode model, in which it is assumed that the earth is a negatively charged surface and that there is a vertical electric field in the atmosphere above the surface. Using a system of 1D electrostatic equations, the steady-state distribution of vertical charge can be found, both in clear air and in a foggy air with prescribed aerosol. The results of these simulations provide the expected electrical charge in an idealised setup, which show appreciable space charge near the surface of the earth, as well as a rapidly decreasing PG with height.

Real-world measurements of the vertical charge distribution in fog up to 55m are made using a miniature electrode sensor and battery powered datalogger which is attached to a tethered balloon. The electrode current is amplified, and changes are apparent if the balloon passes through a sharp vertical gradient in space charge. As a result, vertical profiles of the magnitude and polarity of space charge in the fog layer can be measured and then compared with the modelled ideal case. In this presentation, we will show the measurements made during several fog cases with this setup.

A better understanding through modelling and measurements of the space charge in fog will help to identify cases where PG is especially well suited to fog prediction.