Air-earth current density measurements at Lerwick; implications for seasonality in the global electric circuit

Abstract

Atmospheric electricity measurements were made at Lerwick Observatory in the Shetland Isles (60°09′N, 1°08′W) during most of the 20th century. The Potential Gradient (PG) was measured from 1926 to 84 and the air-earth conduction current ( J c) was measured during the final decade of the PG measurements. Daily J c values (1978–1984) observed at 15 UT are presented here for the first time, with independently-obtained PG measurements used to select valid data. The 15 UT J c (1978–1984) spans 0.5–9.5 pA/m 2, with median 2.5 pA/m 2; the columnar resistance at Lerwick is estimated as 70 PΩm 2. Smoke measurements confirm the low pollution properties of the site. Analysis of the monthly variation of Lerwick J c data shows that winter (DJF) J c is significantly greater than the summer (JJA) J c by 20%. The Lerwick atmospheric electricity seasonality differs from the global lightning seasonality, but J c has a similar seasonal phasing to that observed in Nimbostratus clouds globally, suggesting a role for non-thunderstorm rain clouds in the seasonality of the global circuit.