From AMS Glossary
The transfer of electric charge from the positively charged atmosphere to the negatively charged earth.
This current is made up of the air–earth conduction current, a point-discharge current, a precipitation current, a convection current, and miscellaneous smaller contributions. Of these, the air–earth conduction current is by far the largest. This is not just true locally, but throughout the world where there are no thunderstorms occurring, which is estimated to be 80%–90% percent of the earth. The existence of this quasi-steady current in fair weather and the observed maintenance of the earth's net negative charge are both better established than the nature of the supply current, which must replenish the positive charge in the upper atmosphere and the negative charge on the earth.
Gish, O. H. 1951. Compendium of Meteorology. p. 113.