Thunderclouds are electrified when charge is transferred between small and large ice 
particles colliding in a cloud that contains strong updrafts. The small ice particles rise 
with one type of charge and the large ice particles fall and carry with them downward 
the other type of charge, which is most often negative, so that normally lightning 
lowers negative charge from cloud to the ground. While the mechanism of ice 
charging is well established, the nature of the charge transfer between the colliding ice 
particles is not very well understood on the atomic level, and no present theory can 
explain fully the charge transfer, or even the sign of the charging. Here we propose a 
new charge separation mechanism that is based on molecular simulations of the 
collisions, keeping track of the individual charges as they move in the form of salt 
ions from one ice particle to another. Under normal conditions, when sulfates 
dominate as cloud condensation nuclei, this ionic mechanism is consistent with the 
prevailing negative lightning in thunderclouds. Moreover, with dearth of sulfate 
anions, the present mechanism predicts a shift towards positive charging. This fits 
well to a large range of observations of enhanced positive lightning, connected with 
smoke rich in chlorides and nitrates, that could not be explained satisfactorily 
previously.