  The paper presents a model of electric charge generation in precipitating
clouds due to breaking of large particles in low melting layer. The Earth's
electric field polarises the droplets and they break into small negatively
charged and large positively charged fragments. A negatively charged cloud
region is formed below melting layer. The charged region induces a field, which
increases both polarisation of the breaking droplets and rate of charge
generation. An equation is derived to describe spatial and temporal variability
of the electric field, which depends on size spectrum of hydrometeors and
vertical draft velocity. The model yields an exponential growth of the electric
field strength to 105 - 106 V/m after 15 min of raining at a rate of more than
10 mm/h. The major condition for that is a downdraft with a velocity of 0.5 - 1
m/s within the melting layer. The maximum field strength in clouds is found at
+20C level where lightning strokes to aircraft are observed most frequently. In
conclusion, factors affecting the charge generation rate are analysed and
methods for experimental verification of the proposed model discussed.
