  Strong electric fields in thunderclouds have long been known to accelerate
secondary charged particles. We investigate this effect using three years
(2015-2017) of data from the scalar system of the High Altitude Water Cherenkov
(HAWC) observatory, which is an air shower array deployed 4100 m a.s.l. in
central Mexico. The experimental site is frequently affected by strong
thunderstorms, and the detector's high altitude, large area, and high
sensitivity to cosmic-ray air showers make it ideal for investigating particle
acceleration due to the electric fields present inside the thunder storm
clouds. In particular, the scaler system of HAWC records the output of each one
of the 1200 PMTs as well as the 2, 3, and 4-fold multiplicities (logic AND in a
time window of 30 ns) of each water Cherenkov detectors (WCD) with a sampling
rate of 40 Hz. Using data from this scaler system, we identify approximately
100 increases in the scaler rate which is in time coincidence with
thunderstorms. These events show high cross correlation between the scaler rate
and the electric field, hence can be produced by the acceleration of secondary
particle by the thunderstorm electric fields. In this work we present the
method of identification of these events and their general characteristics.
