  We study the H$\alpha$ and Ca II 8542 \r{A} line spectra of four typical
Ellerman bombs (EBs) in active region NOAA 11765 on 2013 June 6, observed with
the Fast Imaging Solar Spectrograph installed at the 1.6 meter New Solar
Telescope at Big Bear Solar Observatory. Considering that EBs may occur in a
restricted region in the lower atmosphere, and that their spectral lines show
particular features, we propose a two-cloud model to fit the observed line
profiles. The lower cloud can account for the wing emission, and the upper
cloud is mainly responsible for the absorption at line center. After choosing
carefully the free parameters, we get satisfactory fitting results. As
expected, the lower cloud shows an increase of the source function,
corresponding to a temperature increase of 400--1000 K in EBs relative to the
quiet Sun. This is consistent with previous results deduced from semi-empirical
models and confirms that a local heating occurs in the lower atmosphere during
the appearance of EBs. We also find that the optical depths can increase to
some extent in both the lower and upper clouds, which may result from either a
direct heating in the lower cloud, or illumination by an enhanced radiation on
the upper cloud. The velocities derived from this method, however, are
different from those obtained using the traditional bisector method, implying
that one should be cautious when interpreting this parameter. The two-cloud
model can thus be used as an efficient method to deduce the basic physical
parameters of EBs.
