  We make an analytical estimate of the maximum 21 cm absorption signal from
the cosmic dawn, taking into account the inhomogeneity of gas distribution in
the intergalactic medium (IGM) due to non-linear structure formation. The gas
located near halos is over-dense but adiabatically heated, while the gas far
from halos is under-dense and hence cooler. The cumulative effect of adiabatic
heating and cooling from this gas inhomogeneity results in a reduction in the
maximum global 21 cm absorption depth by about 40% as compared with homogeneous
IGM model, assuming saturated coupling between the spin temperature of neutral
hydrogen (HI) and the adiabatic gas kinetic temperature.
