We examine the influence of non-adiabatic effects on the modes of an isothermal stratified magnetic atmosphere. We present new solutions for magneto-acoustic-gravity (or MAG) waves in the presence of a radiative heat exchange based on Newton's law of cooling. An analytic expression for the dispersion relation is derived, which allows the effect of a weak magnetic field on the modes to be studied. The insight so gained proves useful in extending the computations to the moderate-high field case. In the second part we present observations of two sunspots obtained in the EUV wavelength range with the Coronal Diagnostic Spectrometer (CDS) on SoHO. We examine the time series for the line intensities and relative velocities and calculate their power spectra using wavelet transforms. We find oscillations in the chromosphere and transition region above the sunspots in the temperature range log T = 4.6-5.4 K. Most of the spectral power above the umbra is contained in the 5-7 mHz frequency range. When the CDS slit crosses the sunspot umbra a clear 3 min oscillation is observed. The observed oscillation frequencies are compared with the computed frequencies and the observations are interpreted in terms of the slow magneto-acoustic waves.