Pressure waves in the Martian atmosphere could so far be observed only in the presence of clouds. Thanks to the Mars Global Surveyor/Mars Orbiter Laser Altimeter (MGS/MOLA), a direct observation is now possible, with the method presented here. Many more events are thus potentially observable. We compare altimetry measurements from MGS/MOLA and spectral observations from ISM, the infrared spectrometer aboard the Phobos-2 spacecraft, to infer pressure variations in the Martian atmosphere. We identify variations of pressure above Martian volcanoes and we observe lee vortices of wavelength ~200--250 km and amplitude ~0.05 mbar in two different occasions. The application of this technique should constrain Martian meteorology with data from future imaging spectrometers.