Dispersion-direction shift measured in the Na I (589.3 nm) core minus dispersion direction shift measured in the Hα core (656.3 nm) vs. airmass for R150 transit 2. The shift values for each feature were determined by cross-correlating the spectra using a small 100-pixel range around each feature. Overplotted is the expectation from differential atmospheric refraction. The reduced ﹩{\chi }^{2}﹩ for the model fit is 1.