Calculated transmission spectra for the evening and morning limbs of HD 209458b (left panel) and HD 189733b (right panel), where the vertical structure is obtained by averaging the temperature over each limb and adopting the abundance profiles at each limb from the nominal pseudo two-dimensional chemical model. Spectra have been smoothed to a resolving power R = 300. The transit depth is simply calculated as (R_p(λ)/R_∗)², where R_p(λ) is the calculated radius of the planet as a function of wavelength and R_∗ is the stellar radius (see values in Sect. 2). The absolute scale of the transmission spectrum is set by our choice of assigning the value of the planetary radius given in Sect. 2 to the 1 bar pressure level. Since no attempt has been made to reproduce the absolute scale indicated by primary transit observations, calculated transit depths are somewhat higher than given by observations. The insets in both panels compare the transmission spectrum around 4.3 μm as calculated for the evening limb and as computed using the mean temperature profile of the evening limb and the chemical composition corresponding to the morning limb. The most important differences between both spectra occur around 4.3 and 15 μm, due to CO₂.