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Intermediate-velocity clouds (IVCs) are HI halo clouds that are likelyrelated to a Galactic fountain process. In-falling IVCs are candidates for there-accretion of matter onto the Milky Way. We study the evolution of IVCs atthe disk-halo interface, focussing on the transition from atomic to molecularIVCs. We compare an atomic IVC to a molecular IVC and characterise theirstructural differences in order to investigate how molecular IVCs form highabove the Galactic plane. With high-resolution HI observations of theWesterbork Synthesis Radio Telescope and 12CO(1-0) and 13CO(1-0) observationswith the IRAM 30m telescope, we analyse the small-scale structures within thetwo clouds. By correlating HI and far-infrared (FIR) dust continuum emissionfrom the Planck satellite, the distribution of molecular hydrogen (H2) isestimated. We conduct a detailed comparison of the HI, FIR, and CO data andstudy variations of the $X_\rm{CO}$ conversion factor. The atomic IVC does notdisclose detectable CO emission. The atomic small-scale structure, as revealedby the high-resolution HI data, shows low peak HI column densities and low HIfluxes as compared to the molecular IVC. The molecular IVC exhibits a richmolecular structure and most of the CO emission is observed at the eastern edgeof the cloud. There is observational evidence that the molecular IVC is in atransient and, thus, non-equilibrium phase. The average $X_\rm{CO}$ factor isclose to the canonical value of the Milky Way disk. We propose that the twoIVCs represent different states in a gradual transition from atomic tomolecular clouds. The molecular IVC appears to be more condensed allowing theformation of H2 and CO in shielded regions all over the cloud. Ram pressure mayaccumulate gas and thus facilitate the formation of H2. We show evidence thatthe atomic IVC will evolve also into a molecular IVC in a few Myr.

High-resolution HI and CO observations of high-latitude intermediate-velocity clouds