Context: The magnetic and kinematic properties of the photospheric Evershed flow are relatively well known, but not completely understood. The evolution of the flow with time, which is mainly due to the appearance of velocity packets called Evershed clouds (ECs), may provide information to further constrain its origin. Aims: We undertake a detailed analysis of the evolution of the Evershed flow by studying the properties of ECs. In this first paper we determine the sizes, proper motions, location in the penumbra, and frequency of appearance of ECs, as well as their typical Doppler velocities, linear and circular polarization signals, Stokes V area asymmetries, and continuum intensities. Methods: High-cadence, high-resolution, full vector spectropolarimetric measurements in visible and infrared lines are used to characterize the EC phenomenon through a simple line-parameter analysis. Results: ECs appear in the mid penumbra and propagate outward along filaments having large linear polarization signals and enhanced Evershed flows. The frequency of appearance of ECs varies between 15 and 40 min in different filaments. ECs exhibit the largest Doppler velocities and linear-to-circular polarization ratios of the whole penumbra. In addition, lines formed deeper in the atmosphere show larger Doppler velocities, much in the same way as the “quiescent” Evershed flow. According to our observations, ECs can be classified in two groups: type I ECs, which vanish in the outer penumbra, and type II ECs, which cross the outer penumbral boundary and enter the sunspot moat. Most of the observed ECs belong to type I. On average, type II ECs can be detected as velocity structures outside of the spot for only about 14 min. Their proper motions in the moat are significantly reduced with respect to the ones they had in the penumbra. Appendices A and B are only available in electronic form at http://www.aanda.org