Abstract
Using the available NRAO's site characterization data, we study the observing conditions (PWV and Phase Stability) at Llano de Chajnantor during the ``Altiplanic Winter'' and their impact on ALMA scientific operations. The results suggest that during this period ALMA observations will be limited to lower frequency bands and compact array configurations. In terms of only the PWV, observations are possible until Band 6 (~230 GHz) the 70% of the time. Phase stability conditions suggest that atmospheric phase correction is strictly necessary during this period, allowing us to observe in the compact configurations at Band 3 (~100 GHz) about 55% of the time.