Cool Stars 14 - Submitted Abstract # 154 This version created on 05 October 2006 Spitzer: A search for Weather in late-L Brown dwarfs Maria Morales-Calderon, LAEFF-INTA John R. Stauffer, SSC-Caltech J.Davy Kirkpatrick, IPAC-Caltech Sean Carey, SSC-Caltech Chris Gelino, SSC-Caltech David Barrado y Navascues, LAEFF-INTA Luisa Rebull, SSC-Caltech Mark Marley, NASA Ames Research Center We have conducted a photometric monitoring program of 3 field late-L brown dwarfs looking for temporal variability due to the existence of clouds in their atmospheres. The observations were performed using Spitzer/IRAC 4.5 and 8 um bandpasses and were designed to cover at least one rotational period of each object. One-sigma RMS (root mean squared) uncertainties of less than 3 mmag at 4.5 um and around 9 mmag at 8 um were achieved. This observations provide the most sensitive search to date for structure in the photospheres of late-L dwarfs at mid-IR wavelengths, and our photometry provides stringent upper limits to the extent to which the photospheres of these transition L dwarfs are structured. Two out of the three objects studied exhibit some modulation in their light curves at 4.5 um -but not 8 um- with periods of 7.4 and 4.6 hr and peak-to-peak amplitudes of 10 mmag 8 mmag. ----------------------------------