Automated meteorological observations at coastal and island stations, instrumented aircraft flights, and coastal soundings were made between July 1983 and June 1985 to define the variations of the lower atmosphere over the northern half of the Gulf of California during the two important climatic seasons, the mid-latitude winter and the subtropical summer. A marine layer is well defined over water during both seasons but dissipates within a few kilometers inland. The winter large-scale pressure field is dominated by the Great Basin high over the southwestern United States. Modulated by upper level synoptic activity, it causes 3 to 6 days events of northwesterly winds (8–12 m s−1) directed along the gulf's axis, which are coherent over basin scales, and bring cool, dry desert air over the gulf. The vertical scale of these winds is 700–1000 m over land on the western side of the gulf, close to the height of the Baja California mountains, but only 100–400 m over water, defined by the wintertime inversion. Cross-gulf winds, related to topographic effects during upper level trough passages over Baja California, are particularly intense in the northwestern portion of the gulf. The winter marine layer is defined by a 1° to 4.5°C temperature inversion and a dew point temperature of 6°–11°C (a moisture content of 6–8 g kg−1); weak subsidence reduces dew point values to −4°C above the inversion. Winter winds above the marine layer are coherent (0.8) across the width of the gulf. The summer large-scale pressure field is dominated by a thermal low over the southwestern United States, and drives weak (2–5 m s−1) southeasterly winds, also directed primarily along the gulf, which are less spatially uniform than winter winds. Air temperature and moisture content are rather constant, and topographic effects are absent. The summer marine layer is on the order of 200–300 m thick, with dew point temperatures of 26°–28°C (21–24 g kg−1), and capped by a weak temperature inversion (1°–2°C) over water. Aloft, the dew point temperature is 17°–21°C, and winds are weak and uncorrelated across the width of the Gulf. During June and early July 1984, four week-long pulses of southeasterly winds (10 m s−1) support the existence, reported previously, of moisture surges from the gulf into the Sonoran desert. A dynamical analysis of subdiurnal motions shows that the cross-gulf momentum balance is geostrophic, the along-gulf balance is ageostrophic, with the along-gulf pressure gradient opposed by friction at the surface. These results are consistent with a scale analysis of the equations of motion in the marine layer over the Gulf of California.