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The Gulf of California is also a gulf in the lower atmosphere, delimited by the mountains of Baja California and by the Sierra Madre, open to the eastern tropical Pacific to the south and spreading into the Great American Desert of the southwestern United States. The Gulf of California lies asymmetrically under its southwestern portion, and a well-defined Marine Boundary Layer (MBL), the lower atmosphere over the gulf proper, develops over water but dissipates rapidly over land. The flow within the MBL is forced principally by the along-gulf pressure gradient from the Great Basin High over the southwestern United States and by blocking of the isobaric flow by the Baja California mountains; the flow for most of the year is a northwesterly low-level jet with speeds of 8-12 m sec(-1), balanced at the surface by friction in the along-gulf direction, but geostrophically across the gulf, and capped by inversions that slope down from west to 0 east. Moisture in the MBL is kept below 6-8 g kg(-1) by the cold winds drawn from the desert. Modulation of the high pressure over the desert by upper level synoptic activity causes the typical 3 to 6-day wind events. In late spring or early summer, a monsoon sets up as a thermal low develops over the southwestern United States and reverses the along-gulf pressure gradient. Most of the southerly flow now takes place over the lowlands off the Sierra Madre, so the winds in the MBL over the gulf appear weaker and more variable, but remain as a low-level jet in cross-gulf geostrophic balance, under a weaker inversion that slopes down from east to west. The Gulf of California warms considerably; moisture increases dramatically within the MBL to about 21-24 g kg(-1) and spills over the lowlands to the east, feeding the sometimes intense rainfall against the Sierra Madre. Moisture is also driven onto the southwestern North American desert as a source for the summer rains. This sometimes occurs as wind pulses that result possibly front hydraulic self-adjustment of the MBL to changes in the forcing of the flow, and are often a signature of the onset of the monsoon. The monsoon acts like a short tropical summer, which contrasts with the much longer subtropical 'winter', and gives the region's atmosphere its asymmetric, pseudoseasonal character.