During the long days of summer, the sun beats down with a vengeance. Day after day of hot, dry weather keeps people indoors, in the safety of their air conditioners. When it seems like nothing could break the monotony of summer in Sacramento, a cool, humid breeze starts to filter in and life becomes bearable outside.
The Delta Breeze is the local name for a wind coming from the southwest, off of The Delta of the Sacramento River and San Joaquin River. This wind carries with it cooler, more humid air from off of the Pacific Ocean. The effects of this wind are very noticeable during the summer, as this seabreeze can cool the air by more than 10oF (that's 5o C!). This can turn an unbearable summer day to a comfortable one, especially for those of us from cooler climates.
The wind is primarily driven by a sea breeze circulation, which can often become coupled with a mountain breeze to form one large (mesoscale) circulation of air from the Farallon Islands up into the Sierra Nevada. A more detailed meteorological discussion can be found at the National Weather Service under the heading "Central Valley and Delta".
Content modified from Davis Wiki.