Hi John: There is a direct relationship between sea and land temperatures. At its simplest, warm air over the land rises, and the cooler air from over the adjacent ocean fills the vacuum created by the rising warm air. This is known as a sea breeze affect.

At night the process often reverses, with the cooler land air descending and flowing out over the now warmer ocean.

So, in S. California and along the Baja coast this is why you get an afternoon onshore (sea breeze) flow – which often reverses some time after midnight through the early morning hours.

To the extent that there is a pressure gradient present exclusive of the temperature differential, the sea breeze or night time land breeze, will reinforce the gradient or reduce it.

There is an entire section in our Mariner’s Weather Handbook which goes into this in great detail.

Steve

PS-differentials in sea temperature, especially in areas like the Gulf Stream or South Australian current also have a big impact on local weather.