Satellite view Sunday (NASA)

While the eastern Pacific ocean off the west  coast of Baja Peninsula in Mexico  was absolutely draped in cloud cover Sunday and Monday, islands in the midst of these sheets of gray were clear skies oases.  How can this be?

The clouds form over the ocean because the chilly Pacific water creates a layer of cool air at low altitudes. When the air is heated just above the ocean surface, it cools and condenses into clouds and fog forming the so-called marine layer (which is held in place by a temperature inversion, in which the air – divorced from the cool sea surface – warms with altitude).

Satellite view Monday (NASA)

But over the islands, the layer of cool air required for these clouds to form is often absent since land masses and the air above them heat up quickly (thanks to the low heat capacity of land versus water).  And so, unless there is wind  to push the marine layer over the islands, they’re bastions of sunshine.

Wide satellite view Sunday (NASA)

At times, however, westerly flow and/or a sea breeze circulation set up so that the marine layer is pushed into the islands (see case below from Saturday).  This same phenomenon occurs in coastal California – especially in late spring and early summer leading to spells of dreary fog and gray skies, often referred to as “June gloom” .

Satellite view Saturday (NASA)

Further reading: What are Marine Layer Clouds and How Do they Form? | The Marine Layer