onshore flow

[mjb]

what is it and what causes it?

[Nathan Cool]

Hi MJB,

I'll see if I can answer your questions:

Onshore flow is the flow or air from the ocean to the land. An
offshore flow is the opposite (air movement from land to sea). An
onshore flow is typical most afternoons in SoCal, where the onshore
breezes almost inevitably pick up by noon. In this case -- a typical
diurnal pattern -- increasing temperatures over land throughout the
day cause air to rise, and cooler air from the ocean comes in to fill
the void (thus creating the afternoon sea breezes).

An onshore flow though typically refers to a heavier influence of an
onshore effect. This is often the case when a storm approaches the
area, and winds begin to increase onshore. It is also the case when
there is less high pressure around SoCal (and the America West), thus
reducing the offshore flow. When troughing descends from the Gulf, low
pressure tends to dominate the area, and high pressure exits to the
east. This dominance of low pressure with the lack of high pressure,
creates more of an onshore flow, especially since winds from the
approaching low (if located off the coast, as would be the case from a
trough/low coming down from the Gulf) are swirled in from the
counterclockwise spin of the low.

Note though that once a low pressure system passes to the east, the
backside of the counterclockwise spin tends to bring a northerly
element to the winds -- often referred to as an inside-slider.

An onshore flow also tends to bring in more moisture from the ocean,
which can be in the form of low clouds and fog, drizzle, and even
stormy weather. An offshore flow though pushes air from land to sea,
thus pushing moisture back over the ocean, resulting in clearer skies
and warmer temps.

--
Nathan Cool
http://www.NathanCool.com

"Strive not to be a success, but rather
to be of value." -- Albert Einstein