Meteorology
The great thing
about studying
meteorology
(weather) is
that it’s going
on around you
all of the time.
Look out of the
window, there it
is.
Meteorology is a
big subject and
much of it is
abstract and
complex. Don’t
worry, you don’t
have to be a
physicist to
understand
weather. Just
get a grasp of
the basics
(below),
download weather
information from
the web, and tie
it together by
looking out of
the window.
The basics
Fundamentally,
weather is
caused by the
sun. Heat energy
from the sun
warms the
surface it
shines on. That
surface, whether
land or sea,
warms the air
above. The sun
doesn’t warm the
air directly.
The warmed air
rises because it
has become less
dense than the
surrounding air.
Think of a hot
air balloon, it
gets carried
upwards because
the hot air
within wants to
rise. As the air
is trapped it
takes the
balloon up with
it.
That rising air
is replaced at
deck level by an
in-flow of
surrounding air.
That air may
become heated
and will also
rise. You may
get a
circulation
going, where
warmed air rises
and cools and
then descends to
flow back into
the hot spot on
the ground.
Sea Breeze
That is the
basics behind a
sea breeze.
Consider, - a
still day,
sunny, no breeze
at all early on,
by mid morning
it picks up
‘till by
lunchtime it’s
blowing quite
nicely - around
force 3 – 4,
more or less
onshore.
At the same time
you might notice
that a line of
cloud has formed
along the coast.
From offshore it
looks like it’s
a couple of
miles inland.
That line of
cloud is formed
by water that
has been picked
up as vapour by
the in-flowing
air that has
blown across the
sea, (the air
that has been
propelling your
boat), and then
carried aloft by
the warmed air.
But why cloud?
Dewpoint
One interesting
characteristic
of air is that
the warmer it is
the more water
it can carry as
vapour. Warm air
can carry lots
of vapour, cold
air carries
virtually none.
Have you ever
put cheese in
the ‘fridge
without wrapping
it up? Goes all
rubbery because
it’s dried out,
because the air
in the ‘fridge
is so dry, it
just draws the
moisture out of
it.
When a parcel of
air is cooled it
will reach a
temperature at
which it can no
longer carry the
water vapour.
The vapour will
condense to form
droplets. That
temperature is
the dewpoint.
Hence clouds, or
fog if it
happens at deck
level.
So far, in a
fairly
un-scientific
way we have
grasped a couple
of fundamentals.
Warm air rises -
cool air
descends. And
the idea that
warmer air can
carry more water
vapour than
cooler air, and
that when our
parcel of vapour
laden air is
cooled beyond
its ability to
carry vapour,
the vapour
condenses to
form water
droplets.
Fog
There are two
sorts of fog,
land fog and sea
fog, known
officially as,
respectively,
radiation and
advection.
Fog occurs when
a parcel of air,
laden with water
vapour is cooled
below it’s dew
point.
The difference
between the two
types of fog is
how that wet air
got to wherever
it became fog.
Land fog. A
winter time,
still air,
phenomenon. Goes
like this…
Wet, winter time
land is warmed
by the sun,
water evaporates
and forms a
shallow layer of
wet air over the
ground. The sun
disappears by
mid to late
afternoon and
the ground
temperature
rapidly falls.
Our parcel of
air is cooled by
the underlying
cold ground,
eventually
reaching it’s
dew point. The
vapour within
condenses,
forming water
droplets, bingo!
– fog.
Sea or advection
fog is much the
same except the
moisture laden
air wasn’t
formed on the
spot. It has
blown in over
the sea. Rather
than being
cooled by the
underlying cold
ground, as with
land or
radiation fog,
it is the
underlying sea
that chills the
air above. As
the air travels
up from the
tropics it blows
over ever colder
seas until it
cools to its dew
point and fog
forms.
