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against what it could hold at that
temperature or, in other words, the
percentage saturation, which will decrease
if the air gets warmer, as when
subsiding in a high pressure area,
because temperature is raised by
compression, and can absorb more
moisture (exam question). Relative
humidity could change as a result of
the air absorbing more moisture, say
when moving over the sea, but it is
more likely to change quickly
through temperature changes, at
least for our purposes. Water added to
air makes it moister, and less dense, and
therefore more likely to rise.
Mostly, air is made to reach its
saturation point by force, such as
being moved up the sides of
mountains or over large areas of
slower moving air (large scale ascent)
and, if the conditions are right, cloud
will form. In fact, the ways of
cooling air are many. Where
horizontal movement of air over a
cooler surface is involved, it is called
advective, as found over the praires
when air is moving from East to
West, and a frequent cause of fog.
The convection currents we have
already met with cause adiabatic
cooling by lifting air so it expands
and cools, and expansion cooling is
really the same thing caused by
upslope movement or mechanical
turbulence, another name for low level
air being mixed and moved upward.
Radiation cooling tends to happen
overnight with clear skies, when the
Earth’s heat radiates out into space.
If the winds are light, just enough to
stir things up (3-5 kts), fog will form,
predictably enough called radiation
fog. Night-time cooling is less over
water than land, as water traps heat –
this is called the maritime effect. It
could also cause a nocturnal inversion.
What’s known as the cloud effect
reduces the results, because they
absorb some radiation themselves.
The topographical effect results from
cold air getting trapped in valleys
and lower areas.
The process of sublimation occurs
when water vapour goes directly to
the solid state (i.e. ice) without going
through a liquid stage, or vice versa.
Anyhow, the lapse rate changes
when we add moisture to the mix.
The Dry Adiabatic Lapse Rate (DALR)
Weather 91
is 3°C per 1,000 feet, and you use it
to find the cloud base, after which
you switch to the Saturated Adiabatic
Lapse Rate (SALR) of 1.5°C per
1,000 feet (dry, in these
circumstances, just means a relative
humidity of less than 100%).
For example, your ground
temperature is 10°C, and the
dewpoint 7°C. To find the cloudbase
and freezing level, take the DALR
and divide it into the difference
between the temperature and
dewpoint, which in this case is 3,
then divide that by the lapse rate (3),
so your cloudbase would be at 1,000
feet. Then use the SALR from the
cloudbase to count down to zero, so
divide 7°C (the dewpoint) by 1.5 and
add the converted number in
thousands of feet to the cloudbase,
to get 4,660 (from 4.66), so the
freezing level would be at 5660 feet.
The reason for the difference in
lapse rates is latent heat. The word
means undeveloped, implying that an
amount of heat is lurking in the
background waiting to do
something. Converting water from
one state to the other requires
energy, which originally comes from
the Sun’s rays as water is evaporated
in the first place, and is stored with
the vapour. While there, it is known
as latent, and released when the
water condenses, hence the lesser
rate that ascending saturated air
cools at, since the air becomes
warmer as a result. Latent heat
becomes involved when you change
the form of a substance without
changing its temperature. There can
be so much heat released that flight
in normally stable layer cloud can be
bumpy, due to internal eddying.
Latent heat is the reason for the
Chinook, a warm wind common in
the lee of the Rockies around
Calgary, which can raise the air
temperature to 20° in the middle of
Winter. Saturated air made to rise by
the mountains cools at SALR, and
when it descends on the other side,
having dropped its moisture, it
warms at twice that, i.e. DALR, so
you get a dry, warm wind with clear
skies. In France, it is called the
Mistral, and is accepted as a defence
in court for weird actions, as the air
is also ionised. In the Alps, it is
known as the Fohn Effect.
Stability
Remembering the previous mention
　
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