Download Dew Point and Cloud Formation

One atmospheric
factor not mentioned
yet is how wet or dry
the air can be.
There is always
water vapor in the
air, which we
measure as
humidity.
Meteorologists, people who study weather,
use a figure which they call relative humidity
Relative humidity (R.H.)
is the amount of
moisture currently
present in the air
compared to the
maximum amount of
moisture that can
evaporate at that
temperature.
If the air has 40 % of
all of the water vapor
that could form, then
the relative humidity
is 40 %.
When the air is
totally saturated,
the relative
humidity is 100 %.
Warm air has more
ability to evaporate
liquid water into vapor.
Cold air has less
ability to evaporate
liquid water into vapor,
so it is more likely to
condense water vapor
into liquid water.
If warm, moist air is
cooled off suddenly,
the water vapor
condenses. The
temperature at
which the relative
humidity reaches
100 % is called the
dew point.
At the dew point, water vapor condenses
dramatically, forming dew on the ground…
… or clouds and
precipitation in the air.
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1.
2.
The Dew Point is helpful in two
main ways:
Low dew point usually means the air
feels really dry (low humidity), while
High dew point (closer to the air
temperature) means high humidity.
As air rises, it cools. When the air
temperature drops to the dew point,
clouds start to form. This altitude is
called the dew point altitude.
As you go up through the
atmosphere, the
temperature drops off
rather quickly.
With a temperature of 78o
and a dew point of 58o,
the relative humidity on
the ground may be only
40 %.
As the air rises, the
temperature drops while
the dew point often remains
unchanged.
At 1800 feet altitude, the
temperature has dropped
from 78o to 68o, while the
dew point has remained at
58o .
At 3600 feet, the
temperature (58o) has
dropped down to the
dew point (58o). This
means the relative
humidity has risen to
100 %, and clouds will
start to form.
At this height (the
dew point altitude),
the water vapor
condenses and
forms clouds.
Most clouds tend to
have flat bottoms or
bases…
Clouds often have flat bottoms because
water will not condense (at least not very
much) below the dew point altitude.
(R.H. is too low below this altitude.)
100 %
R.H.
The flat bottom of the clouds that forms
at this altitude is called the cloud base.
40 %
R.H.
Clouds often have flat bottoms because
water will not condense (at least not very
much) below the dew point altitude.
(R.H. is too low below this altitude.)
100 %
R.H.
Cloud Base
40 %
R.H.
Cloud droplets are tiny: about 20 microns (mm)
across. Each cloud droplet is built around an even
tinier grain of dust, ash or salt (0.2 mm). These secret
ingredients that make clouds grow are called Cloud
Condensation Nuclei (CCN).
1) Cloud Condensation
Nuclei (CCN) provide
the water vapor
molecules in the air a
surface on which to
collect .
2) As the water vapor
condenses on the CCN
surface a cloud droplet
forms.
3) It may take 1,000,000
(1 million) cloud
droplets to make a rain
drop!! (See next slide.)
(0.2 mm)
(50 mm)
(20 mm)
(2000 mm)
In stable air, rising warm air
pockets cool faster than the
surrounding air (or may be cooler
than the overlying air to begin
with), so they tend to resist
rising.
Stable air rises to the dew point
altitude and then spreads out
sideways to form layered
clouds such as stratus and
cirrus.
Altostratus
Cirrus
Altocumulus
Stratus
In unstable air, rising warm air
pockets cool more slowly than
the surrounding air. They hit the
dew point altitude just keep on
rising!!
Unstable air leads to clouds with
a great deal of vertical
development.
This class of clouds is called
convective clouds.
Cumulonimbus
Cumulus congestus
Towering Cumulus
Clouds also form where
warm air meets cold air
and rises along a front.
Water vapor condenses
as the warm air rises
and cools, causing the
temperature to quickly
drop below the dew
point.
This sudden increase in
relative humidity often
results in precipitation
(rain, snow, sleet, hail, fog).
The uplift along a cold
front is sudden.
This creates very
unstable air and the
clouds along a cold front
are convective clouds
such as cumulus and
cumulonimbus.
Cold Front Clouds
Uplift along a warm front is
more gradual, so the air is
usually more stable.
As a result, clouds along a
warm front are more likely
to be layered clouds such
as stratus, cirrus,
altostratus, altocumulus
and stratocumulus.
Clouds also form when air rises as it passes over
a mountain range. This is called orographic
lifting. Water vapor condenses as the air
temperature drops below the dew point. One
spectacular variety is the lenticular cloud.
Heavy rainfall occurs on the upwind side of the
mountains due to orographic lifting.
As the dry air descends on the downwind (lee)
side of the mountains, it becomes super dry and
often creates desert conditions.