Download Weather & Climate Chapter 5

CHAPTER 5
FORMS OF
CONDENSATION
AND
PRECIPITATION
Clouds
- Buoyant masses of visible droplets of
H2O vapor or ice crystals
- May be as low as the ground (fog), or
as high as the Troposphere (ice
clouds)
- Play role in maintaining heat balance
by reflecting insolation and
terrestrial radiation
Clouds, cont
-
-
effect of cooling and condensation of
H2O in the atmosphere
Forms of precipitation originate here
Sign of atmospheric stability /
instability
Clouds are classified according to
height and appearance
Average man’s key to weather
Cloud Height and Appearance
Height
cirrus - high
alto – middle
stratus – low
clouds of vertical formation
Appearance
cirrus – white; thin
cumulus – globular; pillowy
stratus – sheet; layered
Cloud Height and Appearance,
cont
- Height and appearance are functions
of temperature and H2O vapor
available at each altitude for cloud
formation
- Clouds identified with precipitation
we give the prefix “nimbo” or suffix
“nimbus”
Cloud Height and Appearance,
cont
-
Specialized location conditions may give
rise to sub-groups of clouds
fractus – broken
uncinus – hook shaped
mammatus – rounded/udder-shaped
lenticular – lens-like
banner – banner-like
nacreous – pearly; Stratospheric
nactilucent – wispy; Mesospheric
Cloud Formation
(1)
(2)
(3)
(4)
(5)
Cooling to, and below, the dew point
Cooling is the effect of vertical lifting /
lifting requires a lifting force
Cooling of the parcel of air is controlled by
the adiabatic lapse rate
Rising air becomes stable when it loses
upward buoyancy and has the same
density as surrounding air
Cloud formation assumes condensation
nuclei (hydroscopic particles)… without
which air below dew point becomes
supersaturated
Cloud Classes: High
-
visible because temperature
produces ice crystals
--- cirrus(Ci) – “mares tail”
--- cirrocumulus(Cc) – “mackerel
sky”
--- cirrostratus(Cs) – partially or
totally cover the sky
Cloud Classes: Middle/Low
Middle
--- altostratus(As)
--- altocumulus(Ac)
Low
*may be advective rather than
convective*
--- Stratus (St)
--- Stratocumulus (Sc)
Fog
- Generally accepted to be a cloud with
its base at, or near, ground level
- Clouds are the result of lifting to
saturation and condensation… fog
results from
(1) radiation cooling
(2) cooling of air mass
(3) adding additional moisture
Fog, cont
Cooling fogs
- radiation fog – cool, clear, calm
nights
- advection fog – warm, moist
air blown across a cold
surface
- upslope fog – most like a
cloud; adiabatic cooling of
valley breeze
Fog, cont
Evaporation fogs
- steam fog – cool air passing
over warm water
- frontal fog – result of frontal
wedging
How Precipitation Forms
All clouds are composed of H2O
vapor
Why do some clouds make
precipitation and others just
float there?
How Precipitation Forms, cont
(1)
Cloud droplets are extremely
small – about 0.02 mm dia
… abundance of condensation nuclei
results in many, small droplets
--- gravity has very little effect on them
--- they fall very slowly and evaporate
very quickly in the atmosphere
(average size droplet would take hours
to fall from 1000 m; but it would
evaporate too quickly)
How Precipitation Forms, cont
(2) Billions of cloud droplets are
in competition for available
moisture and growth by
condensation is also very slow
Rain drop large enough to fall to
the ground is about 1 million
times larger than a cloud droplet
How Precipitation Forms, cont
In order to grow large enough to
fall without evaporation, cloud
droplets must coalesce into
large droplets… 2 ways
(1) Bergeron Process
(2) Collision-Coalescence
Bergeron Process
Dependent on the simultaneous existence of
H2O vapor, liquid water droplets and ice
crystals
Dependent on two properties of water
(1) Droplets do not freeze at 0o C,
they freeze at –40o C
(liquid water below 0o C is supercooled)
(2) Saturation vapor pressure over ice
crystals is much lower than over
supercooled liquid droplets
Aside
Saturation vapor pressure is the
increase in atmospheric
pressure resulting from the
motion of H2O molecules in the
air through evaporation
Aside, cont
As more and more molecules
escape, increasing atmospheric
pressure (and slowing molecular
motion through collision), forces
some to return to the liquid
state
Where returning molecules equal
evaporation saturation vapor
pressure exists
Bergeron Process, cont
… air saturated with respect to
water will be supersaturated
for ice crystals
… the ice crystals will then
capture more H2O vapor
then they will lose through
sublimation
(remember: more energy is necessary to
sublimate than evaporate cloud droplets)
Bergeron Process, cont
… continued evaporation provides ice
crystals with moisture “to rob”
… larger ice crystals are produced that
compete with supercooled
droplets for available moisture
and altering the relative
humidity… increasing evaporation
(constantly altering both temp.
and moisture)
Bergeron Process, cont
Growing rapidly, ice crystals begin to
fall:
(1) some are “shattered” to
become freezing nuclei and
rob more liquid water
(2) some crystals collect “shattered”
material and become even larger
*precipitation form these take depends
on atmospheric conditions*
Collision-Coalescence
- Clouds below freezing altitudes
(air temp > 0o C) large cloud droplets
are necessary to produce
precipitation
---these droplets fall quickly,
colliding with smaller droplets and
coalesce together
--- they become larger and fall faster
Collision-Coalescence, cont
-Two opposing forces act of
droplets: gravity and friction
- Gravity pulls the drop to the
Earth surface, and increases as
drop size increases
- Friction (drag) increases as the
drop accelerates – more air
molecule collisions
Collision-Coalescence, cont
Eventually gravity and friction balance and
the drop has reached terminal velocity
--- as droplet size increases, gravitational
force is affected more than drag
… smaller drops have lower terminal
velocity
- Terminal velocity is important because
speed-of-fall determines the probability
that a drop will reach the surface before
evaporation
-
Collision-Coalescence, cont
-
-
Process is more efficient in
clouds of vertical formation
(abundant moisture)
Creation of “super drops” is
limited to about 5 mm by drag
from the air exceeding drop
surface tension
Collision-Coalescence, cont
Presence of “super drops” does
not ensure collision-coalescence
(1) was from falling drop may
push smaller drops away
(2) electrical charge may be a
factor
Precipitation Form
Rain
Sleet
Glaze
Dry Snow
Hail
Wet Snow
Rain