Air stability
About
clouds…
Precipitation
A mass of moist, stable air gliding up and over these
mountains condenses into lenticular clouds. Fig. 5-CO, p.110
…air in unstable equilibrium will move--up/down
Fig. 5-1, p.112
FIGURE 5.3 A stable
atmosphere. An absolutely
stable atmosphere exists
when a rising air parcel is
colder and heavier (i.e., more
dense) than the air
surrounding it. If given the
chance (i.e., released), the air
parcel in both situations
would return to its original
position, the surface.
Adiabatic = w/ no
exchange of heat
from outside!
Dry adiabatic rate = no heat
added from condensation as
air rises.
moist adiabatic rate = less
because latent heat added
during condensation of
rising air
The dry adiabatic rate. As long as the air parcel remains unsaturated, it
expands and cools by 10°C per 1000 m; the sinking parcel compresses
and warms by 10°C per 1000 m.
Fig. 5-2, p.113
Inversion acts as lid
Fig. 5-3, p.113
Absolutely unstable air!
= if rising air is warmer and
less dense than air around it,
once parcels start upward,
they will continue!
Air becomes more unstable as
environmental lapse rate
steepens (air is colder w/ ht)…
Generally, as surface air warms
during the day, the air becomes
more and more unstable…
FIGURE 5.4 Cold surface air, on this morning, produces a stable
atmosphere that inhibits vertical air motions and allows the fog and haze
Fig. 5-4, p.114
to linger close to the ground.
Fig. 5-5, p.115
1
FIGURE 5.6 Unstable air. The
warmth from the forest fire heats
the air, causing instability near
the surface. Warm, less-dense air
(and smoke)bubbles upward,
expanding and cooling as it rises.
Eventually the rising air cools to
its dew point, condensation
begins, and a cumulus cloud
forms.
Condensation level = level above
sfc where cloud first forms
Fig. 5-6, p.115
IOW: if unsaturated,
unstable air is
somehow lifted to a
level where it becomes
saturated, instability
may result
FIGURE 5.7Conditionally unstable air. The atmosphere is conditionally
unstable when unsaturated, stable air is lifted to a level where it becomes
saturated and warmer than the air surrounding it. If the atmosphere
remains unstable, vertical developing cumulus clouds can build to great
Fig. 5-7, p.116
heights.
surface heating and convection
FIGURE 5.8 The primary ways clouds form: (a) surface heating and
convection; (b) forced lifting along topographic barriers;(c)
convergence of surface air; (d) forced lifting along weather fronts.
Fig. 5-8, p.118
Fig. 5-8a, p.118
Fig. 5-8b, p.118
Fig. 5-8c, p.118
forced lifting along topographic barriers
2
Fig. 5-8d, p.118
Cumulus clouds building on
a warm summer afternoon.
Each cloud represents a
region where thermals are
rising from the surface. The
clear areas between the
clouds are regions where
the air is sinking.
Cumulus clouds form as hot, invisible air bubbles detach themselves from
the surface, then rise and cool to the condensation level. Below and within
the cumulus clouds, the air is rising. Around the cloud, the air isFig.
sinking.
5-9, p.118
Cumulus clouds developing
into thunderstorms in a
conditionally unstable
atmosphere over the Great
Plains. Notice that, in the
distance, the cumulonimbus
with the anvil top has reached
the stable part of the
atmosphere.
Fig. 5-10, p.119
<= tropopause
Fig. 5-11, p.119
Orographic uplift, cloud development, and the formation of a rain
shadow.
Fig. 5-12, p.120
The formation of lenticular clouds.
Fig. 5-13, p.120
3
Relative sizes of
raindrops, cloud
droplets, and
condensation
nuclei.
Lenticular clouds (mountain wave clouds) forming over Mt. Rainier,
Fig. 5-14, p.121
Washington.
Fig. 5-15, p.121
Collision and coalescence. (a) In a
warm cloud composed only of small
cloud droplets of uniform size, the
droplets are less likely to collide as
they all fall very slowly at about the
same speed. Those droplets that do
collide, frequently do not coalesce
because of the strong surface tension
that holds together each tiny droplet.
Fig. 5-16, p.122
Fig. 5-16a, p.122
Collision and coalescence. (b) In
a cloud composed of different
size droplets, larger droplets fall
faster than smaller droplets.
Although some tiny droplets are
swept aside, some collect on the
larger droplet’s forward edge,
while others (captured in the
wake of the larger droplet)
coalesce on the droplet’s
backside.
Fig. 5-16b, p.122
A cloud droplet rising then falling through a warm cumulus cloud can
grow by collision and coalescence and emerge from the cloud as a large
Fig. 5-17, p.123
raindrop.
4
The distribution of ice and water in a cumulonimbus cloud. .
Fig. 5-18, p.123
In a saturated environment, the water droplet and the ice crystal are in
equilibrium, as the number of molecules leaving the surface of each
droplet and ice crystal equals the number returning. The greater number
of vapor molecules above the liquid indicates, however, that the
Fig. 5-19, p.124
saturation vapor pressure over water is greater than it is over ice.
Ice particles in clouds.
The ice-crystal process. The greater number of water vapor molecules
around the liquid droplets causes water molecules to diffuse from the
liquid drops toward the ice crystals. The ice crystals absorb the water
vapor and grow larger, while the water droplets grow smaller. Fig. 5-20, p.125
Fig. 5-21, p.125
Natural seeding by cirrus clouds may form bands of precipitation
downwind of a mountain chain.
Fig. 5-22, p.127
The streaks of falling precipitation that evaporate before reaching the
ground are called virga.
Fig. 5-23, p.128
5
Which of the three drops drawn here represents the real shape of a
falling raindrop?
p.129
Table 5-1, p.130
Sleet forms when a partially melted snowflake or a cold raindrop freezes
into a pellet of ice before reaching the ground.
Fig. 5-26, p.131
The dangling white streamers of ice crystals beneath these cirrus clouds
are known as fallstreaks. The bending of the streaks is due to the
Fig. 5-24, p.129
changing wind speed with height.
Computer color enhancedimage of dendrite snowflakes.
Fig. 5-25, p.130
An accumulation of rime forms on tree branches as supercooled fog
droplets freeze on contact in the below-freezing air.
Fig. 5-27, p.131
6
A heavy coating of freezing rain during this ice storm caused tree limbs
Fig. 5-28, p.131
to break and power lines to sag.
The accumulation of small hail after a thunderstorm. The hail formed as
super cooled cloud droplets collected on ice particles called graupel
Fig. 5-29, p.133
inside a cumulonimbus cloud.
This giant hailstone — the largest ever reported in the United States with
a diameter of 17.8 cm (7 in.) — fell on Aurora, Nebraska, during June,
Fig. 5-30, p.133
2003.
5.31Hailstones begin as embryos (usually ice particles) that remain suspended in the
cloud by violent updrafts. When the updrafts are tilted, the ice particles are swept
horizontally through the cloud, producing the optimal trajectory for hailstone growth.
Along their path, the ice particles collide with supercooled liquid droplets, which freeze
on contact. The ice particles eventually grow large enough and heavy enough to fall
Fig. 5-31, p.133
toward the ground as hailstones.
Components of the standard
rain gauge.
Fig. 5-32, p.134
The tipping bucket rain gauge. Each time the bucket fills with onehundredth of an inch of rain, it tips, sending an electric signal to the
remote recorder.
Fig. 5-33, p.135
7
(a) Doppler radar display showing precipitation intensity over Oklahoma
for April 24, 1999. The numbers under the letters DBZ represent the
logarithmic scale for measuring the size and volume of precipitation
Fig. 5-34a, p.136
particles.
(b) Doppler radar display showing1-hour rainfall amounts over Oklahoma
for April 24, 1999.
Fig. 5-34b, p.136
8