Chapter 12 – Clouds & Precip

Eth Sci Chapter 12 – Clouds & Precip
What is the most impor gas in the atmosphere? ______________
Water vapor
I. Changes of State
Water vapor defined as an
Oxygen and nitrogen also change state from gas to liquid to solid, but at
much lower temps than we see on earth…wtr vpr is unique as a gas in
moving from gas to liq to solid at the press and temp we see at earth
Change of state requires heat absorption or release..
Heat is measured by calories –
1 cal = amt heat required to raise temp of 1 g of water 1o C
latent heat (“hidden” heat)
example is when heat is taken up to melt ice to water…all heat is taken
up in breaking solid xl structure to liquid…when all ice melted, then T of
water strts to rise, but not before all ice melted
latent heat important source of much energy, for example clouds, tstorms, trop storms, hurricanes
Fig 12.1 shows specifics of heat absorption/release
- Latent heat of evap…600 cal needed to convert 1 g wtr to wtr vpr
Interesting pt here..higher
-Latent heat of condensation is the reverse, where 600 cal given
off…this energy used in producing heavy weather, violent storms
latent heat of melting … 80 cal req to produce 1 g of water from solid
latent heat of fusion …80 cal released during freezing…reverse of
sublimation – direct conversion of solid to gas – dry ice a good
example of this…another example is unused ice cubes which
eventually sublimate in a freezer..680 cal absorbed by a gram of ice
for it to sublimate
reverse of sublimation is “deposition”…also can be frost..household
example is frost in an older freezer – 680 cal released when frost is
Eth Sci wk 12 p.2
Humidity – water vapor in the air
2 types of humidity – specific, relative
relate these back to concepts of sautration and vapor pressure..
saturation – putting as much wtr vpr into air as it can hold w/out
vapor pressure – that part of atmos press that is due to water vapor, say
above a body of wtr
at saturation, the no. of molecules evaporating = no. of molecules
temp complicates situation
So specific humidity = wtr vpr wt / total wt of mass,
And relative humidity = actual wtr vpr content / potential wtr vpr content
The 2 factors that can change rel humidity –
1. add or subtract water Fig 12.3
2. change temp Fig 12.4
if hold wtr content constant, rise in temp will lower rel humidity (because
poten wtr vpr content goes up)
drop in temp will increase
Fig 12.5
Another impor concept – dew point
Is the temp to which air of
measuring humidity
psychrometer used, which is two thermometers, one dry –air, the
other wet-air. Table used to compare the two readings
Earth Sci wk 12 p. 3
Basis of cloud formation: Adiabatic cooling
Now look at wtr vpr role in weather and fm of clouds…
Fog/dew vs clouds..all 3 require saturation
Radiation cooling causes dew and fog fm.
Clouds different…
Air that is compressed has temp increase (molecules closer
Air that is decompressed expands and cools
These temp changes are not assoc w/heat loss or gain, but with
pressure changes…known as adiabatic changes
As air rises it cools adiabatically, as it drops it warms ..rate of
temp change is known as dry adiabatic rate if air unsaturated
If air rises and cools enough it will reach dew point, and after this
as it continues to rise, its latent heat of condensation will be
released as vapor cools and condenses Fig 12.7
….this is mechanism of cl
Stability of air
Why air rises sometimes, or why cloud sizes differ, has to do with
air stability
Stability is function of temp.
…cool air will sink according to its density, reach stable position
…but warm air will rise, like hot air balloon and be unstable
stability prevails when the environmental lapse rate is less than
the wet adiabatic rate Fig 12.8, Fig 12.9
the rising air being forced upward still would rather sink downward
note the flat clouds, trying to spread out
Unstable air is when envir lapse rate is greater than the dry
adiabatic rate Fig 12.10
Earth Sci wk 12 p.4
Rising air would just as soon keep on rising….note how clouds
billow upward….
Stability and daily weather
Stable air sometimes is forced to rise, even though it is
relatively cool..when this occurs, clouds are pretty flat, have
little precip
Unstable air has towering clouds, heavy precip
For example, on overcast drizzly day, stable air has been
forced up
Contrast this to big upward-growing clouds, probably unstable
Processes that lift air
 orographic lifting Fig 12.11
“mountain lifting” ….precip
example – Olympic Mts in
often see rainshadow, abs
frontal wedging…due to wedge of cool air that warm air overrides
can result in both stable and unstable air Fig 12.13
convergence – where air mases flow together
Condensation and Cloud Formation
Condensation defined- when water vpr in air changes to a liquid
This produces dew, fog, or clouds
Usu occurs because temp cools to dew pt, but sometimes
because water is added to air
Usu need a surface for condensation
Ex dew on grass, car wind
Or in case of clouds, need condensation nuclei (bits of dust)
At start of condensation, w.v. becomes millions of tiny droplets (or
ice crytals) suspended in air
Earth Sci wk 12 p.5
 Cloud classification
Defined as “visible aggregates of minute droplets or xls of water”
Classif based on:
Form – 3 types Fig 12.14
cirrus – high white thin, ptches, sheets, or wispy
cumulus – individual “globs”
stratus – sheets or layers…no individual globs
Other component of cloud classif - height
 3 types – fig 12.15
high – above 6000 m (about 20,000 ft):
- cirrus – thin, delicate mare’s tails
- cirrostratus – thin sheets
- cirrocumulus-fluffy masses
middle (2000 to 6000 meters) “alto” is the prefix,like diff between alto
and soprano voices in choir..alto is middle female voice
- altocumulus – globby clouds that are larger, denser than cirrocum
- altostratus – white to grayish sheets
low –
- stratus – uniform, fog-like, cover much of sky
- stratocumulus – scalloped bottoms, rod-like
- nimbostratus – “rain” + “sheets” , major rain makers, forced
ascent of stable air
clouds with low bases, high tops…all assoc with unstable air
cumulus can grow to cum
Defined- a cloud with its base at or near the ground
Dff betw fog and other clouds are in
Method of fm
Place of fm
Earth Sci wk 12 p.6
Clouds are result of adiabatic cooling,
but fog forms from radiation cooling or mvmt of air over a cold surface
Fog also can form by addition of wv to air (evaporation fog)
A. fog caused by cooling
1. advection fog – Fig 12.16
due to warm air over cool surface,T drops below dew pt
2. radiation fog – Fig 12.17 cool calm nights when earth losing heat
through radiation..
air in contact with ground cooled below dew point..pockets of fog
form in low spots like river valleys
3. upslope fog – humid air moves upslope, cools adiabatically
B. fog caused by evaporation
forms from cool air moving over a warm surface (usu. water), happens
because water evaps from warm surface up into cloud and causes
1. steam fog - rising water vapor immediately recondenses – looks like
steam rising from lakes and rivers
2. frontal fog – warm air lifted over cold air, rain develops, then rain
evaporates to produce fog
What causes it?
How it forms…millions of cloud droplets (about 1/8 diam of human hair)
coalesce into bigger droplets, large enough to fall
2 mechs proposed to explain this –
1. ice-crystal process..based on observation that precip often forms at
temps below freezing in high lat clouds…those droplets that make
contact with a solid particle are the ones that freeze
remaining droplets are supercooled, but are still liquid, not ice
Earth Sci wk 12 p.7
this sets stage for xls collect water, grow into snowflakes.
They start to fall, get bigger as they pick up the supercooled could
droplets that freeze on the flakes… then when these particles get low
enough to warm up, the snowflakes change to water, fall rest of the way
as rain
2. collision-coalescence process
start with condensation nuclei…large droplets form, then collide with
smaller droplets that coalesce with the big ones, ultimately the
droplets become big and heavy enough to fall
forms of precipitation-
1. rain - drops of water at least 0.5 mm in diam, rarely bigger than 5
2. drizzle- drops of water less than 0.5 mm in diam
3. snow – in form of ice xls known as snow flakes, size shae, form
controlled by temp at formation..
Snow (cont)
Snow formed at low T is light, fluffy, due to low moisture content of air..
this is the powder that skiers love….
Above a temp of – 5o C (about 22 deg F), snow is wet , tangled clumps
with high moisture content, ideal for snowballs
4. sleet – small particles of ice that are essentially melted snow which
froze..the melted snow falls through a cold layer of air just above
ground,and becomes sleet
5. glaze is technical term for freezing rain..conds same as for sleet,
except the melted snow (supercooled) doesn’t freeze until it hits
something solid, like a car or tree branch or power line
6. hail – hard pellets of ice, often concentric layers showing progressive
growth from small nuclei to large ball
largest to date rept’d in Kansas, how big?? How heavy??
What town did I live in when a hailstorm ruined my roof?
Hail produced in cumulonimbus , storm clouds, big updrafts, ice particles
form, drop, get picked up, get bigger (new layer of ice), fall again, get
picked up again, get a new layer, process repeats until it finally falls to
7. rime – deposited by supercooled fog