1. Clouds (Condensed PPT)

2. Clouds take different shapes and “looks”
Shape (and “look”) of a cloud determined by its
Altitude (thus temperature)
Water content and distribution
Maritime clouds: fewer, bigger drops
Continental clouds: more, smaller droplets
Vertical velocity
Cumulus clouds (vertically grown, “puffy looking”) indicate instability
Fast(er) upward velocities
Stratus clouds: often “forced” ascent (not buoyant)
Slow(er) upward velocities

3. Cloud droplets form by condensation
“Spontaneous” condensation (vapor  liquid, without anything to condense onto) is rare
Occurs in very cold, very supersaturated conditions
“Regular” condensation onto a cloud condensation nucleus (CCN) is much more common
Requires only supersaturation
Cool the air parcel and/or moisten it will give supersaturation
Lots of processes yield cooling, several processes yield moistening
Larger radii CCN are more efficient at promoting condensation (more surface area

4. Warm clouds (no/little ice)
Maritime clouds: fewer CCN, thus fewer droplets that are larger in radius
Continental clouds: more CCN, thus more droplets that are smaller in radius
Large droplet size
x-axis: number of droplets per volume. More droplets per volume means they are smaller
y-axis: % of droplets a large volume of air that have those # of droplets
High concentrations of CCN

5. As an air parcel rises, it entrains some environmental air into itself
That dry air evaporates some of the smallest cloud droplets
At the edge of a cloud, entrainment can be pretty large, enough to cool the parcel (remember, evaporation takes sensible heat from the parcel and stores it in the H2O molecule as latent heat) and stop it from rising
Parcels rising in the core of the cloud don’t entrain much
Undiluted parcel
Diluted parcel
Rising parcel near the edge of the cloud mixes in drier environmental air as it rises 
significant evaporation/cooling  parcel stops rising
Rising parcel in the center of the cloud mixes in other “cloudy” air as it rises  little evaporation/cooling  parcel continues to rise

6. Typical sizes:
CCN (very small): 0.1 μm
Cloud droplet: μm
Small rain drop: 100 μm
Typical rain drop: 1000 μm

7. Warm rain development How do rain drops form?
Condensation onto CCN  Cloud droplets
Cloud droplets grow by condensation
Cloud droplets bump into each other (collide)
They combine with each other (coalesce)
Bigger droplets bump into smaller droplets
Grow even bigger!
Eventually cloud droplets grow so large they cannot be supported by the cloud updraft
Cloud droplets start to fall, colliding and coalescing with even more smaller droplets
Rain occurs!
Called the “collision-coalescence process”

8. Cold precipitation processes
Involve ice!
Main ideas:
Cloud droplets remain liquid at temperatures well below 0°C
If their temperatures are below 0°C, they’re called “supercooled” droplets
Super-cooled droplets will freeze when they come in contact with an ice nucleus
Lots of things can serve as ice nuclei
The original CCN
A different particle
Other ice crystals / fragments
However, freezing supercooled cloud droplets is not the main process by which snow forms
Instead, a particle called a deposition nucleus favors the direct deposition of water onto it
I.e., the water vapor molecules go directly from vapor  ice

9. More on deposition
One of the neat properties of liquid, and ice, phases of water is that they have different saturation vapor pressures
Liquid water has a higher saturation vapor pressure than solid ice
Consequence #1: supersaturation of an air parcel is higher if ice is present
Consequence #2: vapor molecules “prefer” to change phase directly from vapor  ice
In most clouds, both ice crystals and liquid cloud droplets co-exist
The above 2 consequences mean that the when liquid droplets get close to ice crystals (but not touching), they will:
Evaporate into vapor
Then deposit as ice

10. Some cool properties of cold clouds
The shape of the ice crystal that forms via deposition depends strongly on its temperature
Snowflakes are actually “aggregates of dendrites
Dendrites
~ −10 to −18℃
Sector plates
~ −10 to −40℃
Rosettes
~ below
−40℃
Hexagonal plates
~ 0 to −10℃ subsaturated wrt water
Columns/needles
~ 0 to −10℃ saturated wrt water
Aggregations of ice crystals that fall relative to the ground (at speeds generally about 1 m/s) are what you know as snowflakes

11. Typical clouds The bottom part is all water
The middle part is supercooled water
The top part is all ice
The part that is all ice is called “glaciated”
Sometimes, when an ice fragment touches a supercooled cloud droplet, the droplet freezes from the outside-in
As the inner part freezes, it expands and “blows up” the cloud droplet, creating many tiny ice fragments that can then serve as ice nuclei
ice
Supercooled water

12. Microphysical “pathways” to develop precipitation-sized particles from water vapor can be quite complex, but some common ones are as follows:
Convective precipitation (e.g., in a thunderstorm):
Ice growth by vapor deposition / ice multiplication
Particles held aloft by updraft or recycled into updraft experience heavy riming increasing their mass
Particles eventually fall out of the updraft or overcome it and begin to fall relative to the ground
Particles melt above 0℃
Raindrops reach the surface
Stratiform precipitation (often produced by gentle, large-scale lifting):
Ice growth by vapor deposition
Once formed, ice crystals descend relative to the ground (situations where fall speed of ice > vertical uplift velocities defines stratiform precipitation)
Ice crystals aggregate (and perhaps lightly rime) as they approach the melting layer (if it exists)
Raindrops, snowflakes, or sleet reaches the surface
In extreme cases, ice particles held aloft by strong updrafts can collide with larger supercooled drops that coat their surface before freezing. As this process continues, layers of ice will build up on the particle increasing its weight until it is either lofted out of the updraft or can no longer be supported by it. These are hailstones and in very extreme situations, they can grow quite large

13. What about snow, sleet, freezing rain, and rain?