G109: Weather and Climate 9: Clouds & Precipitation
Importance of Clouds
• For weather & climateRadiative Properties
• Block K↓→
• Absorb and re-radiate L↑ →
Moisture content –
Precipitation
•
• For weather forecasting
G109: Weather and Climate 9: Clouds & Precipitation
Achieving Saturation
• Formation of clouds, fog, or dew, requires:Condensation: water vapor (gas) ⇒ liquid
i.e., Saturation
• Air can become saturated by any one of three processes:
G109: Weather and Climate 9: Clouds & Precipitation
Achieving Saturation
1. Radiation cooling – during evening hoursdew, fog
2. Cooling via – most commonCooled adiabatically (DALR if unsaturated)
0
200
400
600
800
1000
1200
8 10 12 14 16 18 20
Temperature (oC)
Hei
gh
t (m
LCLDALR
SALR
If parcel rises high enough, cools sufficiently → lifting condensation levelAbove this point air continues to rise but now at SALRSALR<DALR → does not cool as rapidly
G109: Weather and Climate 9: Clouds & Precipitation
Achieving Saturation
• Mechanisms that lift aira. Orographic Uplift:
b. Frontal wedging:
• uplift of
• due to air ρ differences
c. Convergence:
d. Localized convection:
G109: Weather and Climate 9: Clouds & Precipitation
Stability and Cloud Type
• Different types of clouds form depending on the stability
• Unstable – air tendency to rise
May get
• Stable - air tendency to resist upward movement
Air forcibly lifted by other means e.g. front, mountains
Precipitation, if any, is
G109: Weather and Climate 9: Clouds & Precipitation
Cloud Classification
• Initial classification of clouds had four basic categories (Howard, 1803)1. Cumulus
• “heaped”• Clouds with vertical development
2. Stratus• “layered”
3. Cirrus• “curl of hair”• High, thin wispy ice clouds
G109: Weather and Climate 9: Clouds & Precipitation
Clouds Classification• Current classification includes 10 cloud
types, grouped by height:1. High Clouds:
Cirrus Cirrostratus Cirrocumulus• Above 6000 m → predominantly
2. Middle Clouds:Altostratus Altocumulus
• 2000-6000 m → mix of 3. Low Clouds:
Stratus Nimbostratus Stratocumulus• Below 2000 m → mostly
4. Clouds of extensive vertical development:Cumulus Cumulonimbus
• Base of cloud is low extend to high altitudes (>6000+ m)
G109: Weather and Climate 9: Clouds & Precipitation
Cloud Classification
G109: Weather and Climate 9: Clouds & Precipitation
G109: Weather and Climate 9: Clouds & Precipitation
Cloud Formation
• Requires condensation (gas ⇒ liquid)• Air must be saturated: cooled to Tdew or
water added• Must be a surface on to which water
condensesWhat surface is in the atmosphere?Cloud Condensation Nuclei (CCN or CN)
•
• Normally large numbers in the atmosphereIf CCN are absent, can have
G109: Weather and Climate 9: Clouds & Precipitation
Cloud Formation
• Air saturated with water + CCN →Condensation
• When condensation takes placeInitial growth of cloud droplets very rapid
Growth rate diminishes quickly because H2O availability decreases
Even in very moist air - growth of these cloud droplets by additional moisture is slow
Also: cloud droplets are so tiny, they remain suspended in the air
G109: Weather and Climate 9: Clouds & Precipitation
Precipitation Formation
• Rain drops: large enough to fall
• Takes about cloud droplets to form 1 rain drop
• But…
→Need a mechanism to merge cloud droplets into rain drops1. 2.
G109: Weather and Climate 9: Clouds & Precipitation
1. Bergeron ProcessOccurs in • All or part of cloud < 0°C• i.e.,
Precipitation Formation
Pure water suspended in air:• Doesn't freeze until -40°C• Below 0 °C, it is liquid water
G109: Weather and Climate 9: Clouds & Precipitation
1. Bergeron ProcessSupercooled water will freeze if it:
• Is agitated sufficiently
• Comes into contact with Freezing Nuclei
Freezing Nuclei (FN): solid particles with ice or crystalline structure
Sparse in the atmosphere
FN generally not active until T < -10°C
Precipitation Formation
Liquid and ice crystals co-exist -10 to -20°C
Clouds primarily supercooled H2O 0 to -10oC
Clouds generally composed entirely of ice crystals<-20oC
Phase of Water in CloudsTemperature
G109: Weather and Climate 9: Clouds & Precipitation
1. Bergeron ProcessGrowth of ice crystals in the Bergeron process, relies on differences in saturation vapor pressure:
es ice < es water
Ice crystals grow
Requires a mix of both liquid water and ice
• i.e., clouds between 0 and -40oC
Precipitation Formation
G109: Weather and Climate 9: Clouds & Precipitation
Precipitation Formation
1. Bergeron ProcessIce crystals grow at the expense of liquid water
• Ice crystals grow
• Fall….collide with supercooled droplets
• Break…forming more FN
• Grow….
• Lifted by vertical ascent of the air
G109: Weather and Climate 9: Clouds & Precipitation
2. Collision – Coalescence ProcessOccurs in
Precipitation Formation
Giant condensation nuclei
• Hygroscopic particles - sea salt
Larger particles fall faster - collide with smaller (slower) particles
• May coalesce
• Millions of collisions ⇒ produces something large enough to fall to the surface without completely evaporating
• Need clouds with great vertical extent
• Need abundant moisture
G109: Weather and Climate 9: Clouds & Precipitation
Precipitation Formation
2. Collision – Coalescence ProcessIf collector drop is much larger than drops below it, then collision efficiency will be low
G109: Weather and Climate 9: Clouds & Precipitation
Precipitation Formation
2. Collision – Coalescence ProcessRaindrops may break up if
• Large size (> 5 mm) when falling 30 km hour-1
Surface tension - holds the drop together
• Surpassed by the drag imposed by the air
G109: Weather and Climate 9: Clouds & Precipitation
Forms of Precipitation
• Will precipitation be liquid or solid?
• RainLiquid water formed in warm clouds (Collision-Coalescence process)Ice crystals from cold clouds (Bergeron process) that melt completely before reaching surface
• Snow Ice crystals formed by the Bergeron process that do not melt before reaching surfaceForm (plate/column/needle-like) depends on history: T, humidity
G109: Weather and Climate 9: Clouds & Precipitation
Forms of Precipitation
• SleetSmall clear translucent particles of iceBegins as ice crystal (Bergeron process)Melts as it passes through warm layer of airThen re-freezes in the air above the surface
G109: Weather and Climate 9: Clouds & Precipitation
Forms of Precipitation
• Freezing Rain (glaze)Process and vertical T similar to that for sleet
BUT subfreezing air near the ground is not thick enough to allow raindrops to freeze
Rain becomes supercooled
Freezes when collides with solid objects
G109: Weather and Climate 9: Clouds & Precipitation
Forms of Precipitation
• HailConcentric shells of different densities and degrees of opaquenessForm in cumulonimbus clouds • Clouds with vertical extent & strong updrafts• Abundant supply of supercooled H20• Size depends on strength of updrafts → length
of path through the cloud
G109: Weather and Climate 9: Clouds & Precipitation
Fog
• FogAir is
Depth: ~1 meter to tens of meters
Visibility < 1 km
Formation different to cloud
Method of formation is used to distinguish types
a. Radiation Fog
b. Advection Fog
c. Upslope Fog
d. Evaporation Fog
G109: Weather and Climate 9: Clouds & Precipitation
Fog
1. Radiation FogCause:Forms in a shallow layer of moist airSurface cools rapidly via L↑• Air above surface cools• Air becomes saturated→ Fog forms
Most likely to form:•••
•Valley bottoms – cool, moist air drains down
G109: Weather and Climate 9: Clouds & Precipitation
Fog
2. Advection FogCause:Surface must be sufficiently cool to reduce the T to Tdew
• Involves • Surface H20 near the coast is colder than
surface water offshoreWind:• Will not dissipate fog• Brings in more moisture
Common along coastlines of lakes or oceans• e.g., San Francisco in summer
G109: Weather and Climate 9: Clouds & Precipitation
Fog
3. Upslope Fog: Cause:
Air gradually
• If sufficient cooling → fog
G109: Weather and Climate 9: Clouds & Precipitation
Fog
4. Evaporation Fog: Cause:
Mix two air parcels together
•
•
Air becomes saturated – because saturation vapor pressure increases rapidly with T
Examples
• Breath on a cold day
• Steam fog above a heated swimming pool
G109: Weather and Climate 9: Clouds & Precipitation
