1. Clouds, Precipitation, and Weather Radar
Chapter 7
Clouds, Precipitation, and Weather Radar

2. Clouds
A cloud is the visible product of condensation or deposition of water vapor in the atmosphere
Need more than just saturation to form clouds
When the relative humidity of the air is >100% we say the air is supersaturated:
and a cloud forms; vapor in excess of 100%

3. Nuclei
Recall that that the atmosphere is composed of gases and aerosols
Nuclei – naturally occurring particles that promote condensation or deposition in the atmosphere
Nuclei have a radius greater than 1.0 μm - droplets grow at RH near 101%, which does occur in the atmosphere
Sources: volcanoes, forest fires, pollution, soil erosion, and sea spray

4. Nuclei Cloud Condensation Nuclei (CCN) Ice Forming Nuclei (IN)
Condensation of water vapor at temperatures above and below the freezing point of water
Ice Forming Nuclei (IN)
Formation of ice crystals at temperatures well below freezing
Freezing nuclei – water vapor condenses and freezes
Deposition nuclei – water vapor deposits directly as ice

5. Supercooled Water
Water that cools below freezing, but does not freeze (as cold as –38.2oF)
Homogeneous Nucleation
Supercooled water drops collect on a tiny ice crystal spontaneously at a temperature less than –38.2oF
Heterogeneous Nucleation
Supercooled water drops collect on a foreign particle at a temperature less than freezing, but warmer than –38.2oF

6. Classification of Clouds
General Appearance
Altitude of Cloud Base
Stratiform: high, middle, or low level
Cumuliform: clouds with vertical development
Temperature
Warm cloud > 0oC
Cold cloud at or below 0oC
Composition
Ice crystals, supercooled droplets, or water droplets

7. International Visual Cloud Classification-
• Cumulus (literally, heap or pile)
• Stratus (literally, flattened out or covered with a layer)
• Cirrus (literally, a lock of hair or a tuft of horsehair)
• Nimbus (precipitating cloud)
• Altum (height)

8. High Clouds Altitude: above 5000m - stratiform Temperature: -25oC
Composition: almost entirely ice crystals
Appearance:
Thin and wispy
Transparent to allow sunlight through
Rarely cover the entire sky
No precipitation

9. Cirrus (Ci)
Cirrostratus (Cs)
Cirrocumulus (Cc)

10. Middle Clouds Altitude: 2000-7000m Temperature: Between 0oC and -25oC
Composition: ice crystals, water droplets, or a combination of both
Appearance:
Thicker and larger than cirrus clouds
Sun is dimly visible
Completely or partially cover the sky
Rarely produce precipitation that reaches the ground

11. Altostratus (As)
Altocumulus (Ac)

12. Low Clouds Altitude: ground to 2000m
Temperature: temperatures above -5oC
Composition: mostly water droplets
Appearance:
Low lying thick gray clouds
Sun is obscured
Completely cover the sky
Generally light, but steady precipitation

13. Stratocumulus (Sc)
Stratus (St)
Nimbostratus (Ns)

14. Clouds With Vertical Development
Altitude: height of Convective Condensation Level (CCL)
generally about m
Clouds tops can be as high as 20,000m (stratosphere)
Composition: water drops, supercooled water drops, and ice crystals
Appearance:
White puffy clouds
Cotton, Cauliflower
No precipitation with “fair weather” cumulus
Significant storms with cumulonimbus

15. Cumulus (Cu)
Cumulus Congestus (CuCon)
Cumulonimbus (Cb)

16. Mountain Wave Cloud

17. Cap Cloud

18. Lee-Wave Clouds

19. Lenticular

20. Mammatus

21. Nacreous Clouds

22. Noctilucent Cloud

23. Fog Fog The air needs to be saturated for fog to develop
a cloud (stratus) in contact with the ground
Restricts visibility to 1000m or less
If this visibility restriction is not met, then it is called mist
The air needs to be saturated for fog to develop

24. Radiation Fog Air becomes saturated due to radiational cooling
Conditions for development:
Clear night sky
Light winds (calm winds would favor dew)
Humid air at the ground with dry air aloft
Generally occurs over land where rain or snowmelt has occurred the day before
Is often burned off by the sun a few hours after sunrise

25. Advection Fog Air becomes saturated due to advective cooling
Conditions for development:
Warm humid air advecting over a cold surface
The cold surface chills the air to its saturation point at the lowest layers
Warm air flowing over snow covered ground or a cold water surface (Great Lakes)

27. Steam Fog Air becomes saturated due to the addition of water vapor
Conditions for development:
In the winter when cold dry air flows over an unfrozen lake
The lower layer warms and becomes more humid due to evaporation  this mixes with the cold dry air aloft to form fog
Fog resembles smoke coming out of a smokestack

29. Upslope Fog Air becomes saturated due to expansional cooling
Conditions for development
As humid air ascends up a mountain it expands and cools, thus reaching saturation
Sometimes the fog reaches the top of the hill and spreads as a stratus cloud over a valley – this is called high fog

30. Precipitation Processes
Most clouds do not bring any rain or snow
For clouds to precipitate the cloud particles must be large enough for their terminal velocity to be greater than the updraft in the cloud
For this to happen drops need to be about 2mm in diameter, but cloud drops are only 10-20μm in diameter – so how does it rain (or snow)?

32. Warm Air Clouds Collision-Coalescence Process
Droplets that grow by colliding and then coalescing (merging) with one another
Droplets with larger diameters have a larger terminal velocity, so as they move through the cloud, they “pick up” smaller droplets
As droplets become large enough they fall out of the cloud as precipitation

33. Cold Air Clouds Bergeron-Findeisn Process
The growth of ice crystals in a cloud at the expense of supercooled water droplets
Same idea as warm air clouds  as the frozen particles grow they overtake more droplets and fall out of the cloud

35. Virga
Once a large droplet leaves the base of the cloud there is no guarantee that it will reach the surface
Often the drop will evaporate
Virga
Water or ice particles that vaporize before they reach the earth’s surface

36. Types of Precipitation
Rain
Diameters between 0.5 and 6mm
Drops break apart if diameter gets too large
Drizzle
Diameters between 0.2 and 0.5mm
Generally occurs in stratus clouds
Occur with fog and contribute to low visibility

37. Types of Precipitation
Snow
An agglomeration of ice crystals in the form of flakes
Crystals come in 4 types
Needles
Dendrites
Plates
Columns
Snow flakes vary in size, but can be as big as 5-10 centimeters in diameter
Snow Pellets: supercooled droplets that collide and freeze on an ice crystal
Snow Grains: like drizzle, except they freeze before reaching the ground

39. Types of Precipitation
Ice Pellets
Also called sleet
Snowflakes that partially or completely melt and then refreeze before hitting the ground
Freezing Rain (or drizzle)
Liquid drops that supercool and partially freeze on contact on cold surfaces at the ground
This forms a coat of ice on road, trees, and stuff
Hail
Chunks of ice
Forms in thunderstorms with strong updrafts that cause ice rock to grow
Mostly melt before hitting the surface

42. Weather Radar
A remote sensing tool for determining the location, movement, and intensity of areas of precipitation
National Weather Service uses a WSR-88D
WSR – weather surveillance radar
Reflectivity Mode
Location, movement, and intensity of areas of precipitation
Maximum range of 285 miles
Velocity (Doppler) Mode
Air motions directly toward or away from the radar associated with the circulation of the weather system
Maximum range of 143 miles

43. Doppler Effect
A shift in the frequency of sound waves emanating from a moving source
How the sound of a train or ambulance changes as it moves towards and then away from you
Doppler radar monitors the motion of precipitation toward or away from the radar
Meteorologists can detect circulations and rotations (tornados) and thus give advanced warnings

44. Reflectivity Mode
Doppler Effect

45. Radar Stuff Clear Air Mode Ground Clutter Very sensitive radar setting
Radar can detect dust particles or swarms of bugs that collect along boundaries of air masses
These boundaries are potential sites for thunderstorms development
Ground Clutter
Nearby objects (buildings, trees) that reflect back to the radar