1. Chapter 7 Clouds, Precipitation, and Weather Radar
Weather Studies Introduction to Atmospheric Science American Meteorological Society
Chapter 7
Clouds, Precipitation, and Weather Radar
Credit: This presentation was prepared for AMS by Michael Leach, Professor of Geography at New Mexico State University - Grants

2. Case-in-Point Contrails may be affecting weather and climate
They are formed when hot, humid air from the jet’s exhaust mixes with cold, drier air at high altitudes
Exhaust turbulence causes mixing
May dissipate within minutes or hours, or they may spread laterally, formally cirrus clouds that last for a day
Effect on local radiation budget
During day, reflect solar radiation and cool Earth’s surface
At night, absorb/emit infrared radiation from below and enhance greenhouse effect by warming Earth’s surface
May stimulate local precipitation by providing ice crystal nuclei for lower clouds
Effect on climate may increase in the future with the increase in global air traffic

3. Driving Question How do clouds and precipitation form?
Clouds are products of condensation, or in the case of ice clouds, deposition, within the atmosphere
Clouds are essential to the global water cycle
Without clouds, there would be no precipitation
Yet most clouds do not yield precipitation
This chapter covers:
Development and classification of clouds and fog
Formation and types of precipitation
Weather radar
A valuable tool for monitoring air motion within weather systems
Methods for measuring precipitation

4. Cloud Formation Water vapor is invisible
Clouds are visible products of condensation or deposition of water vapor within the atmosphere
Clouds are increasingly likely to form as air nears saturation
Clouds do not form in laboratory clean air unless it is supersaturated due to the curvature effect

5. Cloud Formation The curvature effect
The curvature of a water surface affects the ability of a water molecule to evaporate from the surface
The smaller the droplet, the greater the water vapor concentration needed for droplet growth
Smaller droplets have greater curvature than large droplets
They also have the fewest surrounding molecules, so they are more weakly bonded
Water vapor more readily escapes a curved surface than a flat surface
There is more surface area on a curved surface
Saturation vapor pressure is greater surrounding a small droplet vs. a large one
For droplets with a radius of micrometer, saturation vapor pressure is up to 3.4 times greater than for larger surface → this equates to 340% relative humidity
For droplets with a radius > 1.0 micrometer, RH only slightly higher than 100% is required
What then makes cloud formation possible?

6. Cloud Formation Role of Nuclei
Outside the laboratory, clouds form readily as the relative humidity nears 100%
Suspended in the air are solid or liquid particles, known as nuclei, which provide a relatively large surface area for condensation or deposition to occur
Nuclei have radii > 1 micrometer
When condensation starts on the surface of the nuclei, they become comparably sized cloud droplets (or ice crystals) and additional growth is more likely
Nuclei are essential for cloud formation
They are abundant within the atmosphere
Sources include volcanic eruptions, wind erosion of soil, forest fires, and ocean spray

7. Cloud Formation Role of Nuclei, continued Types of nuclei:
Cloud condensation nuclei (CCN) promote condensation at temperatures both above and below the freezing point of water
Cloud droplets can remain liquid even at temperatures well below 0 °C (32 °F) - supercooled droplets
Ice-forming nuclei (IN) promote ice crystal formation at temperatures well below freezing
Freezing nuclei – surfaces on which water first condenses, then freezes
Active at temperatures below about -9 °C (16 °F)
Deposition nuclei – surfaces on which ice deposits directly from vapor
Not fully active until temperature is below -20 °C (-4 °F)

8. Cloud Formation Role of Nuclei, continued Types of nuclei, continued:
CCN are much more common than IN
Hygroscopic nuclei – special category of CCN
They have a chemical attraction for water molecules
Clouds form more readily in their presence, for example, magnesium chloride in ocean spray can promote condensation at relative humidity as low as 70%
Many sources exist in urban-industrial areas
Cities also spur clouds and precipitation development by contributing water vapor (raising the relative humidity) and adding heat (adding to the buoyancy of air)
Rainfall has been shown to be greater downwind from an urban-industrial area

9. Cloud Formation Supercooled water
Supercooling = fresh water cooled below its freezing point remains liquid
Within very small droplets, kinetic energy is sufficient to prevent growth of ice embryos to the critical size needed for continued growth and freezing
Hence, water droplets can remain liquid even at temperatures well below freezing
As droplet temperature falls, the probability of an ice embryo growing to critical size increases because average kinetic molecular activity decreases

10. Cloud Formation Supercooled water, continued Homogeneous nucleation
The formation of ice embryos of critical size due to the chance aggregation of water molecules
If no foreign particles acting as IN are present, a cloud droplet can cool to as low as -39 °C (-38.2 ° F) without freezing (Schaefer point)
Additional cooling leads to homogeneous nucleation
Heterogeneous nucleation
If a supercooled droplet contains (or comes in contact with) foreign particles that are IN, the cloud droplet will freeze at a temperature below freezing, but well above the Schaefer point
Water molecules collect on the IN and form an embryo that is close enough to critical size that additional growth causes the droplet to freeze

11. Cloud Classification Clouds are classified based on:
General appearance
Cirriform – wispy, fibrous
Stratiform – layered
Cumuliform – heaped, or puffy
Altitude of cloud base
High, middle, or low
Temperature
Warm cloud – 0 °C (32 ° F)
Cold cloud – at or below 0 °C (32 ° F)
Composition
Corresponds to temperature
See Table 7.1 (next slide)

12. Cloud Classification

13. Cloud Classification

14. High Clouds
Bases above 5000 m (16,000 ft) where average temperatures are typically below -25 °C (-13 °F)
Clouds are composed almost exclusively of ice crystals and have fibrous appearance
Names include the prefix cirro
cirrus
cirrostratus
cirrocumulus

15. Middle Clouds
Bases between 2000 and 5000 m (6600 and 16,000 ft) where average temperatures are typically between 0 °C and -25 °C (32 °F and -13 °F)
Clouds are composed of supercooled water droplets or a mixture of droplets and ice crystals
Names include the prefix alto
altostratus
altocumulus

16. Low Clouds
Bases from Earth’s surface (fog) to 2000 m (6600 ft) where average temperatures are typically above -5 °C (23 °F)
Clouds are composed of mostly water droplets
Include stratocumulus, stratus, and nimbostratus
Usually only drizzle falls from stratus, but significant amounts of rain or snow may fall from thicker nimbostratus
stratocumulus
stratus

17. Clouds Having Significant Vertical Development
cumulus
cumulus congestus
cumulus clouds on a
visible satellite image
cumulonimbus

18. Sky Watching
Cumuliform and stratiform clouds are often observed in the sky together
Clouds at different levels may move in different directions at different speeds
Caused by wind shear – change in wind direction or speed with distance
Strong vertical shear in horizontal wind speed or direction may cause cloud streets to form
Cloud holes may be formed by aircraft
Turbulence may cause expansional cooling of supercooled droplets
Droplets freeze to ice crystals that fall out of the cloud

19. Unusual Clouds
Stationary disk-shaped clouds (altocumlus lenticularis) are generated when a prominent mountain range disturbs large scale winds
Mountain-wave clouds – lenticular clouds situated over the mountain
Lee-wave clouds - lenticular clouds formed in the wave crests downwind of the mountain

20. Unusual Clouds
In a stable atmosphere, waves and wave-type clouds can develop along the interface between two layers of air moving horizontally at different speeds
In the figure, a layer of relatively warm air moving horizontal more rapidly overlies a colder, denser air layer
Vertical shear in the horizontal wind creates Kelvin-Helmholtz waves along the boundary
Billow clouds are formed in the wave crests

21. Unusual Clouds
Noctilucent clouds form in the upper mesosphere above an altitude of about 80 km (50 mi)
Seldom observed, and then only at high latitudes
Key to formation is exceptionally low temperatures; ice clouds can form at extremely low vapor pressures
Water vapor can be from volcanic eruptions or chemical reactions involving methane
Nacreous clouds occur in the stratosphere at altitudes of 20 to 30 km (12 to 19 mi) and form on sulfuric acid nuclei

22. Fog Fog is simply a cloud in contact with the ground Types:
Restricts visibility to 1000 m (3250 ft) or less
Otherwise, the suspension is called mist (light drizzle)
Types:
Radiation – radiational cooling causes humid air near the ground to saturate

23. Fog Types: Advection fog
The advecting air passes over a relatively cool surface causing condensation to occur as the air cools
This happens in the summer over the surface of the Great Lakes

24. Fog Types of fog: Steam fog Also called Arctic sea smoke
Develops in late fall or winter when extrememly cold and dry air flows over a large unfrozen body of water
Evaporation and sensible heating cause the lower portion of the air mass to become more humid and warmer than the air above
The air is destabilized, and the consequent mixing of mild, humid air with cold, dry air causes saturation and fog formation
It looks like steam or smoke coming off the water

25. Fog
Types of fog:
Upslope fog – fog develops on mountainsides or hillsides as the ascending humid air reaches saturation

26. Precipitation Processes
Precipitation is water in solid or liquid form that falls from clouds to Earth’s surface under the influence of gravity
Terminal velocity
Terminal velocity is defined as the constant downward-directed speed of a droplet (or other particle within a fluid) due to a balance between gravity acting downward and air (or fluid) resistance acting upward
Helps explain why tiny water droplets and ice crystals composing clouds will not fall as precipitation unless they are sufficiently large
The two basic methods for cloud particle growth are collision-coalescence (warm clouds) and the Bergeron-Findeisen process (cold clouds)

27. Precipitation Processes
The terminal velocity of a particle falling through the air increases with the size of the particle

28. Precipitation Processes
Warm-Cloud Precipitation (collision-coalescence process)
Warm cloud = cloud at temperatures > 0 °C
Droplets may grow by colliding and coalescing with one another
Takes place in a cloud with a mixture of droplet sizes, ideally with some having diameters of at least 20 micrometers
Laboratory simulations demonstrate that colliding droplets will not coalesce unless they are of significantly different sizes
Eventually, the droplets become large enough that their terminal velocity overcomes updrafts
Then, precipitation will occur
A key factor is collision efficiency
This is the fraction of all droplets in the path of the falling larger droplet that come into contact with the larger droplet
Collision efficiency increases with the size of the larger droplet

29. Warm-Cloud Precipitation
The relatively large droplet falls through a cloud of much smaller droplets
The larger drop falls faster and collides with the smaller droplets in its path
The larger drop grows via coalescence
This is the collision-coalescence process

30. Precipitation Processes
Cold-Cloud Precipitation (Bergeron-Findeisen process)
Cold cloud = clouds or portions of clouds at temperatures < 0 °C
Most middle and high latitude clouds form precipitation in an environment of supercooled water and ice crystals
At sub-freezing temperatures, water molecules more readily vaporize from a liquid water surface than an ice surface because water molecules have stronger bonds in the solid phase
At the same sub-freezing temperature, the saturation vapor pressure is greater over water than over ice (see next slide showing Table 6.3)
A vapor pressure that is saturated for water droplets is actually supersaturated for ice crystals, hence the water vapor deposits on the ice and the crystals grow at the expense of surrounding water droplets

31. Precipitation Processes
Bergeron-Findeisen process, cont.
Terminal velocity increases with increased ice crystal size
Larger crystals overtake, collide, and agglomerate with smaller crystals and water droplets
May fall out of cloud base when large enough, and may reach Earth’s surface as snow or rain depending on ambient air temperatures below the cloud

32. Precipitation Processes
Once a falling raindrop or snowflake leaves the base of a cloud, it enters unsaturated air and begins to vaporize
Amount that vaporizes increases with cloud base height and decreased relative humidity of ambient air
Virga is streaks of water and ice crystals falling from a cloud that mostly vaporize before reaching Earth’s surface

33. Forms of Precipitation
Precipitation – water in solid or liquid form that falls from clouds to the Earth’s surface
Occurs in many forms
Liquid (rain, drizzle)
Freezing (freezing rain and freezing drizzle)
Frozen (snow, snow pellets, snow grains, ice pellets, hail)
Liquid precipitation
Rain (diameters of 0.5 – 6 mm or 0.02 to 0.2 in.)
Flattened spheres (not teardrop shaped)
Unstable at larger diameters and break apart
Fall mostly form nimbostratus and cumulonimbus clouds
Drizzle (diameters of 0.2 – 0.5 mm or 0.01 to 0.1 in.)
Originates mostly in stratus cloud and has limited growth by collision-coalescence

34. Forms of Precipitation
Frozen precipitation
Snow (aggregate diameters can reach 5 – 10 cm or 2 – 4 in.)
An agglomeration of ice crystals in the form of flakes
Vary in size depending on water vapor concentration and the temperature in the portion of the cloud where they grow

35. Forms of Precipitation
Frozen precipitation, continued
Snow pellets (diameters of 2 – 5 mm or 0.08 – 0.2 in.)
Soft conical or spherical white ice particles
Form when supercooled cloud droplets collide with an ice crystal and freeze
Snow grains (diameters < 1 mm or 0.04 in.)
Flat or elongated opaque white ice particles
Form in similar way to drizzle, but freeze prior to reaching the ground
Ice pellets (sleet) (diameters < 5 mm or 0.02 in.)
Spherical or irregularly shaped transparent or translucent ice particles
Form when snowflakes partially or completely melt below cloud base and then refreeze into ice particles before striking the ground

36. Forms of Precipitation
Frozen precipitation, continued
Freezing rain (freezing drizzle)
Supercooled liquid precipitation that freezes (totally or partially) on contact with subfreezing objects
Forms a coating of ice (glaze) on exposed surfaces
Sounding favorable to freezing rain formation

37. Forms of Precipitation
Frozen precipitation, continued
Hail
Characterized by concentric, onion-like layers of ice
Develops within severe thunderstorms characterized by vigorous updrafts, abundant supercooled water droplet supply, and great vertical cloud development
Updrafts lift ice pellets into higher portion of cloud, they grow by collecting supercooled droplets and eventually may exit the cloud base

38. Forms of Precipitation
Schematic cross-section of the lower atmosphere showing temperature conditions required for the formation of frozen, freezing, and liquid forms of precipitation

39. Acid Deposition Rain and snow are naturally slightly acidic
They dissolve some atmospheric CO2, producing weak carbonic acid H2CO3
When air is polluted with oxides of sulfur and oxides of nitrogen, acid rain may form
These gases interact with moisture in the atmosphere to form tiny droplets of sulfuric acid H2SO4 and nitric acid HNO3
These acids dissolve in precipitation and may increase its acidity by as much as 200 times
Without precipitation, sulfuric acid droplets convert to acidic aerosols that reduce visibility and may cause human health problems – acidic aerosols may settle to ground as dry deposition
Acid deposition is the combination of acid precipitation and dry deposition

40. Acid Deposition
pH Scale (note that scale is logarithmic)
An acid is a hydrogen-containing compound that releases hydrogen ions (H+) when it dissolves in water
An alkaline substance releases hydroxyl ions (OH-) when dissolved in water
Acidity and alkalinity are expressed in terms of pH, a measure of the hydrogen ion concentration
Rain with a pH < 5.6 is considered acid rain

41. Weather Radar: Locating Precipitation
Radar is an acronym (radio detection and ranging)
Emits microwave signals and receives reflected signals from targets as it continually scans a large volume of the lower atmosphere
Can detect potentially dangerous weather, such as a tornado’s circulation within a parent thunderstorm
Monitors rainfall rates and cumulative rainfall totals
Widespread use began after WWII
Mid-1950s, Congress allocated funds to purchase long-range radar units for meteorological purposes following several tornado and hurricane disasters
1959, coastal hurricane radar stations established (WSR-57)
Late 1960s – common to television weather reporting
1974 – major upgrade (WSR-74)
1990s – implementation of Doppler radar network (WSR 88-D)
Operates in a reflectivity or velocity mode
Reflectivity – detects location, movement, and intensity of areas of precipitation
Velocity – detect motion directly toward or away from the radar unit

42. Weather Radar: Locating Precipitation
Reflectivity mode
WSR 88-D emits short pulses of microwave energy with wavelengths of 10 to 11.1 cm
At these wavelengths, radar pulses are scattered readily by rain, snow, or hail, but not significantly by cloud droplets
Falling precipitation reflects some of radar signal back to a receiving unit, where it is processed and displayed on a computer screen as a radar echo
Some radar echoes may be caused by fixed objects on ground (ground clutter) – typically this is electronically subtracted from display

43. Weather Radar: Locating Precipitation
A radar reflectivity product in which echo
intensity is graduated by color
The radome for a WSR-88D unit houses a
rotating radar dish antenna

44. Weather Radar: Locating Precipitation
Doppler Effect
The shift in frequency of sound or electromagnetic waves that accompanies the motion of the wave source(s) or wave receiver
(A) = The sound wave source is stationary and wave frequency is uniform everywhere
(B) = The wave source is in motion so that wave frequency is greater ahead of the source than behind the source

45. Weather Radar: Locating Precipitation
Velocity (Doppler) mode
Determines horizontal air motions within a weather system
There is a shift in frequency of sound and electromagnetic waves coming from a moving source
Doppler radar can detect this frequency shift, and displays it as motion
Doppler radars monitor movement of precipitation particles directly toward or away from the radar unit
Doppler radar allow advance notification of severe approaching weather

46. Weather Radar: Locating Precipitation
Doppler image – greens and blues indicate motion directly towards the radar, while reds and yellows indicate motion directly away from the radar

47. Measuring Precipitation
Rainfall and snowfall are routinely measured in terms of accumulation depth over a specified time interval, usually hourly and every 6 hrs and 24 hrs
Measurements are made with gauges or remotely by weather radar or satellite sensors
Standard non-recording rain gauge
Cylinder with a cone-shaped funnel opening
Can resolve rainfall into increments as small as 0.01 in. (0.25 mm)
A graduated stick measures depth of water in cylinder
Rainfall is measured manually at a fixed time and then the gauge is emptied
Weighing-bucket rain gauge
calibrates the weight of rainwater in terms of water depth
Marks a chart on a clock-driven drum that sends an electronic signal to a computer
May melt frozen precipitation with antifreeze or a heater

48. Measuring Precipitation
A standard NWS non-recording rain gauge
Tipping-bucket rain gauge
Consists of a free-swinging container partitioned into 2 compartments, each of which can collect 0.01 in. of rainfall
Each compartment alternately fills with water, tips, and spills its contents, tripping an electronic switch that marks a chart or sends a signal to a computer
NWS will be replacing heated tipping-bucket gauges with weighing bucket gauges
A new weighing-bucket rain gauge

49. Measuring Precipitation
Snowfall Measurement
Meteorologists are interested in the depth of snow that falls over a certain time period, the melt water equivalent of the snowfall, and the depth of snow on the ground at observation time
New snowfall accumulates on a board placed on top of the old snow cover and is measured with a ruler
The melt water equivalent can be determined by weighing-bucket gauge measurements or melting snow in a non-recording gauge
As a very general rule, 10 cm of fresh snow melts to 1 cm of water
Actual ratio depends on crystalline form of snowflakes and the temperature of the air through which the snow falls
The depth of snow on the ground is determined by an average of ruler measurements at several locations

50. Measuring Precipitation
Remote Sensing – Weather Radar
Reflectivity is proportional to raindrop surface area and rainfall rate is proportional to the volume of the raindrops
Reflectivity data is converted to rainfall rates by estimating raindrop size distribution
Remote Sensing – Satellite
Tropical Rainfall Measuring Mission (TRMM) satellite uses radar, a microwave imager, and a visible/IR scanner to estimate rainfall
Cumulative rainfall as determined by a computer analysis of radar echoes