class #4: stability, cloud development, and precipitation

Class #4 Monday, July 12, 2010 1

Class #4: Stability, cloud development, and precipitation

Chapters 6 and 7

Stability & Cloud development

Chapter 6

Fig. 6-CO, p. 140

Fig. 6-1, p. 142

Importance of Clouds

• Release heat to atmosphere• Help regulate energy balance• Indicate physical processes

Atmospheric Stability

• Clouds from as air rises and cools• Adiabatic processes: change in temperature

without giving or removing– Dry rate = 10°C/1000m– Moist rate = 6°C/1000m

• Stability is a state of equilibrium in terms atmospheric movement; no vertical movement occurs

Determining Stability

• Warm air rises or is unstable• Cool air sinks or is stable• Compare air parcel lapse rate to

environmental lapse rate

Table 6-1, p. 143

• Stable environment– Environmental lapse rate less than moist lapse

rate– If an air parcel is forced it will spread horizontally

and form stratus clouds– Usually a cool surface (radiation, advection)– Inversion: warm over cool.

Fig. 6-6, p. 145

• Special Topic: Subsidence Inversions– Strong subsidence exacerbates air pollution due to

the lack of vertical motion. – Pollution is not diluted.

• An Unstable Atmosphere– Environmental lapse rate greater than the dry

adiabatic lapse rate– As air parcel rises it forms a vertical cloud– Convection, thunderstorms, severe weather

• A Conditionally Unstable Atmosphere– Moist adiabatic lapse rate is less than the

environmental lapse rate which is less than the dry adiabatic lapse rate

– Stable below cloud unstable above cloud base– Atmosphere usually in this state

Fig. 6-13, p. 149

Fig. 6-14, p. 149

Fig. 1, p. 150

• Causes of Instability– Cool air aloft (advection, radiation cooling in

clouds)– Warming of surface (insolation, advection, warm

surface)

Cloud Development

• Clouds develop as an air parcel rises and cools below the dew point.

• Usually a trigger or process is need to initiate the rise of an air parcel.

Cloud Development

• Convection– Differential land surface heating creates areas of

high surface temperature.– Air above warm land surface heats, forming a

‘bubble’ of warm air that rises or convection.– Cloud base forms at level of free convection.

Class #4 Monday, July 12, 2010 32Stepped Art

Fig. 6-16, p. 152

Fig. 2, p. 155

Cloud Development

• Topography– Orographic uplift– Orographic clouds– Windward, leeward, rain shadow– Lenticular clouds

Cloud Development

• Topic: Adiabatic charts– Adiabatic charts show how various atmospheric

variables change with height: pressure, temperature, humidity.

Fig. 3, p. 158

Fig. 4, p. 158

Fig. 5, p. 158

Fig. 6, p. 159

Fig. 7, p. 159

Cloud Development

• Changing cloud forms– Stratus clouds can change to cumulus clouds if the

top of the cloud cools and the bottom of the cloud warms.

– Alto cumulus castellanus: towers on alto stratus– If moist stable air without clouds is mixed or

stirred it can form stratocumulus clouds.

Precipitation

Chapter 7

Fig. 7-1, p. 166

Fig. 7-2, p. 166

Fig. 7-3, p. 167

Table 7-1, p. 168

Precipitation Processes

• Precipitation is any form of water that falls from a cloud and reaches the ground.

• How do cloud drops grow?– When air is saturated with respect to a flat surface

it is unsaturated with respect to a curved droplet of water.• Super saturated

• Collision & Coalescence– Droplets of different sizes collide and coalesce into

larger droplets; warm cloud process– Ice-Crystal Process• Cold clouds a mixture of ice & water• Ice crystals grow at expense of surrounding water

droplets• Saturation vapor pressure greater over water as

compared to ice.

Fig. 7-5, p. 169

Fig. 1, p. 171

Fig. 7-10, p. 173

• Topic: Freezing of Cloud Droplets– Spontaneous or homogeneous freezing– Ice embryo

• Cloud Seeding– Inject cloud with small particles that act as

condensation nuclei, starting the precipitation process.

– NEED CLOUDS: seeding does not generate clouds– Cold clouds with a low seed ration best– Dry ice, silver iodide

Fig. 7-12, p. 174

Precipitation in Clouds

• Starts quickly• Most Precipitation formed through accretion• Many times rain starts as ice

Fig. 7-12, p. 174

Stepped Art

Fig. 7-12, p. 174

Precipitation Types

• Rain: falling drop of liquid water– Drizzle less than 0.5 mm– Virga– Cloudburst

• Snow: frozen water falling from sky (crystal or flake)– Most precipitation starts as snow– Freezing level, snow & cloud appearance, fall streaks,

drifting snow, blizzard– A blanket of snow is a good insulator

Table 7-2, p. 176

Fig. 2, p. 177

Fig. 7-16, p. 178

Fig. 7-17, p. 178

Table 7-3, p. 178

Fig. 3, p. 179

Fig. 7-18, p. 180

Table 7-4, p. 180

Precipitation Types

• Topic: Tear Drops– Raindrops not tear shaped– Shape is size dependent• Less than 2 mm = sphere• Greater than 2 mm = flattened sphere

Precipitation Types

• Topics: Sounds and snow– A blanket of snow will act like an acoustic tile and

absorb sound waves.• Topics: Snow with Temperature above

Freezing– Unsaturated wet bulb temperature below or equal

to 0°C, rain cooled by evaporation forms snow despite environmental temperature above freezing.

Precipitation Types

• Sleet: air below freezing, then travels through a layer of air above freezing, begins to melt and then falls through a layer of air below freezing just above the ground surface.

• Freezing Rain: ground surface is freezing as rain hits the surface it freezes.

Fig. 7-21, p. 182

Fig. 7-22, p. 182

Fig. 7-23, p. 182

• Precipitation is any form of water that falls from a cloud and reaches the ground.

• How do cloud drops grow?– When air is saturated with respect to a flat surface

it is unsaturated with respect to a curved droplet of water.• Super saturated

Precipitation Types

• Observation: Aircraft Icing– Aviation hazard is created by the increase in

weight as ice forms on the body of the airplane.– Spray plane with anti-freeze.

Precipitation Types

• Snow Grains: solid equivalent of drizzle, no bounce or shatter

• Snow Pellets: larger than grains, bounce, break, crunch underfoot

• Graupel: ice particle accumulation with rime• Hail: graupel act as embryo in intense

thunderstorm, grow through aggregation as pushed up by updraft.

Fig. 7-29, p. 185

Measuring Precipitation

• Instruments– Rain gauge: standard, tipping bucket, weighing• Snow: average depth at 3 locations, 10:1 water

equivalent

• Doppler Radar– Transmitter generates energy toward target,

returned energy measured and displayed• Brightness of echo = amount/intensity of rain

– Doppler: measures speed of horizontal rain

Fig. 7-33, p. 188

Measuring Precipitation

• Measuring from space– Specific satellites designed to assess clouds,

atmospheric moisture, and rain• TRMM• CloudSat

class #4: stability, cloud development, and precipitation

stable air

air pollution

dense air

stabilitywarm air

rising air parcel

cold surface air

precipitation class

unstablecool air sinks

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