Chapter 5

Cloud Development and Precipitation

Equlibrium

Atmospheric Stability

• Air is in stable equilibrium when after being lifted or lowered, it tends to return to its original position – resists upward and downward air motions.

• Air Parcel- balloon like blob of air• As air rises its pressure decreases and it

expands and cools• As air sinks pressure increases and it is

compressed and warms

Adiabatic Process

• If an air parcel expands and cools, or compresses and warms, with no interchange of heat with its outside surroundings the situation is called an adiabatic process.

• Dry Adiabatic lapse rate – 10˚C per 1km or 5.5˚F per 1,000 feet. (applies to unsaturated air)

• Moist Adiabatic lapse rate - ~6˚C per 1km or 3.3˚F per 1,000 ft (applies to saturated air). Not a constant. Varies greatly. This number is used to keep things simple.

Determining Stability

• Determine stability by comparing the temperature of a rising parcel to that of its surrounding environment.

• If it is colder than its environment it will be more dense (heavier) and tend to sink back to its original level. This is called stable air because the parcel resists moving away from its original position.

• If the parcel is warmer (less dense) than its environment, it will continue to rise until it reaches the same temperature of its environment. This is called unstable air because the parcel continues to move away from its original position.

Stable Air

• Environmental Lapse Rate – rate at which the air temperature of the environment would be changing if we were to climb upward into the atmosphere.

• Absolutely stable – the lifted parcel of air is colder and heavier than air surrounding it (its environment).

• Stable air strongly resists upward vertical motion, it will, if forced to rise, tend to spread out horizontally.

• Atmosphere is stable when the environmental lapse rate is small – when there is relatively small difference in temperature between the surface air and the air aloft.

• The atmosphere stabilizes as the air aloft warms or as the air near the surface cools.

Stable Air

Dry air example

Stable Air

Saturated air example

Cold surface air, on this morning, produces a stable atmospherethat inhibits vertical air motions and allows fog and haze to linger close to the ground.

Unstable Air

• Atmosphere is unstable when the air temperature decreases rapidly as we move up into the atmosphere.

• Absolutely unstable atmosphere – when considering both moist and dry air – the rising air is warmer than the environmental air around them.

• Atmosphere becomes unstable when:– Daytime solar heating of the surface– An influx of warm air brought in by the wind near the

surface– Air moving over a warm surface

Unstable Air

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Dry air example

Unstable air. The forest fire heats the air, causing instability near the surface. Warm, less-dense air (and smoke) bubbles upward, expanding and cooling as it rises.

Conditionally unstable air.

The atmosphere is stable if the rising air is unsaturated...

Conditionally Unstable Air• Suppose an unsaturated, but humid air parcel is forced to rise from

the surface.• As it rises, it expands and cools at the dry adiabatic rate until it cools

to its dew point.• The elevation above the surface where the air is saturated and

clouds form is called the condensation level.• Above the condensation level rising air cools at the moist adiabatic

rate. • Conditionally unstable atmosphere – the condition for stability being

where (if anywhere) the rising air becomes saturated. If unsaturated stable air is lifted to a level where it becomes saturated, instability may result.

(See text figure 5.7 on page 116)

When the environmental lapse rate is greater than the dry adiabatic rate, the atmosphere is absolutely unstable. When the environmental lapse rate is less than the moist adiabatic rate, the atmosphere is absolutely stable. And when the environmental lapse rate lies between the dry adiabatic rate and the moist adiabatic rate (shaded green area), the atmosphere is conditionally unstable

Cumulus clouds developing into thunderstorms in a conditionally unstable atmosphere over the Great Plains. (Note the anvil in the distance)

Level of free convection

• The level of the atmosphere where an air parcel, after being lifted, becomes warmer than the environment surrounding it. This air can then rise on its own and the atmosphere is unstable.

Convection and Clouds

• Some areas of the earth surface absorb more sunlight than others, and thus heat up more quickly. (Discuss examples)

• Thermal – a hot bubble of air that breaks away from the surface and rises, expanding and cooling as it ascends.

• As a thermal rises, it mixes with cooler, drier air aloft and gradually looses its identity. But, if it cools to its saturation point, the moisture inside will condense and the thermal becomes a cumulus cloud.

Thermals forming cumulus clouds

Four primary means of convection(ways to form clouds)

• Surface heating (thermals)

• Topographic (forced) lifting

• Convergence at the surface

• Frontal (forced) lifting

Topography and Clouds

• Orographic lift – forced lifting along a topographic barrier (mountains)

• Rain Shadow – the region on the leeward side of a mountain, where precipitation is noticeably low and the air if often drier

• Lenticular clouds – (mountain wave clouds) form on the lee side of mountains. Resemble waves that form in a river downstream from a large boulder.

• Rotor clouds – Form beneath lenticular clouds. In the large swirling eddy associated with the mountain wave, the rising part may cool and condense enough to form a cloud.

Orographic lift, cloud development, and the formation of a rain shadow

The air's stability greatly influences the growth of cumulus clouds.

The air's stability greatly influences the growth of cumulus clouds.

The air's stability greatly influences the growth of cumulus clouds.

The formation of lenticular clouds

Lenticular clouds (mountain wave clouds) over Mount Shasta in Northern California

Collision and Coalescence Process

• In clouds with tops warmer than 15oC collisions between droplets can play a significant role in producing precipitation.

• Large drops form on large condensation nuclei or through random collisions of droplets.

• As the droplets fall (larger drops fall faster than smaller drops) the larger droplets overtake and collide with smaller drops in their path.

• The merging of cloud droplets by collision is called coalescence. (Note: collision does not always guarantee coalescence)

Relative sizes of raindrops, cloud droplets and condensation nuclei

Collision and Coalescence

• In a warm cloud composed only of small cloud droplets of uniform size, the droplets are less likely to collide as they all fall very slowly at about the same speed. Those droplets that do collide, frequently do not coalesce because of the strong surface tension that holds together each tiny droplet.

Collision and Coalescence

• In a cloud composed of different size droplets, larger droplets fall faster than smaller droplets. Although some tiny droplets are swept aside, some collect on the larger droplet's forward edge, while others (captured in the wake of the larger droplet) coalesce on the droplet's backside.

Warm Clouds

• A cloud droplet rising then falling through a warm cumulus cloud can grow by collision and coalescence, and emerge from the cloud as a large raindrop.

Factors in cloud formation and raindrop production

• The cloud’s liquid water content

• The range of droplets sizes

• The cloud thickness

(heaviest precipitation occurs in those clouds with most vertical development)

• The updrafts of the cloud

• The electric charge of the droplets and the electric field in the cloud

Ice Crystal (Bergeron) Process

• Process of rain formation proposes that both ice crystals and liquid cloud droplets must co-exist in clouds at temperatures below freezing.

• This process is extremely important to rain formation in the middle and high latitudes where cloud tops extend above the freezing level (cold clouds)

The distribution of ice and water in a cumulonimbus cloud.

Supercooled water

Collison coalescence occurs here

Ice Nuclei

• Ice-forming particles that exist in subfreezing air

• Small amount of these available in atmosphere

• Clay materials, bacteria in decaying plant leaf material and other ice crystals

Saturation Vapor PressureIce vs Water

• In a saturated environment, the water droplet and the ice crystal are in equilibrium, as the number of molecules leaving the surface of each droplet and ice crystal equals the number returning. The greater number of vapor molecules above the liquid indicates, however, that the saturation vapor pressure over water is greater than it is over ice.

Ice Crystal (Bergeron) Process• The ice-crystal process. The

greater number of water vapor molecules around the liquid droplets causes water molecules to diffuse from the liquid drops toward the ice crystals. The ice crystals absorb the water vapor and grow larger, while the water droplets grow smaller.

• It takes more vapor molecules to saturate the air directly above the water droplet than it does to saturate the air directly above the crystal.

• Ice crystals grow at the expense of the surrounding water droplets.

Accretion

• In some clouds ice crystals might collide with supercooled liquid droplets. Upon contact, the liquid droplets freeze into ice and stick to the ice crystal – accretion or riming.

• The icy matter that forms is called graupel or snow pellets.

Secondary Ice particles

• In colder clouds the ice crystals may collide with other ice crystals and fracture into smaller ice particles or tiny seeds which freeze hundreds of supercooled droplets on contact.

Aggregation

• As the crystals fall, they may collide and stick to one another forming an aggregate of crystals called a snowflake.

Cloud Seeding

• To inject (or seed) a cloud with small particles that will act as nuclei, so that the cloud particles will grow large enough to fall to the surface as precipitation.

• First experiments in late 1940s using dry ice.• Silver Iodide is also used today because it’s

structure is similar to that of ice crystals.• Natural seeding – cirriform clouds lie directly

above a lower cloud deck, ice crystals descend into lower clouds.

Natural seeding by cirrus clouds may form bands of precipitation downwind of a mountain chain.

Precipitation Types

• Rain– Drizzle– Virga– Showers

• Snow– Snow grains and snow pellets– Fallstreaks– Flurries– Squalls– Blizzard

• Sleet and Freezing Rain• Hail

Rain

• Falling drop of liquid water that has a diameter equal to or greater than .5 mm (.02 in)

• Drizzle – drops too small to qualify as rain• Virga – raindrops that fall from a cloud but

evaporate before reaching the ground• Shower – intermittent precipitation from a

cumuliform cloud usually of short duration but often heavy intensity

• Acid rain – rain that is mixed with gaseous pollutants (sulfur, nitrogen) and becomes acidic

Virga

Snow

• A solid form of precipitation composed of ice crystals in complex hexagonal form

• Much of the precipitation reaching the ground actually begins as snow.

• Fallstreaks – Ice crystals and snowflakes falling from high cirrus clouds. Behave similar to Virga – fall into drier air and disappear before reaching the ground. Change from ice to vapor (sublimation)

• Flurries – light snow showers that fall intermittently for short durations. Light accumulation.

• Squall – more intense snow shower, brief but heavy snowfall.

• Blizzard – severe weather condition. Low temperatures and strong winds (greater than 30 kts) bearing a great amount of falling or blowing snow.

Fallstreaks

Dendrite snowflakes – most common form of snow.

Sleet and Freezing Rain

• Sleet – type of precipitation consisting of transparent pellets of ice 5 mm or less in diameter (ice pellets)

• Freezing Rain/drizzle – rain/drizzle that falls in liquid form and then freezes upon striking a cold object or ground. (glaze)

• Rime – an accumulation of white or milky granular ice. Formed when supercooled cloud or fog droplets strike an object whose temperature is below freezing.

Sleet forms when a partially melted snowflake or a cold raindrop freezes into a pellet of ice before reaching the ground.

Rime -An accumulation of rime forms on tree branches as supercooled fog droplets freeze on contact in the below-freezing air.

A heavy coating of freezing rain during this ice storm caused tree limbs to break and power lines to sag.

Vertical temperature profiles (solid red line) associated with snow.

Vertical temperature profiles (solid red line) associated with sleet.

Vertical temperature profiles (solid red line) associated with freezing rain.

Vertical temperature profiles (solid red line) associated with rain.

Hail

• Hailstones are pieces of ice either transparent or partially opaque, ranging in size from that of a small pea to that of a golf ball or larger.

• Produced in cumulonimbus clouds when graupel, large frozen raindrops or just about anything (insects) act as embryos that grow by accumulating supercooled liquid water droplets.

• Golf ball size hail has remained aloft for between 5 and 10 minutes.

Hailstones• Hailstones begin as embryos

(usually ice particles) that remain suspended in the cloud by violent updrafts. When the updrafts are tilted, the ice particles are swept horizontally through the cloud, producing the optimal trajectory for hailstone growth. Along their path, the ice particles collide with supercooled liquid droplets, which freeze on contact. The ice particles eventually grow large enough and heavy enough to fall toward the ground as hailstones.  

The accumulation of small hail after a thunderstorm. The hail formed as supercooled cloud droplets collected on ice particles called graupel inside a cumulonimbus cloud.

The giant Coffeyville hailstone first cut then photographed under regular light... September 1970 weighed 1.67 lbs.

Measuring Precipitation

• Rain gauge – instrument used to collect and measure rainfall.

• Trace – an amount of precipitation less than .01 in

• Snow depth – determined by measuring in three or more representative areas and taking an average.

• Water equivalent – generally about 10 inches of snow will melt down to about 1 inch of water. Varies greatly and depends on texture and packing of snow.

Standard Rain Gauge

• A nonrecording rain gauge with an 8 inch diameter collector funnel and a tube that amplifies rainfall by ten.

Tipping Bucket Rain Gauge

• The tipping bucket rain gauge. Each time the bucket fills with one-hundredth of an inch of rain, it tips, sending an electric signal to the remote recorder.

Doppler Radar

• Radar – radio detection and ranging• Used to examine the inside of clouds• Doppler Radar – a radar that determines the

velocity of falling precipitation either toward or away from the radar unit by taking into account the Doppler shift

• Doppler shift (effect) the change of frequency of waves that occurs when the emitter or the observer is moving toward or away from the other

Doppler radar display showing precipitation intensity over Oklahoma for April 24, 1999. The numbers under the letters DBZ represent the logarithmic scale for measuring the size and volume of precipitation particles

Doppler radar display showing 1-hour rainfall amounts over Oklahoma for April 24, 1999.