Download Chapter 5: Forms of Condensation and Precipitation Clouds

Chapter 5: Forms of Condensation and Precipitation
Clouds – visible aggregate of minute droplets of water or tiny crystals of ice or a mixture of both
Cloud formation: major factor is adiabatic cooling reaching saturation level
Lifting condensation level – height at which a rising parcel cools to the dew point
Condensation has two requirements
Air must be saturated (or nearly so)
There must be a surface on which condensation can occur
On the ground, blades of grass serve as the surface for dew formation
In the air there must be small particles of dust
Cloud condensate nuclei: The name for small surfaces suspended in the air on which
condensation can form
Especially near oceans, salt forms the major hygroscopic nuclei particles
Suspended salt occurs from evaporation of sea spray droplets
Dust storms, volcanic eruptions, and pollen are major sources of particles on land
Hygroscopic nuclei – water seeking particles that allow condensation to begin
Hygroscopic nuclei can begin to form water droplets at a relative humidity below 100%
Water droplets on hygroscopic particles tend to grow quickly and become large
Hydrophopic nuclei – water repelling particles that allow condensation to begin
Hydrophobic nuclei tend to form droplets only when the relative humidity is 100%
Water droplets on hygrophobic particles tend to grow slowly and are generally smaller
Clouds are classified on the basis of two criteria
Form
Cirrus – high, white, and thin (cirrus is from the Latin word meaning curl or filament)
They are separated or detached and form delicate veil-like patches or extended wispy
fibers that often have a feathery appearance
Cumulus – globular, individual cloud masses
Normally they have a flat base and appear as rising domes or towers and are frequently
described as having a cauliflower-like structure
Stratus – sheets or layers that cover much or all of the sky
While there may be minor breaks, these clouds have no distinct individual units
Height
High – above 6000 meters (20 000 feet) usually ice
Cirrus – wispy horsetails
Cirrostratus – transparent, fibrous or smooth, forming halos around the sun or moon
Cirrocumulus – white patches composed of very small cells or ripples (mackerel sky)
Middle – 2000 to 6000 meters usually water droplets
Altocumulus – large patches of rounded masses or rolls that may or may not merge
cells usually have a more distinct outline because they are formed of water droplets
instead of ice
Altostratus – formless layer of grayish clouds covering all or most of the sky
the sun is usually visible through the cloud layer with indistinct limbs
Low – below 2000 meters (6500 feet)
Stratus – a uniform layer that frequently covers much of the sky and occasionally
produces light precipitation
Stratocumulus – stratus clouds that develop a scalloped bottom or long parallel rolls or
broken globular patches
Nimbostratus – from the Latin nimbus (rain cloud) and stratus (to cover with a layer)
as the name implies, these clouds are one of the major precipitation producers and
start as middle clouds (from thickening altostratus) but lowers into low category as
precipitation continues
Clouds of vertical development – have bases in the low height range but extend upward
into the middle or high altitudes
Cumulus – individual masses that develop domes or towers
Usually form on clear days when unequal surface heating causes parcels of air to rise
convectively above the lifting condensation level and are obvious because of their
flat cloud bottom
Often called ‘fair weather clouds’
Cumulonimbus – dark, dense, billowy clouds of considerable vertical extent forming
huge towers and often exhibit an anvil head
The tops of these clouds turn to ice and appear fibrous
Cloud varieties extend the 10 basic cloud types with adjectives
(See Cloudwise from NOAA at http://www.srh.noaa.gov/srh/jetstream/clouds/cloudwise/types.html or
Jetstream Online School for Weather http://www.srh.noaa.gov/jetstream/clouds/m2.htm for better pictures)
Uncinus – hook-shaped
Cirrus uncinus appear like commas resting on their sides and are bad weather precursors
Fractus – broken into smaller pieces
Mammatus – having rounded protuberances on the under side
Associated with stormy weather in cumulonimbus clouds
Lenticular – lens shaped
Most frequently, lenticular altocumulus clouds form on the lee side of mountains as air
forms waves that lift crests above the lifting condensation level
Fog – a cloud with its base at or very near the ground
Fog formed by cooling: occurs when the temperature of the air close to the ground falls below
the dew point
Radiation fog – results from radiative cooling of the ground and adjacent air
Calm air results in patchy fog less than a meter deep
Light breezes up to 3 to 5 km/hr create enough turbulence to carry the fog 10 to 30 meters
upward without dispersing the fog
Air containing radiation fog is cool and dense and tends to flow downhill or into valleys
These fogs usually dissipate 1 to 3 hours after sunrise
Fog lifts: actually, radiation fog does not lift, rather the sun heats the ground and air just
above it resulting in temperatures above the dew point – the warm level of the air
increases in depth until the fog is gone
Advection fog – results from air moving horizontally over a cold surface below
A certain amount of turbulence is, again, necessary for development of advection fog
Winds between 10 and 30 km/hr because turbulence facilitates cooling through a thicker
layer of air and also lofts the fog to greater heights
Upslope fog – results from relatively humid air moving up a gradual sloping plain or, in
some cases, up a steep mountainside
A certain amount of turbulence is, again, necessary for development of
Fog formed by evaporation
Steam fog – if cool air moves over warm water, enough moisture may evaporate to saturate
the air immediately above the water and when rising meeting cold air above condenses to
form fog that looks like steam
An extreme example called arctic sea smoke occurs when very cold arctic air moves over
quite warm ocean water (sometimes as much as a 30°C temperature difference)
Frontal fog – (sometimes called precipitation fog) if warm air rising over fairly humid cool
air drops rain into the cooler air, enough rain can evaporate to saturate the cooler air and
form fog
Dew and frost
Dew – the condensation of water vapor on objects that have cooled enough to drop below the
dew point of the surrounding air
White frost – (sometimes called hoar frost) forms when the dew point of the air is below
freezing so that deposition occurs
How precipitation forms
Cloud droplets are very tiny – typically 0.02 mm (20 μm: compare to human hair at 75 μm)
This occurs because condensation nuclei are abundant and available water is distributed
among numerous droplets instead of concentrating into fewer large droplets
Cloud droplets are so tiny they fall incredibly slowly
It would take several hours to fall 1000 m
But droplets this size would evaporate in just a few meters
Typical raindrops are 2 mm in diameter – in order to fall, droplets must increase volume by a
factor of 1 000 000 times
Two processes account for droplet accumulation to form precipitation
The Bergeron Process (precipitation from cold clouds) typical for middle latitudes
Even on sweltering days, the temperature at cloud tops can be –50°C
The Bergeron process depends upon the co-existence of water vapor, liquid cloud-droplets,
and ice crystals
Cloud droplets do not freeze at 0°C as might be expected
Pure water suspended in the air typically supercools and will not freeze until –40°C
Alternately, supercooled water will freeze it if impacts an object
This explains why planes flying through liquid cloud droplets ice up
It also explains why falling liquid water drops instantly freeze when they hit tree
branches or the ground (ice storms)
Supercooled water will also freeze on contact with solid particles that have a crystal
structure similar to that of ice (AgI, for example)
Such materials are called freezing nuclei (which are a requirement just as condensation
nuclei are required for condensation during fog formation)
In contrast to condensation nuclei, freezing nuclei are rare and do not become active
until the temperature reaches –10°C with 100% saturation levels of humidity
100% relative humidity with respect to liquid water at –10°C is the same as 110%
relative humidity with respect to ice
Given these facts, a cloud at –10°C where each ice crystal is surrounded by many
thousands of liquid droplets and with air at 100% saturation wrt water is supersaturated,
110% wrt ice
Under these conditions the ice crystals will collect more water from surrounding droplets
than they lose to sublimation
This situation will generate ice crystals that become large enough to fall
During the descent of such ice crystals, these crystals will enlarge as they intercept cloud
drops that freeze on them
Air movement breaks up these delicate crystals which then act as freezing nuclei for other
liquid droplets – a chain reaction that produces many snow crystals
Accretion of 10 to 30 snow crystals forms a large mass called a snowflake
If the surface temperature is above 4°C, these snowflakes melt and continue to fall as rain
The Collision–Coalescence Process (precipitation from warm clouds) typical for the tropics
Clouds made entirely of liquid droplets must contain some droplets larger than the typical
20 μm if precipitation is to form
Large droplets form only when ‘giant’ condensation nuclei are present or when
hygroscopic particles (such as sea salt) exist (which begin to form droplets below
100% relative humidity)
Because rate of fall is size dependent, these ‘giant’ droplets fall most rapidly
As ‘giant’ droplets fall through a cloud they coalesce with the smaller, slower droplets and
become even larger causing them to fall faster and grow even larger
If such large droplets can collect the equivalent of a million normal droplets they will
become large enough to fall to the surface without evaporating
This large number of required collisions requires clouds that have great vertical thickness
and updrafts will help add to this process
Remember, larger drops fall faster but higher speeds increase friction with the air
At a typical 2 mm size, raindrops begin to flatten on the bottom
At 4 mm, raindrops form depressions
At 5 mm, raindrops will form a toroid (donut), and break apart into many smaller drops
These smaller drops will updraft and build the chain reaction to form a rainstorm
Forms of precipitation
Rain
Rain – drops of water that fall from a cloud and have a diameter of at least 0.5 mm
(This size excludes drizzle and mist which both have smaller droplets)
Much of the world’s rainfall begins as snow, graupel, or hail which melts before reaching
the surface
Warm cloud rain (especially over the ocean) has fewer condensation nuclei thus forming a
range of droplet size where raindrops quickly form by collision-coalescence
Drizzle – fine, uniform drops of water having a diameter less than 0.5 mm
Often formed by stratus or nimbostratus clouds where precipitation may occur for hours
Mist – precipitation containing the finest droplets able to reach the ground
These tiny droplet appear to float and their impact is almost imperceptible
Virga – rain that enters unsaturated air and evaporates before reaching the ground
Snow
Snow – precipitation in the form of snowflakes or, more often, aggregates of ice crystals
The size, shape, and concentration of snowflakes depend to a great extent on the
temperature at which they form
At very low temperatures moisture content is very low resulting in small, individual, sixsided ice crystals called ‘powder’
At temperatures warmer than about –5°C, ice crystals join together into clumps of
composite snowflakes
These flakes tend to be heavy and high in moisture content (ideal for snowmen and
snowballs)
Sleet and Glaze
Sleet – the fall of small particles of ice that are clear to transparent
Sleet requires an above-freezing layer of air overlying a subfreezing layer of air near the
ground
Glaze – or freezing rain occurs when an above-freezing layer of air overlies a thin
subfreezing layer of air near the surface so that the raindrops do not have time to freeze
Hail – precipitation in the form of hard, rounded pellets of irregular lumps of ice
Large hailstones, when cut in half, reveal nearly concentric shells of ice of differing densities
and degrees of opaqueness
The layers of ice accumulate as the hailstone travels up and down in a strong convective
cloud
Clear layers form in warmer parts of the cloud where liquid water accumulates and freezes
slowly while rapid freezing of supercooled droplets in cold parts of a cloud traps air
bubbles giving that layer a milky appearance
Most hail is 1 – 5 cm in diameter (pea to golf ball size)
The heaviest authenticated hailstone was 14 cm 766 g (1.67 lb)
Hail is only produced in large cumulonimbus clouds where updrafts can reach speeds of
160 km/hr
Rime – a deposit of ice crystals formed by the freezing of supercooled fog or cloud droplets on
objects whose surface temperature is below freezing
Rime has a unique, feathery appearance
Wind may cause rime to form only on the windward side of objects
Precipitation measurement
Standard instruments
Standard rain gauge – a 20 cm funnel opening narrows to a tube with a cross section onetenth that size thereby magnifying the depth of the rain to increase accuracy and allowing
the measurement of rain depth of 0.01 inches
Trace of precipitation: report made for rain depths less than 0.01 inches
Tipping-bucket rain gauge – similar to a standard gauge for collection but contains see-saw
dual bucket that will tip each time 0.01 inches of rain is collected the data being recorded
on a drum chart (or electronically) so that both amount and rate of rainfall can be
recorded
Weighing gauge – another rate recording gauge that works by collecting rain in cylinder that
rests on a spring balance the data being recorded in real time on a chart
Measuring snowfall
Two measurements are required to record snowfall accurately
Depth
Usually measured with a calibrated stick but best done with several averaged values
taken in an area free of obstructions to prevent errors caused by drifting
Water equivalent
Usually measured by melting snow and weighing the resulting water mass
Measurement errors
Rain gauges
These can measure low for multiple reasons
Splashing: some rain splashes out of the gauge
Wetting: some water doesn’t run down the funnel
Evaporation: in dry climates, evaporation can be a factor
Proper exposure is also critical
Buildings, trees, other tall objects may block rain falling at an angle – gauges should be
twice as far from such objects as the objects are high
Wind causes turbulence – windscreens are often placed around the gauge
In the US, errors are thought to range from 7 to 20% – in higher latitudes errors are thought
to be as much as 80%
Weather radar
Radar between 3 and 10 cm will penetrate cloud droplets but are reflected by raindrops, ice
crystals, and hailstones
Such radar is not only useful to detect where precipitation occurs, but by the strength of
the reflected image the amount of precipitation can also be accurately assessed
Intentional weather modification
Weather modification strategies fall into three broad categories
1. Employment of energy to forcefully alter weather
Example: use of helicopters to mechanically mix air to disperse fog at an airport
2. Modifying land and water surfaces to alter natural interaction with lower atmosphere
Example: spread dark substance on land to increase absorption/radiation (untried)
3. Triggering, intensifying, or redirecting atmospheric processes
Example: cloud seeding with dry ice or silver iodide (cheap and has been done)
Cloud seeding was first accomplished in 1946 by Vincent J. Schaefer who spurred growth
of ice crystals by seeding a cloud with dry ice
It was later discovered that AgI could do the same thing
Certain conditions must be met for successful cloud seeding
a. Clouds must be present
b. A portion of the cloud must contain supercooled droplets
Orographic cloud seeding has been tried at Vail and Beaver Creek, Colorado
A paper mill in South Africa was putting Na and K chlorides into the air and seeding
clouds
Fog and cloud dispersal
Dry ice seeding of supercooled fog can open clear areas
Seeding will not work with warm fog
Warm fog has been dispersed using mechanical mixing (helicopters) with warmer and
dryer air above to fog
Turboclair (underground jet engines) installed at Orly Airport (Paris, France) heat the air
and can clear 900 meters of the airstrip
Hail suppression
Hail cannons in Europe fired large smoke rings into clouds to prevent hail (late 1800s
early 1900s)
Russia reported great success by seeding hail clouds (increased nucleation should reduce
size of hailstones)
US made similar seeding tests in Colorado
Studies showed all of these methods to be ineffective
Frost prevention: reduce losses in agriculture
Large daytime air masses where temperature drops below 0°C (difficult to combat)
Or small low lying areas in night time radiation cooling (easier to combat)
Methods used
Covering plants with insulating materials
Warming methods
Sprinklers: latent heat of freezing can prevent crop damage
Mixers: large windmills can mix warmer air higher up with cooler air below
Heaters: probably the most successful but also the most expensive method