Learning Center
Plans & pricing Sign in
Sign Out


VIEWS: 101 PAGES: 10


As discussed in the previous lecture, there are six important processes that make
up the water cycle. These are:

Evaporation is the process where a liquid, in this case water, changes from its
liquid state to a gaseous state. Liquid water becomes water vapor. Although
lower air pressure helps promote evaporation, temperature is the primary factor.
For example, all of the water in a pot left on a table will eventually evaporate. It
may take several weeks. But, if that same pot of water is put on a stove and
brought to a boiling temperature, the water will evaporate more quickly.
During the water cycle some of the water in the
oceans and freshwater bodies, such as lakes and
rivers, is warmed by the sun and evaporates. During
the process of evaporation, impurities in the water
are left behind. As a result, the water that goes into
the atmosphere is cleaner than it was on Earth.

Condensation is the opposite of evaporation. Condensation occurs when a gas is
changed into a liquid. Condensation occurs when the temperature of the vapor

When the water droplets formed from condensation are very small, they remain
suspended in the atmosphere. These millions of droplets of suspended water
form clouds in the sky or fog at ground level. Water condenses into droplets only
when there are small dust particles present around which the droplet can form.

When the temperature and atmospheric pressure are right, the small droplets of
water in clouds form larger droplets and precipitation occurs. The raindrops fall to
Earth. As a result of evaporation, condensation and precipitation, water travels
from the surface of the Earth goes into the atmosphere, and returns to Earth

Surface Runoff
Much of the water that returns to Earth as precipitation runs off the surface of the
land, and flows down hill into streams, rivers, ponds and lakes. Small streams
flow into larger streams, then into rivers, and eventually the water flows into the
ocean. Surface runoff is an important part of the water cycle because, through
surface runoff, much of the water returns again to the oceans, where a great deal
of evaporation occurs.
Infiltration is an important process where rain water soaks into the ground,
through the soil and underlying rock layers. Some of this water ultimately returns
to the surface at springs or in low spots downhill. Some of the water remains
underground and is called groundwater. As the water infiltrates through the soil
and rock layers, many of the impurities in the water are filtered out. This filtering
process helps clean the water.

One final process is important in the water cycle. As plants absorb water from the
soil, the water moves from the roots through the stems to the leaves. Once the
water reaches the leaves, some of it evaporates from the leaves, adding to the
amount of water vapor in the air. This process of evaporation through plant
leaves is called transpiration. In large forests, an enormous amount of water will
transpire through leaves.


Clouds are aggregation of minute particles of water or ice suspended in the air.
Clouds are formed when air containing water vapor is cooled below a critical
temperature called the dew point and the resulting moisture condenses into
droplets on microscopic dust particles (condensation nuclei) in the atmosphere.
The air is normally cooled by expansion during its upward movement. Upward
flow of air in the atmosphere may be caused by convection resulting from intense
solar heating of the ground; by a cold wedge of air (cold front) near the ground
causing a mass of warm air to be forced aloft; or by a mountain range at an angle
to the wind. Clouds are occasionally produced by a reduction of pressure aloft or
by the mixing of warmer and cooler air currents.

Cloudiness (or proportion of the sky covered by any form of cloud), measured in
tenths, is one of the elements of climate. Clouds have become an important
focus in the study of global warming or cooling, including how the increase or
decrease in cloud cover can affect the amount of radiation reflected from the
earth back into space.

In 1803, Luke Howard, an English scientist, devised a classification that was
adopted by the International Meteorological Commission (1929), designating
three primary cloud types, cirrus, cumulus, and stratus, and their compound
forms, which are still used today in modified form. Today's classification has four
main divisions: high clouds, 20,000 to 40,000 ft (6,100–12,200 m); intermediate
clouds, 6,500 to 20,000 ft (1,980–6,100 m); low clouds, near ground level to
6,500 ft (1,980 m); and clouds with vertical development, 1,600 ft to over 20,000
ft (490–6,100 m).
High cloud forms include cirrus, detached clouds of delicate and fibrous
appearance, generally white in color, often resembling tufts or featherlike plumes,
and composed entirely of ice crystals; cirrocumulus (mackerel sky), composed of
small white flakes or very small globular masses, arranged in groups, lines, or
ripples; and cirrostratus, a thin whitish veil, sometimes giving the entire sky a
milky appearance, which does not blur the outline of the sun or moon but
frequently produces a halo.

Intermediate clouds include altocumulus, patchy layer of flattened globular
masses arranged in groups, lines, or waves, with individual clouds sometimes so
close together that their edges join; and altostratus, resembling thick cirrostratus
without halo phenomena, like a gray veil, through which the sun or the moon
shows vaguely or is sometimes completely hidden.

Low clouds include stratocumulus, a cloud layer or patches composed of fairly
large globular masses or flakes, soft and gray with darker parts, arranged in
groups, lines, or rolls, often with the rolls so close together that their edges join;
stratus, a uniform layer resembling fog but not resting on the ground; and
nimbostratus, a nearly uniform, dark grey layer, amorphous in character and
usually producing continuous rain or snow.

Clouds having vertical development include cumulus, a thick, detached cloud,
generally associated with fair weather, usually with a horizontal base and a
dome-shaped upper surface that frequently resembles a head of cauliflower and
shows strong contrasts of light and shadow when the sun illuminates it from the
side, and cumulonimbus, the thunderstorm cloud, heavy masses of great vertical
development whose summits rise in the form of mountains or towers, the upper
parts having a fibrous texture, often spreading out in the shape of an anvil, and
sometimes reaching the stratosphere. Cumulonimbus generally produces
showers of rain, snow, hailstorms, or thunderstorms.

Clouds form when the water vapor condenses into small particles. The particles
in clouds can either be liquid or solids. Liquid particles suspended in the
atmosphere are referred to as cloud droplets and the solid particles are often
called ice crystals. As a volume of unsaturated air cools, its relative humidity
increases. If sufficiently cooled, the relative humidity becomes 100%, the
temperature equals the dew point.

Cloud Forming Processes

Condensation or deposition of water above the Earth's surface creates clouds. In
general, clouds develop in any air mass that becomes saturated (relative
humidity becomes 100 %). Saturation can occur by way of atmospheric
mechanisms that cause the temperature of an air mass to be cooled to its dew
point or frost point. The following mechanisms or processes can achieve this
outcome causing clouds to develop:
(1). Orographic uplift occurs when air is forced to rise because of the physical
presence of elevated land. As the parcel rises it cools as a result of adiabatic
expansion at a rate of approximately 10° Celsius per 1000 meters until
saturation. The development of clouds and resulting heavy quantities of
precipitation along the west coast of Canada are mainly due to this process.

(2). Convectional lifting is associated with surface heating of the air at the ground
surface. If enough heating occurs, the mass of air becomes warmer and lighter
than the air in the surrounding environment, and just like a hot air balloon it
begins to rise, expand, and cool. When sufficient cooling has taken place
saturation occurs forming clouds. This process is active in the interior of
continents and near the equator forming cumulus clouds and or cumulonimbus
clouds (thunderstorms). The rain that is associated with the development of
thunderstorm clouds is delivered in large amounts over short periods of time in
extremely localized areas.

(3). Convergence or frontal lifting takes place when two masses of air come
together. In most cases, the two air masses have different temperature and
moisture characteristics. Oneof the air masses is usually warm and moist, while
the other is cold and dry. The leading edge of the latter air mass acts as an
inclined wall or front causing the moist warm air to be lifted. Of course the lifting
causes the warm moist air mass to cool due to expansion resulting in saturation.
This cloud formation mechanism is common at the mid-latitudes where cyclones
form along the polar front and near the equator where the trade winds meet at
the intertropical convergence zone.

(4). Radiative Cooling occurs when the sun is no longer supplying the ground
and overlying air with energy derived from solar insolation (e.g., night). Instead,
the surface of the Earth now begins to lose energy in the form of longwave
radiation which causes the ground and air above it to cool. The clouds that result
from this type of cooling take the form of surface fog.

Of course these causes of cloud development do not always act in a singular
fashion. It is possible to get combinations of all four types, such as when
convection and orographic uplift cause summer afternoon cloud development
and showers in the mountains.

The potential for cloud formation (and precipitation) depends on the amount of
water vapor in the atmosphere. As a parcel of air rises, the moisture it contains
cools and condenses out onto small particles of dust called cloud condensation
nuclei until a cloud forms.

Lifting, also referred to as adiabatic cooling, is the most common method of
humidification of air to form clouds. As air rises it expands because pressure
decreases with altitude. Kinetic energy is converted to potential energy and the
parcel temperature decreases, and the relative humidity increases.

The method of vertical lifting (orographic, convective, convergence, or frontal)
and the stability of the atmosphere determines the type of cloud. Cumulus clouds
tend to form in unstable atmospheres. Layered clouds form in more stable
environments in which large layers of air are slowly lifted.

The two main large scale lifting processes that result in cloud formation are
convection and advection of air. Convection refers to air rising vertically in the
atmosphere due to heating. Advection is the horizontal transfer of air that usually
results in warmer air being forced up over cooler air. Both advection and
convection results in the formation of clouds.

Clouds play a crucial role in our global climate. Clouds reflect shortwave solar
energy back into space and tend to cool the earth. On the other hand, clouds
absorb longwave terrestrial radiation and warm the planet.

Precipitation Formation Processes

The Collision-Coalescence Process occurs in warm clouds where
temperatures are above freezing. It takes 1 million cloud droplets to form a rain
drop. Larger droplets descend faster than smaller droplets. As they do, they
collide and merge with other droplets growing larger in the process. Eventually
the droplet grows too large to be supported by cloud updrafts. Droplets reach
terminal velocity. If updrafts counter terminal velocity then droplet remains
suspended. The larger the droplet the greater the updraft required for

The Bergeron Process occurs in cold clouds where the upper parts of the cloud
are below freezing. This process requires coexistence of water vapor, ice crystals
and supercooled water droplets. In the Bergeron Process as cloud temps drop
below 0oC cloud droplets become supercooled. At temperatures between -10oC
and -30oC clouds are composed of mixed ice crystals and supercooled water
Below -30oC, clouds are composed of just ice crystals. Water vapor does not
deposit to form ice crystals above -10oC (ice-forming nuclei are not active above
-10oC). Ice crystals grow at the expense of supercooled water droplets until they
fall, melt and leave cloud as rain drops. Some evaporate before reaching the

To conclude, clouds form when an airmass is forced towards saturation.
Precipitation becomes likely when the force pushing the airmass towards
saturation continues after the airmass has become saturated.

   1. Clouds are made of liquid water droplets that condense out of the
      water vapor in the saturated airmass (relative humidity 100%).
   2. The droplets condense onto cloud condensation nuclei (CCN) that are
      made of tiny specks of dust, soot, and sea salt. CCN are about 1 tenth of
      a micrometer (100 nanometers) in diameter.
   3. Clouds droplet diameters range from about 10 micrometers (stratus) to
      about 100 micrometers (cumulus). They are held aloft by air currents.
      Cloud droplets get progressively larger by absorbing more and more
      water vapor, and by coalescing with other cloud droplets.
   4. Cloud droplets are often supercooled, i.e. in a liquid state even though
      their temperature is below zero degrees C.

A small rain drop is about 200 micrometers in diameter, and a typical rain drop
is about 1 millimeter (1000 micrometers) in diameter. Like cloud droplets, rain
drops are often supercooled when they form, and may warm above the freezing
point if they fall through warmer regions of the atmosphere. Rain drops begin
as large cloud droplets, and get progressively larger by absorbing more and
more water vapor, and by coalescing with more cloud droplets and other
raindrops. Eventually, they get too heavy to be held aloft by air currents, and fall
to Earth.

Snow flakes begin as tiny crystals of frozen water molecules that grow as
more and more water molecules are added to the structure. Like rain drops,
they eventually get too heavy to be held aloft by air currents, and fall to Earth.

Thus, precipitation is water in the solid or liquid form that falls from clouds.
Drizzle is liquid precipitation with drops 0.2 to 0.5 mm in diameter. Rain is liquid
precipitation that falls from nimbostratus and cumulonimbus clouds. And the drop
size is 0.5 to 6 mm. Freezing Rain is liquid precipitation that freezes upon
contact. Freezing rain can cause an Ice Storm. Ice Pellets or Sleet is frozen rain
drops 0.5 mm or less in diameter. Snow is an assemblage of ice crystals in the
form of flakes. Snow Pellets are a form of frozen granular precipitation while hail
are lumps of ice that fall from thunderstorms. Doppler Radar can not only
measure the intensity of precipitation but also it horizontal motion.


      Formal definition: any form of water that falls from the atmosphere and
       reaches the ground
      Three main processes for formation:
          o Condensation/deposition (well understood, beginning of rain droplet
          o Collision/coalescence (warm clouds processes; not well
          o Bergeron process (cold clouds processes; not well understood)


      High humidity: condensation exceeds evaporation: cloud particle grows
      Growth is fast initially, then slower
      Particle needs to be 0.2 millimeter (200-micrometer) in diameter
           o If much smaller, doesn't fall
           o If somewhat smaller, falls but evaporates
      It would take days to make precipitation by this process
      This process only important until cloud particle become cloud droplets
       (roughly 20 micrometer in size)
      1 million average size cloud droplets needed to produce an average size
       rain drop of 2000 micrometers

Collision-coalescence process

      Only important in warm clouds (technically the temperature everywhere in
       the cloud is > 0 °C)
      Large cloud droplets fall but encounter air resistance
           o Depends on size of drop and falling speed
           o Speed increases until air resistance equals pull of gravity -->
               terminal velocity
      Larger drops have smaller surface-area-to-weight ratio --> fall faster -->
       collide with smaller drops


      Merging of cloud droplets
      Large drops fall faster, overtake smaller drops
          o Some small drops moved out of way
          o Rest collide
      Some break up (more likely with small drops: higher surface tension)
      Others merge together: coalescence
      Collection efficiency: # coalesced/droplets in path
      Coalescence enhanced in thunderstorms where strongly charged droplets
       exist in strong electrical field

Residence time of a droplet

      Still air:
           o Large cloud droplet takes:
                     12 minutes to travel through 500 m thick cloud
                     1 hour to travel through a 2500 m thick cloud
      Strong updraft maximizes amount of time in the cloud --> larger drops
      For example:
           o Stratus: 500 m thick, updraft 0.1 m/s, r 200 mm, drizzle
           o Tropical cumulus cloud: for typical updraft of 6.5 m/s, cloud droplet
                of 100 mm rises, collides, grows to 1000 mm, falls again and grows
                to 5000 mm
           o Largest droplets fall out in beginning of rain shower

Bergeron or ice-crystal process

      Important in mid- and high-latitudes: cold clouds (can also occur in tropics
       as well)
      If T > 0 °C: water droplets only
      If -40 °C < T < 0 °C :
            o Supercooled water droplets
            o Ice particles
      If T < -40 °C : ice particles only

Ice nuclei

      Ice nuclei rare compared to CCN
      Ice nuclei examples:
           o Clay minerals
           o Bacteria
           o Ice crystals formed by freezing of supercooled water

Saturation (the Bergeron process)

          o   Water and ice at same temperature: equilibrium vapor pressure
              less over ice (fewer water vapor molecules in air near ice)
          o   When water and ice mix: water vapor will attempt to balance -->
              move from near water to near ice
                  more water vapor near ice means supersaturation: so
                  Less water vapor near water: not saturated: so evaporation
                  Ice grows at expense of water
                    Can be 1 million water droplets for every ice particle: ice
                     particle becomes very large

One falling ice crystal to rain and snow

      Ice crystal begins to fall
      Collides with supercooled droplets
           o Freeze on contact and stick together: accretion or riming
           o Creates graupel
           o Graupel falls and splinters
           o Collision with other crystals may freeze hundreds of supercooled
      Aggregation: ice crystals bond to create snowflakes at T > -10 °C
      Snow may melt before reaching ground: rain
      Much of the rain in middle- and high-latitudes begins as snow

Conditions for precipitation from cold clouds

      Ratio of 1:100,000 to 1:1,000,000 ice crystals to water droplets
          o If fewer: crystal grows, falls, leaves rest of cloud untouched (very
              little precipitation)
          o Otherwise lots of small ice crystals that don't fall

Radar and precipitation

      Radar works by sending energy (usually microwaves -- long wavelengths)
       at object and see how much gets reflected back
           o If object is moving can detect Doppler shift
           o By the time the radar hits something, beam usually several hundred
              meters wide
                   Sees one image of cloud: cloud contains water drops, hail,
                     etc. (can't see cloud droplets)
                   If wavelength is large compared to condensate, amount
                     reflected related to number and sizes of particles
                   Tells us how much "stuff" is in cloud -- but not what it actually
                   Relationships have been made to compare reflected power
                     with rainfall rates, hail, lightning potential, etc.

Precipitation types: Snow

      Precipitation either as individual crystals or clumps of crystals
          o Generally less than 5 mm
      Conditions for snow
          o Cold clouds
          o   Snow must not evaporate or sublimate before reaching surface
                  Large crystals or flakes
                  Moist atmosphere below cloud
          o Cold atmosphere below cloud (or very shallow if warm)
      Cold clouds: light and fluffy snow: low moisture content
      Warmer clouds: ice crystals join together --> large flakes, high moisture

Too cold to snow?

      Never! Very cold air: little water for precipitation
      Heaviest snowfalls usually close to freezing: maximum amount of water in
      Too warm to snow? Almost never! Can snow at 50 °F
           o Technically: wet-bulb temperature must be below freezing

Water droplets

      If radius > 0.5 mm: rain
            o Originates as either rain or snow
            o If originating as snow, must fall through warm atmosphere and melt
               before reaching surface
      If radius < 0.5 mm: drizzle
            o Generally from stratus or nimbostratus clouds

Other cold precipitation types

      Sleet (ice pellets)
          o Frozen raindrops
          o At some point during fall to earth, must be in warm layer (to melt),
              then cold layer (to freeze)
          o Generally smaller than 5 mm
      Freezing rain
          o Rain that freezes on contact with surface
          o Requires warmer air aloft, shallow layer of cold air near surface
      Hail: precipitation of rounded pellets or irregular lumps of ice
          o Larger than 5 mm
          o Produced in large cumulonimbus clouds with large updrafts (up to
              100 mph)
          o Need supercooled water
      Shower: sudden rainfall produced by changes in updrafts in cumulus cloud
          o Originally: updraft in clouds exceeds terminal velocity of drops.
              Then: updraft weakens or becomes downdraft --> suspended drops
              fall to ground
      Cloudburst: a very heavy shower

To top