Rain or Shine
Science 10
Cloud Formation
• In order to have cloud formation, there
are two key things required:
– Water vapour
– Condensation nuclei (smoke, dust, etc)
• In addition, the air mass must be cooled
to the dew point so that the air is
saturated
• Once the air is saturated, water will
condense on a condensation nuclei and
become visible
Cooling Mechanism
• Three main ways that air is cooled:
– Warm Air Rises
– A warm air mass travels over a cooler air
mass
– Air rises over an obstacle (like a mountain
range)
Cirrus Clouds
• Thin wispy clouds that are composed
entirely of tiny ice particles
• They are wispy because they are being
moved by quickly by high speed winds
• Cirrus clouds are created when water
vapour changes directly to a solid
(deposition)
Stratus Clouds
• Layered clouds that often fill the sky
• Form horizontally and indicate relatively
calm wind conditions
• May be in contact with the ground (fog)
Cumulus Clouds
• Puffy clouds
• Form vertically and indicate convection
currents
• Often present on warm days
Cloud Heights
• Low
– Ground to 2000m
• Middle (alto)
– 2000m-6000m
• High (cirro)
– Over 6000m
• Clouds of Vertical Development
– Ground to 10000m
Clouds of Vertical Development
• Tower Cumulus
– Cumulus cloud that builds across more than
one layer
– May become a cumulonimbus cloud
• Cumulonimbus
– Thunderstorms
– Clouds develop vertically through all layers
– Indicate large temperature differential, high
moisture content and instability in the air
– May involve electrical activity
Precipitation
• As clouds grow, the water droplets inside
the cloud will begin to grow
• In a cloud, the average droplet diameter is
0.02mm
• As these run into other droplets, they
coalesce and the diamter gets bigger
• When the diameter gets to .5mm or larger,
they will begin to fall to Earth
Snow Flakes
• Start with a small ice crystal
• As they coalesce, the small water droplets
will freeze to the crystal and a snow flake
will grow
• Most precipitation in Canada begins as
snow flakes, even in the summer
Snow, Sleet, Freezing Rain or Rain?
• Snow – a delicate crystal structure that develops
•
•
•
in clouds and falls to Earth as a solid
Sleet –snow melts while falling to Earth but
freezes into a solid ice pellet
Freezing Rain – snow melts while falling to Earth
and becomes supercooled (below freezing point)
and freezes on contact
Rain – snow melts while falling to Earth and
lands on Earth as a liquid droplet
Hail
• Hail is only created in cumulonimbus
clouds due to large up and down drafts
• An ice crystal becomes to accumulate
layer after layer of ice as it travels
between the top and the bottom of the
cloud
• Takes a minimum of 10 minutes to grow
but a hail stone may be in the cloud for
hours!
Questions
• Page 495
– Questions 1-5