Download Clouds and Precipitation

Clouds and Precipitation
Section 11.3
Cloud Formation
nucleus – a small particle
in the atmosphere around which water
droplets form; < .001 mm in diameter
 A cloud is visible when the number of
these droplets gets large
 Stable air resists rising
 Air mass stability determines the types of
clouds that form and weather patterns
 Condensation
Stable Air
 Stability
of air depends on how the
temperature of the air mass changes
compared to the atmosphere
 Fair weather clouds form under stable
conditions
Unstable Air
 The
atmosphere is considered unstable
when air masses continue to rise because
the air mass is less dense than the
surrounding air
 Unstable clouds produce thunderstorms
Atmospheric Lifting
 Clouds
form when moist air rises,
expands, and cools
 Convective lifting occurs when air rises
when it is heated and becomes warmer
than the surrounding air
 Describe Orographic lifting and
convergence.
Atmospheric Lifting
 Orographic
lifting occurs when an air
mass is forced to rise over a topographic
barrier; Sierra Nevadas and heavy rain
 Convergence occurs when air flows into
the same area from different directions;
common at mid-latitudes and equator
Types of Clouds
 Cloud
shapes are due to differences in
how clouds form
 Cloud formation occurs at different
altitudes
 First classified in 1803
 Classified by altitudes formed and shape
 Low (<2000 m), Middle (2000 m – 6000
m), and High (>6000 m)
Low Clouds
– puffy, lumpy looking clouds
 Stratus – a layered, sheetlike cloud that
covers most of the sky in an area; fog that
has lifted
 Cumulus
Middle Clouds
 Altocumulus
and altostratus are made
up of ice crystals and water droplets due
to lower temps
 Altocumulus – white or gray in color and
form large round masses or wavy rows
 Altostratus clouds are gray and form
sheets of clouds
 Sometimes produce mild precipitation
High Clouds
 Made
up of ice crystals
 Cirrus – have a wispy indistinct
appearance
 Cirrostratus – a continuous layer that
can cover the sky; vary in thickness
Vertical Development Clouds
 Cumulonimbus
clouds can reach into the
tropopause; the top is composed entirely
of ice;
 These giant clouds can produce torrential
rains, strong winds, and hail
Precipitation
 All
forms of water that fall from the clouds
to the ground
 Rain, snow, sleet, and hail
 Coalescence occurs when cloud droplets
collide and join together to form a larger
droplet
 Rain is precipitation that reaches Earth’s
surface as liquid; between 0.5 – 5 mm
Snow, sleet, and hail
 Ice
crystals form in clouds with a temp.
below freezing causing snow to fall
 Sleet forms when air currents move
droplets to move up and down between
freezing and non-freezing air
 If this up and down motion occurs over a
large area then hail forms (large ice
pellets)
The Water Cycle
 97%
of Earth’s water is in the oceans
 The constant movement of water between
the atmosphere and Earth’s surface
 Radiation causes evaporation of water
 Water rises and condenses into clouds
 Water falls to Earth as precipitation
 Cycle continues and nourishes living
things