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1. Clouds are made up of:
A.
B.
C.
D.
Liquid water droplets
Ice Crystals
Water Vapor
A combination of liquid water, ice, and water vapor
2. The liquid water droplets and ice crystals which make up clouds
are denser than the surrounding air molecules. Why don’t they fall to
the ground?
A.They are attracted to each other by electrical forces and water
tension.
B.They are far enough apart that the density is not actually greater
C.Upward movement of air keeps them aloft
D.It is too warm at ground level
5. Why does upward movement of air create clouds (and
precipitation)?
A.
B.
C.
D.
E.
Air cools as it rises
Relative humidity decreases as air cools
Cooling of saturated air results in condensation of water vapor
A, and C
A, B, and C
3. What “lifting mechanism” occurs when rising air remains warmer
(less dense) than the surrounding air?
A.Convection
B.Orographic Lift
C.Convergence
D.Weather Fronts
4. Air converging towards an area of _______________ is forced
upward, often creating clouds and precipitation.
A.
B.
C.
D.
Warm temperature
Low pressure
High pressure
Cold temperature
Precipitation Processes: Growth of Cloud Drops
•A raindrop is 100 times larger than a cloud droplet.
•A cloud droplet is 100 times larger than a condensation nucleus.
•1,000,000 cloud droplets = 1 raindrop
Two Important Precipitation
Processes
• Collision and Coalescence (warm
cloud) Process
– Strictly liquid cloud droplets
• Ice-Crystal (Bergeron) Process
– ice crystals and liquid cloud droplets
both must co-exist at temperature
below freezing
Collision & Coalescence Process
Warm Cloud Raindrop Formation Processes
•Collision and coalescence operates in warm clouds to produce rain, and is
affected by the:
•1) clouds liquid water content (LWC)
•2) droplet sizes
•3) cloud thickness
•4) updrafts
•5) drop electrical charges.
Ice Crystal (Bergeron) Process
“important precipitation process in the middle and high latitudes”
•Cloud droplets are still composed
of liquid water below the freezing
level (supercooled).
COLD
CLOUDS
•The pure water in a small amount
has a lower freezing temperature.
(small cloud droplets)
•Cold clouds may drop below -40°
C before small droplets freeze into
ice embryos that can serve as
condensation nuclei (ice nuclei).
more water droplets than ice
crystals
WARM
CLOUDS
Molecules from Water to Ice
Ice crystals have lower saturation
vapor pressures than liquid droplets,
creating a gradient of high to low
water molecules from liquid to ice that
encourages ice growth.
Ice Particle Changes
Accretion or
Riming
Aggregation
Graupel
•As ice crystals fall and collide with super cooled drops, they get bigger by accretion.
•Falling icy matter is called graupel (snow pellet), and aggregation describes the joining of two ice
crystals into snowflakes.
•If the snowflake melts while falling, it continues down as a raindrop.
•Much of the rain, even in the summer, in the middle and northern latitude falls as snow initially.
Precipitation Types
• Rain
• Snow
• Sleet and Freezing Rain
• Snow Grains and Snow Pellets
• Hail
Rain
• A falling drop must have a diameter  0.5 mm
to be considered as rain.
• Uniform drops with diameter < 0.5 mm are
called drizzle.
• Showers are considered intermittent
precipitation, usually short in duration but
often heavy.
Evaporating Rain
Rain falling
into low
humidity air
below will
cause the
drops to
decrease in
size, possibly
evaporating
into streaks of
dry air as in
this virga.
Snow
• Can fall as much as 300 m (1000 ft) below the
freezing level before melting completely.
• In a drier environment, snowflakes can reach
the ground even if the surface layer
temperature is above freezing due to
evaporative cooling effect.
• It is never “too cold” to snow.
Average Annual Snowfall
Sleet & Freezing Rain
For each case, determine whether the air will be above or below freezing at each level in order that
precipitation of the selected type will occur. There may be more than 1 possible answer. Try this
based on how each precipitation type was described in class, rather than using your book.
RAIN
3000m
SNOW
SNOW
* * * * * * 3000m
(Below Freezing)
SNOW
* (Below
* * Freezing)
* * *
FREEZING
RAIN
SLEET
3000m
SNOW
* (Below
* * Freezing)
* * *
3000m
SNOW
* * * * * *
(Below Freezing)
2000m
2000m
2000m
2000m
1000m
1000m
1000m
1000m
Surface
Surface
Surface
Surface
Hailstones & Damage
Radar Rainfall
•Doppler radar uses microwave transmission and reception to
measure backscatter intensity, or reflectivity for large geographic
areas.
•This signal is converted into maps of precipitation intensity, while
phase shift data provide information on storm movement.