1
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2
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- Clouds form when air rises and
becomes saturated in response to adiabatic cooling.
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3
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4
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- Ge = environmental
lapse rate
- Gd = dry adiabatic
lapse rate
- Gm = moist
adiabatic lapse rate
- Absolutely Stable
- Ge < Gm
- Absolutely Unstable
- Ge > Gd
- Conditionally Unstable
- Gm < Ge < Gd
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5
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6
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- When boundaries between air of unlike temperatures (fronts) migrate,
warmer air is pushed aloft.
- This results in adiabatic cooling and cloud formation.
- Cold fronts occur when warm air is displaced by cooler air.
- Warm fronts occur when warm air rises over and displaces cold air.
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7
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8
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9
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10
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11
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- High clouds have low cloud temperature and low water content and consist
most of ice crystal.
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12
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- Middle clouds are usually composite of liquid droplets.
- They block more sunlight to the surface than the high clouds.
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13
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- Low, thick, layered clouds with large horizontal extends, which can
exceed that of several states.
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14
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- They are clouds with substantial vertical development and occur when the
air is absolute or conditionally unstable.
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15
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16
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17
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18
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19
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20
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21
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22
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23
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24
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25
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- Collector drops collide with smaller drops.
- Due to compressed air beneath falling drop, there is an inverse
relationship between collector drop size and collision efficiency.
- Collisions typically occur between a collector and fairly large cloud
drops.
- Smaller drops are pushed aside.
- Collision is more effective for the droplets that are not very much
smaller than the collect droplet.
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26
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- When collisions occur, drops either bounce apart or coalesce into one
larger drop.
- Coalescence efficiency is very
high indicating that most collisions result in coalescence.
- Collision and coalescence
together form the primary mechanism for precipitation in the tropics,
where warm clouds dominate.
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27
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28
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29
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30
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31
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32
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33
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- Rain is associated with warm clouds exclusively and cool clouds when
surface temperatures are above freezing
- Rainshowers are episodic precipitation events associated with
convective activity and cumulus clouds
- Drops tend to be large and widely spaced to begin, then smaller drops
become more prolific
- Raindrop Shape begins as spherical
- As frictional drag increases, changes to a mushroom shape
- Drops eventually flatten
- Drops split when frictional drag overcomes the surface tension of
water
- Splitting ensures a maximum drop size of about 5 mm and the
continuation of the collision-coalescence process
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34
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- Snowflakes have a wide assortment of shapes and sizes depending on
moisture content and temperature of the air.
- Snowfall distribution in North America is related to north-south
alignment of mountain ranges and the presence of the Great Lakes.
- Lake effect: snows develop as the warm lake waters evaporate into cold
air.
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35
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- Sleet begins as ice crystals which melt into rain through a mid-level
inversion before solidifying in colder near surface air
- Freezing Rain forms similarly to sleet, however, the drop does not
completely solidify before striking the surface
- When sleet hits the surface, it bounces and does not coat objects
with a sheet of ice, as feezing
rain does.
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36
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- Graupel are ice crystals that undergo extensive riming
- Lose six sided shape and smooth out
- Either falls to the ground or provides a nucleus for hail
- Hail forms as concentric layers of ice build around graupel
- Formed as graupel is carried aloft in updrafts
- At high altitudes, water accreting to graupel freezes, forming a layer
- Hail falls but is eventually carried aloft again by an updraft where
the process repeats
- The ultimate size of the hailstone is determined by the intensity of
the updraft.
- Great Plains = highest frequency of hail events
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37
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38
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39
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- The objective is to convert some of the supercooled droplets in a cool
clouds to ice and cause precipitation by the Bergeron process.
- Two primary methods are used to trigger the precipitation process.
- Dry ice is used to lower cloud temperature to a freezing point in order
to stimulate ice crystal production leading to the Bergeron process.
- Silver iodide initiates the Bergeron process by directly acting as
freezing nuclei.
- Under ideal conditions, seeding may enhance precipitation by about 10%.
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40
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- Standard raingages, with a 20.3 cm (8”) collected surface and 1/10 area
collector are used to measure liquid precipitation
- Depth of water level conveys a tenfold increase in total precipitation
- Automated devices provide a record of precipitation amount and time of
the event
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41
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- Raingages are inadequate for measuring frozen precipitation
- Measurements of accumulated snow are used
- Water equivalent of snow, a 10 to 1 ratio is assumed
- Automated snow pillows are common in many locations
- Detect snow weight and convert
directly to water equivalent
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