Chapter #7 Flashcards
(15 cards)
The difference between a cloud droplet and a rain drop
- Determined by size.
- Cloud droplets are 20 micro meters big
- Rain droplets are 2000 micro meters big
How collision-and-coalescence produces precipitation
- Known as the ‘Warm Rain Process’
- Temperatures must be above freezing
- Droplets are different sizes due to varying updrafts
- Coalescence occurs when larger droplets collide with smaller ones
- Droplets grow in size and then fall down when they cannot be supported by updrafts
Is the saturation vapor pressure greater over the super-cooled water droplet or over the ice crystal?
- Saturation pressure above a water surface is greater than the saturation pressure above a water surface.
- It is easier for water molecules to escape from a water surface than an ice surface
How the ‘Ice Crystal Process’ or the ‘Cold Rain Process’ increases the size of the crystals
- Ice crystals grow larger by taking water from the super-cooled water around them
- Caused by difference in vapor pressure between ice crystals and water droplets
Can the ‘Collision-Coalescence Process’ and the ‘Bergeron Process’ occur in the same cloud?
- Yes, if it has a vertical structure that allows for regions of ice crystals, super-cooled rain droplets, and liquid droplets
- Cumulonimbus clouds can do so
Why are there more cloud droplets than ice crystals at -14 degrees Celsius?
- There is a limited number of ice-forming nuclei
- Particles of clay become active around -15 degrees Celsius
Three ways ice crystals can form in a cloud
1) Homogeneous Freezing: freezing of pure water w/o use of a nucleus (-40 degrees Celsius)
2) Deposition: deposition nuclei allow water vapor to deposit ice on surfaces in cold, saturated air, bypassing the liquid phase
3) Contact Freezing: super-cooled liquid water freezes on contact with contact nuclei. Dominant production of ice crystals in some clouds.
Why not perform a cloud seeding operation when the sky is clear?
- Seeding does not generate clouds
- For best results:
- clouds must be cold enough to contain super-
cooled water - lacking in ice crystals and ice nuclei
- clouds must be cold enough to contain super-
Define the precipitation type: ‘Virga’
- Precipitation that falls from the sky but evaporates before reaching the ground
What is necessary for snow to stay as snow and not melt when falling into a surface that is slightly above freezing?
- Air must be unsaturated and the wet-bulb temperature must be at freezing or below
Difference between a ‘Ground Blizzard’ and a ‘Blizzard’
Ground Blizzard
- Occurring in open country due to the blowing and the drifting of existing snow
- Snow does not have to be falling
- Visibilities in cities are better than in open country
Blizzard:
- Winds sustained/frequently reach 35 MPH with snow falling
- Reduces visibility to 1/4 mile or less
- Conditions happen for a minimum of 3 hours
How are atmospheric conditions that produce sleet different from those that produce hail?
Sleet:
- Layer of above freezing temperatures below cloud base, but surface temperatures are below freezing
- Below-freezing temperature must be large enough to re-freeze the drop
Hail:
- Produced inside a cumulonimbus cloud by accretion because of a strong updraft
- Continued updrafts cause hail to increase in size until updrafts cannot hold it
What is the difference between sleet and freezing rain?
Sleet:
- Usually transparent
- When it hits the ground it can bounce
Freezing Rain:
- Freezes on contact with objects
Determining Factor:
- Depth and coldness of the subfreezing layer near the surface
What conditions are favorable for aircraft icing?
- Between 0 degrees Celsius and -10 degrees Celsius
- When clouds have a significant amount of super-cooled water droplets
Difference between glaze and rime icing?
Glaze Icing:
- Large, super-cooled liquid drops strike the aircraft
- Spreads out and then freeze
- Forms quickly and heavy
Rime Icing:
- Small, super-cooled water droplets
- Freeze before they spread out
