Visibility Flashcards
Obscurity
Caused by what?
Visibility associated with each
Caused water droplets, solid particles, ice
Water: fog <1000m, mist 1000-5000m
Solid: Haze <5000m
Radiation fog or valley fog
Process
Conditions
Evolution
Ground cools faster than air, air in contact cools fast than air above.
Dew point is reached, condensation happens.
Clear sky, light wind, high relative humidity
Up to 500ft agl
Becomes stratus or stratocumulus upon dispersion
Advection fog
What do you need
Process
When
Wind value
High moisture content!!!! warm air moving over cold surface Air cools, dew point is reached Coastal, winter time Wind up to 15kts
Evaporation, attic smoke, steaming fog
Process
Conditions needed
Dispersal
Condensation!!!
Warm land, dry cold air
Dispersal with wind
Frontal fog
Process
Distance
Dispersal
Below warm front up to 200nm
Dispersal when front passes
Orographic fog
Process
Air Force to rise, cools, dew point, fog
Ice fog
Temperature
What it is
How it happens
-35C up to -45C
Ice crystals in suspension in the air
Sublimation happens
Freezing fog
What
How
Evolution
Super cooled water droplets
Creates rime ice!!!!!!!
Visibility in clouds
High
Middle
Low
High level 1000m+
Mid level 20-1000m
Low level 10-30m
Visibility in precipitation
Mod rain Heavy rain Mod snow Drizzle Heavy snow Blowing snow
Mod rain 3-10km Heavy rain <1000m Mod snow <1000m Drizzle <500m Heavy snow <50m Blowing snow few meters
Haze
How tog et it
Pressure system associated
High pressure system, particles trapped due to subsidence
Dust storm
Speed
Height
Visi
15-20kt wind up to 3000ft agl
Severe dust storm, less than 200m visi
Visibility reporting
0-800m 50m steps
800-5000m 100m steps
5000-10000m 1000m steps
If visi less than 1500m or half total visi, 2 indications
RVR
Instrument
Updates
Transmissometer Updated every minute U: ceiling moves up D: moves down N: no change
3captors, 2 at each end (300m) one at midpoint
P1500: sup at 1500m
M0050: less than 50m
Cat I
DH 200ft
RVr not less than 550m
