VISIBILITY Flashcards
Define visibility
the greatest horizontal distance at which a black object can be seen and recognised against the sky at the horizon in daylight.
Ground based observation only
Define prevailing vis
Vis as reported in METAR AUTOs, TAFs and TRENDs.
The max horizontal vis covering at least half the horizon.
Why does illumination from the sun or moon have no effect on visibility?
Vis is not a function of illumination, but rather the transparency of the air. (i.e. even if it is dark, the vis may still be great, even without illumination)
What are “forward scatter meters”?
How do they work?
The vis sensors used in AWS
Work by firing out a beam of IR light which is scattered by minute particles in the air. Some of the beam will be scattered back to the sensor. From the amount received, the sensor will measure the turbidity of the air & then can calculate the Vis.
Limitations of a forward scatter meter?
- If there is localised mist/fog it may give false indications where prevailing vis is actually much better & vice versa.
- Approaching poor vis (e.g. rain shower) will not be recorded until it reaches the sensor.
- Cannot determine directions
- Limited to 20km
Describe the effect in Vis of the following precip;
- Light
-Moderate
-Heavy
- Light: little reduction
-Moderate: 3000m - 10km
-Heavy: <3000m
What does the visibility reduction as a result of rain depend on?
Difference with drizzle or snow?
Both the droplet size & distribution in a given volume
Drizzle & snow reflect more light which causes further vis rreduction
Describe the effect in Vis of the following Drizzle or snow intensity;
- Light
-Moderate
-Heavy
- Light: >8000m
-Moderate: 500m - 8000m
-Heavy: <500m
What conditions do drizzle, snow & sleet get associated with?
warm & cold fronts, CBs
Describe the effect in Vis from fog vs. mist
Fog: <1000m
Mist: >1000m
What is vis in fog & mist a function of?
How many water droplet are suspended in the atmosphere & therefore how much light scatter there is
Describe the effect in Vis from Haze & smoke
Vis <5000m
What is haze/smoke made up of?
Sold particles of dust, smoke, sea salt & other chemical pollutants in suspension in the atmosphere
What makes vis reductions due to haze & smoke considerably worse?
Presence of low level inversion & airfeild in close proximity to the cause of smoke/haze
How does sea spray cause reductions in vis?
Another problem sea salt aerosols cause?
In rough sea condxns, breaking waves cause droplets of sea water to be thrown up, where they mix with turbulent air. The water evaporates, leaving small aerosols of sea salt.
Stick to every surface - filmy (e.g. windsheild)
What is the height of BLSN a function of?
wind speed, snow state (wet or dry), how old the snow is, and the stability within the lower atmosphere.
What makes vis reductions by BLSN worse?
Often in the case of BLSN, a very low inversion exists (approx 100ft above the surface)
Wind speeds required for BLSN?
New, dry snow: 10kts for vis reductions
Old or wet snow: 40kts
BLSN vs. snow storm?
BLSN: vis reductions confined to low layer above the surface (clear above)
Snow storms: Vis reductions can be marked up to FL200.
How does percieved visibiility get affected by slant range & fog.
High level = increased slant range & able to see the ground through the fog for example (i.e. in the circuit)
Low level (i.e. on approach): slant range is significantly reduced and you are unlikely to be able to see the field through the fog.
List the 5 types of fog
- Radiation
- Advection
- Steam
- Frontal
- Upslope
Define radiation fog
Caused by earth cooling at night due to loss of terrestrial radiation to space
Define Advection fog
Caused by moist air from the sub-tropics moving south over colder waters & being cooled form below
Define steam fog
Formed when cold air moves over warm water. High rate of evaporation from the water quickly saturates cold air above & wispy fog forms above
Define frontal fog
On rare occasions, where there is prolonged periods of rain associated with a warm front, air near the ground can become saturated due to the rain evaporating into it. The cloud base will lower until it reaches the ground.
Define upslope fog
Simply cloud formation on the windward side of a range (essentially low level stratus)..
What are the most common types of fog in NZ?
Radiation & advection
list the 5 conditions required for formation of radiation fog
- Clear sky & low humidity aloft to maintain it
- Location over land
- Overnight or early morning
- High humidity near the ground
- Light SFC wind of 2-7 kts
How is radiation fog formed?
Earths heat is released (longwave radition) back into the atmosphere.
Ground cools & so does air above (conduction)
If sky remains clear & cooling continues & air is sufficiently moist, condensation will occur.
Radation fog: cooling in calm conditions vs. light breeze
Calm: cooling effects shallow layer near the ground & only dew or frost will form.
light breeze of 1-2 kts: air is ixed & cools through a deeper layer & droplets are suspended in the air = fog.
How does radiation fog clear?
increase in wind speed above 7-8kts results in mixing with warmer, dried air from above, lowering the relative humidity.
Or when the sun rises & the land begins to heat, water droplets will evaporate from the ground up.
5 Conditions required for Advection fog to form?
- Clear or cloudy skies
- Over land or sea
- At any time of day or night
- With high humidity near the ground
- in all wind speeds up to 20-25kts
How does advection fog form?
Air must have a northerly component to it, bringing warm, moist air down from the tropics over colder land/sea. Cooled from below.
If cooled to its dew point over a depth of a few 100ft, fog will form.
How does advection fog clear
Can last for days, clearance occurs when there is a complete change in airmass (i.e. front passes through with drier airmass behind).
How do katabatics enhance or inhibit radiation fog?
If katabatic is <8kts (and are often moist from valleys), this will enhance the formation of fog.
If valley system is big enough however, katabatic may become too strong & kill any chance of fog formation
