Define Visibilty
The greatest distance at which objects of suitable dimensions can be seen and identified.
What height is visibility reported at?
Eye level (defined at 1.8m (6ft) above the ground).
Define Prevailing Visibility
The maximum visibility value common to sectors comprising one-half or more of the horizon circle.
Define Point of Observation
Any site at which meteorological instruments are exposed or from which visual observations are taken.
Define Variable Visibility
When the visibility is observed to be fluctuating rapidly and increasing and decreasing from a mean value by 1/4 or more of the mean value, the visibility is said to be “variable”. Use the mean (average) of all observed values as the prevailing visibility.
Units for Reporting Visibility
Land Stations-Statute miles (SM)Ocean Stations-Nautical miles
Where are Ocean stations located?
They are stations onboard a ship (not to stations along a coast or shoreline) .
Reportable values of visibility (increments of 1/8 mile)
0, 1/8, 1/4, 3/8, 1/2, 5/8, 3/4
Reportable values of visibility (increments of 1/4 mile)
1, 1 1/4, 1 1/2, 1 3/4, 2, 2 1/4, 2 1/2
Reportable values of visibility (increments of 1 mile)
3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15
Reportable values of visibility (if suitable markers beyond 15 miles are lacking)
15+
Reportable Values of visibility (increments of 5 miles)
20, 25, 30, 35 (shall be used only if suitable visibility markers are available)
What does a Visibility Chart consist of?
Charts marked in degrees of azimuth and in distances (statute miles), for three different ranges.
Visibility Markers During Daylight
- Viewed against the sky- 1/2° in size (distant chimney too small)
Visibility Markers During Darkness
- Unfocused lights/moderate intensity
Method for determining Prevailing Visibility
- Evaluate the visibility in each sector.2. Begin with the sectors having the greatest visibility in common.3. Determine how much of the horizon circle is covered by those sectors. - If the coverage is 180 or more, the visibility common to those sectors is the prevailing visibility. - If the coverage is less than 180, add in those sectors having the next lower visibility.4. Continue repeating step 3 until the prevailing visibility is determined.5. Record the corresponding reportable value in the Observer’s Notebook.
Determination of Prevailing Visibility
For this purpose, the horizon circle shall be divided into as many sectors as there are different values of visibility. The highest visibility value that is common to sectors which cover one-half or more of the horizon circle shall be taken as the prevailing visibility.
Determining Prevailing Visibility During the Day
- Don’t use binoculars/dark-adaptor goggles- Note Sharpness of Farthest visible marker - Sharp Outlines with little or no blurring - Visibility much greater than distance of reference object - You are seeing much farther! - “Ball-park” figure = twice as far (at least0- Blurred or indistinct object - Visibility not much more than distance of reference object
Should optical devices (such as binoculars, etc.) be used by the observer when determining visibility?
NO
Sharp outlines (with little or no blurring of colour) of a reference object indicate…
That the visibility is much greater than the distance of the reference object.
A blurred or indistinct object indicates…
The presence of haze or some other obstruction to vision that has reduced the visibility to not much more than the distance to the object.
