W07 - Visibility

Question 1

Define Visibilty

Answer 1

The greatest distance at which objects of suitable dimensions can be seen and identified.

Question 2

What height is visibility reported at?

Answer 2

Eye level (defined at 1.8m (6ft) above the ground).

Question 3

Define Prevailing Visibility

Answer 3

The maximum visibility value common to sectors comprising one-half or more of the horizon circle.

Question 4

Define Point of Observation

Answer 4

Any site at which meteorological instruments are exposed or from which visual observations are taken.

Question 5

Define Variable Visibility

Answer 5

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.

Question 6

Units for Reporting Visibility

Answer 6

Land Stations-Statute miles (SM)Ocean Stations-Nautical miles

Question 7

Where are Ocean stations located?

Answer 7

They are stations onboard a ship (not to stations along a coast or shoreline) .

Question 8

Reportable values of visibility (increments of 1/8 mile)

Answer 8

0, 1/8, 1/4, 3/8, 1/2, 5/8, 3/4

Question 9

Reportable values of visibility (increments of 1/4 mile)

Answer 9

1, 1 1/4, 1 1/2, 1 3/4, 2, 2 1/4, 2 1/2

Question 10

Reportable values of visibility (increments of 1 mile)

Answer 10

3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15

Question 11

Reportable values of visibility (if suitable markers beyond 15 miles are lacking)

Answer 11

15+

Question 12

Reportable Values of visibility (increments of 5 miles)

Answer 12

20, 25, 30, 35 (shall be used only if suitable visibility markers are available)

Question 13

What does a Visibility Chart consist of?

Answer 13

Charts marked in degrees of azimuth and in distances (statute miles), for three different ranges.

Question 14

Visibility Markers During Daylight

Answer 14

• Viewed against the sky- 1/2° in size (distant chimney too small)

Question 15

Visibility Markers During Darkness

Answer 15

• Unfocused lights/moderate intensity

Question 16

Method for determining Prevailing Visibility

Answer 16

1. 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.

Question 17

Determination of Prevailing Visibility

Answer 17

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.

Question 18

Determining Prevailing Visibility During the Day

Answer 18

• 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

Question 19

Should optical devices (such as binoculars, etc.) be used by the observer when determining visibility?

Answer 19

NO

Question 20

Sharp outlines (with little or no blurring of colour) of a reference object indicate…

Answer 20

That the visibility is much greater than the distance of the reference object.

Question 21

A blurred or indistinct object indicates…

Answer 21

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.