BÀI GIẢNG KHÍ TƯỢNG THỰC HÀNH CHƯƠNG 6 VÀ 7

CHAPTER 6 Observation of Visibility

CHAPTER 6 Observation of Visibility
Visibility shows us not only to what extent an object can be seen from the observer but also a
simple way to know the atmospheric stability.
Visibility is decreased by such floating substances in the atmosphere as water droplets in the
form of fog, rain or snow and solid particles in the form of sand dust, smoke or crystallized salt
from sea water spray. Visibility is also affected by the refraction and diffusion of light due to the
inhomogeneity of air.
Visibility observations are therefore utilized not only for meteorological analysis but also for
information on visibility troubles of public transportation or for data for the management of
atmospheric pollution.
6.1 Definition of visibility
Visibility (horizontal visibility) is defined as the greatest distance at which a black object of
suitable dimensions can be seen and recognized against the sky at the horizon during daylight or
could be seen and recognized during the night if the general illumination were raised to the normal
daylight level.
If visibility is different according to directions, adopt the shortest distance. Visibility should
be observed with unaided eyes or glasses but not with a telescope, binoculars or a sextant.
See "Guide to Ships' Weather Reports" for coding the observed results.
6.2 Daytime observation
If you can see a ship, an island or a buoy at the limit of visibility, the most precise way is to
use a radar for the measurement of visibility. However you can rarely utilize a fixed object in the
distance on the ocean and so you need to be well trained and experienced for visibility
observation.
In general, a simple method is applied by making use of the relation between the height of
observation above the sea surface (h in (m)) and the distance to the horizon (L in (km)):
L

= 3.6Jh

as shown in Tab.6.1.

Table 6.1 The hei ht of observation above the sea surface and the distance to the horizon
2 4 6 8 10 15 20 25 30
Hei ht of observers' e es above sea level m

5

Distance of ob'ects on the horizon at sea km

7

9

40

10 11 14 16 18 20 23

For example, if you can not see the horizon clearly from a bridge at 15 m above the sea
!urface but can clearly see it from the deck at 8 m above the sea surface, you can assume the
risibility from the table around 12 km between 14 km and 10 km.

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CHAPTER 6 Observation of Visibility

6.3 Night-time observation
The estimation of visibility at night is not easy. What the meteorologist is interested in
knowing is the degree of transparency of the atmosphere. But the distance seen at night

depends on the amount of illumination; and the distance at which a light is seen depends on its
intensity or candle-power. If there is no obvious change in meteorological conditions, the
visibility just after dark will be the same as that recorded just before dark irrespective of the
fact that one may not be able to see as far. A deterioration in visibility can sometimes be
detected afterwards and the visibility code figure adjusted accordingly. In doing this, care must
be taken not to confuse the effect of a decrease in visibility. The presence of a "loom" around
the vessel's navigation lights is frequently a guide to deteriorating visibility. In addition, it is
necessary to allow for the period (usually from 5 to 15 minutes) of adaptation during which the
eyes become accustomed to the dark.

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CHAPTER 7

Observation of Atmospheric Phenomena and Weather

takes place from or inside a cloud or, less often, from high structures on the ground or from
mountains.
Thunder T: A sharp or rumbling sound which accompanies lightning. It is emitted by rapidly
expanding gases along the channel of a lightning discharge.

7.2

Observation and recording of atmospheric phenomena
Observation of atmospheric phenomena includes the estimation of their intensity.

Intensity is classified into 3 ranks; light, moderate and heavy.

To make your observation on an atmospheric phenomenon, you need also to record the
following;
1) Time of appearance
2) Name of the phenomenon
3) Situation
4) Time of ending

7.2.1 Intensity of rain
Light: Scattered drops that do not completely wet an exposed surface, regardless of duration,
to a condition where individual drops are easily seen; slight spray is observed over the
decks; puddles form slowly; sound on roofs ranges from slow pattering to gentle swishing;
steady small streams may flow in scuppers and deck drains.
Moderate: Individual drops are not clearly identifiable; spray is observable just above deck and
other hard surfaces; puddles form rapidly; sound on roofs ranges from swishing to gentle
roar.
Heavy: Rain seemingly falls in sheets; individual drops are not identifiable; heavy spray to
height of several inches is observed over hard surfaces; visibility is greatly reduced; sound
on roofs resembles roll of drums or distant roar.

7.2.2 Intensity of drizzle with visibility
Light: Visibility 1 km or more.
Moderate: Visibility less than 1 km but not less than 0.5 km.
Heavy: Visibility less than 0.5 km.

7.2.3 Intensity of snow
Light: Visibility 1 km or more .
. Moderate: Visibility less than 1 km but not less than 0.2 km.
Heavy: Visibility less than 0.2 km.

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CHAPTER 7

7.2.4

Observation of Atmospheric Phenomena and Weather

Intensity of hail and Ice pellets

Light: Few stones or pellets falling with little, if any, accumulation.
Moderate: Slow accumulation.
Heavy: Rapid accumulation.

7.2.5 Intensity of thunder, thunderstorm and lightning
Light thunder: Slightly perceived as a distant thunder.
Moderate thunder: Thunder with considerably large sound so as to vibrate a window pane.
Heavy thunder: Thunder with deafening and astonishing sound and with effect of rattling
windows.
Light thunderstorm: Light thunder with a flash of light.
Moderate thunderstorm: Moderate thunder with a flash of light.
Heavy thunderstorm: Heavy thunder with a flash of light.
Light lightning: Slightly perceived in the daytime and clearly visible in the nighttime.
Moderate lightning: Perceived without confrontation in the daytime and perceived from inside
a lighted room.
Heavy lightning: A flash of light brightens the surrounding area in the daytime and in the
nighttime everything is dazzled by a heavy flash of light.

7.3

Present we?ther and past weather

Weather should be recorded by codes under present weather and past weather as
described in "Ships' Weather Code Card" and "Guide to Ship's Weather Reports".
The present weather is defined as the state of the atmosphere at the time of observation or
during the hour (60 minutes) preceding it.
Table 7.1 shows the relationship between weather
summary and codes of present weather, total cloud amount, and visibility.
The past weather refers to the type(s) of weather which occurred since the previous main
synoptic hour (00, 06, 12 or 18UTC).
Table 7.2 shows the period to be covered by the past
weather at each observation time. And table 7.3 shows the past weather codes.
The code figures for the past weather and the present weather should be selected in such a
way that they form a description as complete as possible of the weather consistent between the
past and the present.

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