Details of method for day and night long-distance hydrological observation in Vietnam

Details of method for day and night long-distance hydrological observation in Vietnam (Illustrative image)

To be specific, Article 7 of Circular 08/2020/TT-BTNMT stipulates the method for day and night long-distance hydrological observation in Vietnam as follows:

For day-time long-distance visibility observation in Vietnam:

- Long-distance visibility observation towards the mainland is conducted at the regulated position; sequentially observe long-distance visibility over 9 focal points, beginning from the closest to the farthest focal point; determine which focal point is visible and which is not; the visibility range is determined based on the distance between two consecutive focal points and defined from level 0 to 9 as stipulated in Table 2 of Appendix II issued together with Circular 08/2020/TT-BTNMT;

- In case there are enough focal points for sea visibility observation, the focal points chosen can be capes, islands, buoys, lighthouses, and ship smokestacks or other predetermined landmarks; in the absence or shortage of focal points towards the sea, the visibility range is determined based on the clarity of the horizon as stipulated in Table 3 of Appendix II issued together with Circular 08/2020/TT-BTNMT;

- If long-distance visibility does not allow for seeing the horizon, the sea surface visibility must be estimated by eye or binoculars;

- If the long-distance visibility varies in different directions, the observation log will record the worst visibility and the visibility limit;

- When the long-distance visibility is less than 4 km (from level 5 downwards), the log must include additional symbols of phenomena limiting visibility (fog, mist, rain).

For night-time long-distance visibility observation in Vietnam:

- Long-distance visibility observation towards the mainland and the sea when there are focal points: The observer goes to the regulated position to acclimate to the darkness for 10 to 15 minutes before conducting the observation; the observation method is implemented according to the provisions in point a, clause 1, Article 7 of Circular 08/2020/TT-BTNMT;

- In the absence of sufficient focal points, the visibility must be determined one or two hours before sunset (depending on weather conditions); if no phenomenon reduces visibility at the observation time, 7 pm’s visibility will be taken; the observation method is implemented according to the provisions in point a, clause 1, Article 7 of this Circular.

Note: Long-distance visibility observation with automatic equipment: data is automatically measured and transmitted 24 times/day or as required.

For details, refer to Circular 08/2020/TT-BTNMT, which comes into force in Vietnam from October 26, 2020.

Le Vy

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