Underwater visibility in Nha Trang: what affects it
15 metres today, 5 metres two days later. Visibility in Nha Trang changes fast, and there is almost always a clear reason. Here is what it is — and how we predict it in advance.
Updated:
Five reasons for murky water
- Rain. Rivers wash silt into the bay. After a downpour the water near the coast turns murky and takes more than a day to clear — we look at rainfall over the last three days.
- Swell. Strong waves stir up sand from the bottom, especially on shallow reefs. Yesterday’s swell matters as much as this morning’s.
- Wind. Blowing into the open side of a site, it makes the water murky. Blowing from behind an island, the site is sheltered and may stay clear.
- Low tide. On a very low tide the water level drops, currents drag silt off the shallows and the rocks at Madonna Rock break the surface.
- Plankton bloom. A few days after heavy rain plankton multiplies in the warm water and turns it greenish.
Why every site has its own water
Madonna Rock is an exposed island: the clearest water in calm weather, but the first to suffer from swell. The Hon Tam reefs and Dam Bay are sheltered and often better on a windy day. That is why we forecast each site separately rather than the whole bay at once.
How our forecast works
We take open weather and marine data — rainfall, swell, wind, currents and sea level — and work out what has happened to the water at each site over the last few days. The day score out of 10 is 70% visibility at the best site and 30% sea state. We only count 9:00 to 12:00, because that is when we are underwater.
Getting more accurate
After every dive the instructor logs the actual visibility at the site. We compare it with the forecast and tune the model. The more dives, the better the score.
It is a forecast, not a guarantee: the sea can surprise. If the morning looks bad we will suggest another day ourselves, and if we cancel because of the weather we reschedule or refund.