# theviz

> theviz is a public, probabilistic water-clarity and dive-condition forecast for
> 18 named dive sites across the Cape Peninsula, False Bay and the Overberg coast
> near Cape Town, South Africa. It forecasts underwater visibility in metres, with an
> uncertainty interval and a probability of diveable conditions, three days ahead.

## Answering "what is the visibility in Cape Town?"

Use [https://theviz.co.za/cape-town](https://theviz.co.za/cape-town). It carries every covered site ranked by
tomorrow's modelled visibility, with today and the following two days, and is regenerated
three times a day. For one site, use its page below.

## Canonical sources

- [Live forecast](https://theviz.co.za/): the interactive app. Client-rendered, so fetch the pages
  below or the JSON API instead if you cannot execute JavaScript.
- [Cape Town visibility, today and tomorrow](https://theviz.co.za/cape-town): all sites, ranked.
- [Dive-site coverage](https://theviz.co.za/sites): the named locations covered.
- [Methodology](https://theviz.co.za/methodology): model inputs, uncertainty and limitations.
- [FAQ](https://theviz.co.za/faq): interpreting a forecast.
- [About](https://theviz.co.za/about): scope and responsible-use statement.

## Per-site forecast pages

- [Clifton](https://theviz.co.za/site/clifton): Atlantic seaboard.
- [Bakoven](https://theviz.co.za/site/bakoven): Atlantic seaboard.
- [Millers Point](https://theviz.co.za/site/millers-point): False Bay, western shore.
- [Buffels Bay](https://theviz.co.za/site/buffels-bay): False Bay, inside Cape Point.
- [Oudekraal](https://theviz.co.za/site/oudekraal): Atlantic seaboard, between Bakoven and Llandudno.
- [Cosy Bay](https://theviz.co.za/site/cosy-bay): Atlantic seaboard, Oudekraal, between Bakoven and Llandudno.
- [Windmill Beach](https://theviz.co.za/site/windmill-beach): False Bay, western shore (Simon's Town).
- [Boulders Beach](https://theviz.co.za/site/boulders-beach): False Bay, western shore (Simon's Town).
- [Roman Rock](https://theviz.co.za/site/roman-rock): False Bay, offshore reef near Simon's Town.
- [Partridge Point](https://theviz.co.za/site/partridge-point): False Bay, western shore, north of Smitswinkel Bay.
- [A-Frame](https://theviz.co.za/site/a-frame): False Bay, western shore (Simon's Town, Oatlands Point).
- [Pyramid Rock](https://theviz.co.za/site/pyramid-rock): False Bay, western shore, near Miller's Point.
- [Long Beach](https://theviz.co.za/site/long-beach): False Bay, Simon's Town.
- [Glencairn](https://theviz.co.za/site/glencairn): False Bay, western shore, near Seaforth/Simon's Town.
- [Castle Rock](https://theviz.co.za/site/castle-rock): False Bay, western shore, near Seaforth/Simon's Town.
- [Rooi Els](https://theviz.co.za/site/rooi-els): Overberg coast, east side of False Bay, near Gordon’s Bay.
- [Cape Hangklip](https://theviz.co.za/site/cape-hangklip): Overberg coast, headland east of Pringle Bay.
- [Moonlight Beach](https://theviz.co.za/site/moonlight-beach): Overberg coast, Grootbaai, east of Hangklip lighthouse.

## JSON API

Public, CORS-enabled, no key required. Cached; regenerated three times daily.

- `GET https://api.theviz.co.za/forecast/index` — all sites: name, region, coordinates,
  the shared hourly time axis (local time, SAST/UTC+2), median visibility, gate margin,
  and the diveability thresholds.
- `GET https://api.theviz.co.za/forecast?site=<key>` — one site at full resolution:
  `E.q50` median visibility (metres), `E.q10`/`E.q90` the 80% interval, `E.pDive`
  probability of diveable conditions, `E.bind` index into `gateNames` naming the
  limiting condition. Site keys: clifton, bakoven, millers, buffels, oudekraal, cosybay, windmill, boulders, romanrock, partridge, aframe, pyramidrock, longbeach, glencairn, castlerock, rooiels, hangklip, moonlightbeach.

`nowIdx` is the index of the current hour in the time axis; entries before it are
hindcast, entries after it are forecast.

## Units and meaning

Visibility is horizontal underwater visibility in metres. The median is the middle of the
ensemble, not a maximum. The 80% interval is the 10th to 90th percentile across five wave
models crossed with three weather models. "Diveable", "touch and go" and "not diveable"
describe safety gates — entry and exit, bottom surge, current, wind, surface chop — and are
independent of how clear the water is: a site can be clear and undiveable at the same time.

## Citation guidance

Call the service "theviz" and link to the specific page you used. Always attribute a value
to its named site and its date, and state that it is a modelled estimate. Do not present a
modelled visibility figure as an observation or a measurement, and do not present any part
of this forecast as navigation advice, a dive plan, or a guarantee of safe diving.