CAVOK vs 9999: Both Mean Good, Only One Means Clear

CAVOK vs 9999 confuses people because both turn up on nice days and both feel like shorthand for fine weather. They are not the same kind of thing at all.

One of them is a number in a slot. The other one deletes three slots and replaces them with a promise.

What CAVOK vs 9999 each replace

9999 sits in the visibility slot and nowhere else. It means visibility of ten kilometres or more. That is the top of the metric scale, so anything better than ten kilometres is still written 9999.

The rest of the report carries on as normal around it. Weather group, cloud groups, all present. A report can read 9999 -RA BKN008 quite happily. Good visibility, light rain, low broken cloud.

CAVOK does something else entirely. It stands for ceiling and visibility OK, and it takes the place of the visibility group, the weather group and every cloud group at once. One word instead of three sections.

So 9999 tells you one thing. CAVOK tells you three things by leaving them out.

The three conditions CAVOK requires

A station cannot use CAVOK because the day feels pleasant. All three tests have to pass at the same time.

Visibility must be ten kilometres or more. That is the same threshold 9999 marks, so every CAVOK report has at least 9999 visibility built into it.

No cloud below 5000 feet, or below the minimum sector altitude if that is higher. And no cumulonimbus or towering cumulus at any height at all, even far above that floor.

No significant weather. Nothing in the present weather list: no precipitation, no thunderstorm, no dust, no mist.

Fail any one and CAVOK is off. The station goes back to writing the groups out.

What CAVOK still allows

This is where the word misleads. CAVOK is not clear sky.

A field can report CAVOK with a solid overcast at 8000 feet. The cloud is above the floor, it is not convective, so it does not block the code. Somebody reading CAVOK as blue sky will be surprised on the climb.

It also allows quite a lot of haze, as long as visibility holds at ten kilometres. And it allows strong and gusting wind, because wind is reported in its own group and CAVOK never touches it.

Read it as no low cloud, no storms, and you can see a long way. Nothing more than that.

Where each one appears

Neither appears in a US report, and the reason is units.

US stations report visibility in statute miles, so ten miles is 10SM. There is no four-digit metric group to hold 9999. CAVOK is not used in the United States either, which is a rule rather than a consequence of units.

Everywhere else, metric visibility is normal. Europe, most of Asia, Africa, South America and Australia all use metres, so 9999 is one of the most common groups you will meet abroad.

CAVOK turns up in the same places, most often at quieter fields on settled days, and very often in TAFs where a forecaster wants to say the whole afternoon is fine without writing three groups per period.

Reading a low metric visibility

If you can read 9999 you can read the rest of the scale, and the rest of the scale is where the risk lives.

Four digits mean metres, read literally. 0800 is 800 metres. 4000 is four kilometres. 2500 is two and a half.

Some services drop below the four-digit form for very poor conditions and use 0000, meaning less than 50 metres. Others add a directional minimum, so you may see a second lower figure with a compass point after it.

A rough conversion helps if you think in miles. 1600 metres is about a mile. 800 metres is about half. 5000 metres is about three miles, which is the usual line between marginal and instrument conditions.

CAVOK in a forecast

CAVOK is common in TAFs, and there it carries a small trap.

In a forecast it means the same three tests are expected to pass. Good visibility, no low cloud, no significant weather. What it does not carry is any sense of how close the call was.

A field forecast CAVOK all afternoon may be sitting at exactly ten kilometres and 5100 feet. One notch either way and the word disappears, replaced by three ordinary groups that look far worse on the page.

So a change from CAVOK to written-out groups in a later period is not always a big change in the weather. Sometimes it is one number crossing one line.

Verdict on CAVOK vs 9999

Treat 9999 as a number and CAVOK as a summary. The number tells you about visibility alone and leaves the rest of the report to speak for itself.

CAVOK is the stronger statement and the easier one to over-read. It rules out low cloud and storms and poor visibility together, and it says nothing at all about what sits at 8000 feet or how hard the wind is blowing.

Last updated August 29, 2026