AIRMET Guide: Sierra, Tango and Zulu Explained

An AIRMET warns of weather that is hazardous to some aircraft. Light ones, aircraft without ice protection, pilots without an instrument rating.

That is the line that separates it from a SIGMET, which warns of weather hazardous to all aircraft. This AIRMET guide covers the three types, what fires each, and the two things an AIRMET cannot tell you.

The three types

The hazards are split between three named products, issued together.

The names are phonetic letters, and there is a rough logic to them. Sierra is about seeing. Tango is about being thrown around. Zulu is about ice.

All three go out on the same schedule, so a difficult day produces the full set at once.

The thresholds

Each type has firm triggers rather than a judgement call.

Sierra fires on ceilings below 1000 feet or visibility below three statute miles, or on terrain being obscured by cloud, mist, smoke or precipitation.

Tango fires on moderate turbulence, on sustained surface wind of 30 knots or more, or on low-level wind shear that no thunderstorm caused.

Zulu fires on moderate airframe icing. Its freezing level section is published whether or not any icing is expected.

Every one of those needs the conditions to cover at least 3000 square miles.

Why moderate is the ceiling

None of the three types covers the severe end, and that is by design.

Severe turbulence is a SIGMET. Severe icing is a SIGMET. The products split at the point where a hazard stops troubling only some aircraft and starts troubling all of them.

So an AIRMET is never the worst case. If you find one on your route, check whether a SIGMET covers the same air, because that would be a real step up in severity and a different decision.

The six hour window

An AIRMET runs six hours and goes out four times a day, so the set covers the whole day without gaps.

Between issues they can be amended. A new one arriving off-cycle means somebody saw the weather turn, which is worth noticing on its own.

Each carries an outlook that looks a further six hours ahead. If you are flying later in the day, that outlook matters more than the main body.

Six hours is a long time for weather. Treat the advisory as the outline of the day, and expect the detail to have moved.

What an AIRMET cannot tell you

Two limits matter, and both follow from the area test.

First, an AIRMET does not describe your airfield. Inside a Sierra there will be fields in clear air, because the advisory only needs enough of the region to be affected.

Second, and more dangerous, no AIRMET does not mean no hazard. One valley in fog will never reach 3000 square miles. Neither will a single ridge with cloud on it, or ice in one small band of cloud.

So use AIRMETs to understand the system, and use METARs, TAFs and PIREPs to understand your route and your destination.

Reading them with everything else

The sensible order puts AIRMETs in the middle of a briefing.

SIGMETs come first, because they decide routing and exempt nobody. AIRMETs come next, because they tell you what the flight will be like and whether visual flight is available.

PIREPs come last and carry the most weight for the next hour. They are the only current thing in the set, and where a pilot report disagrees with a six hour advisory, the report is fresher.

Abroad, expect this order to break. Many countries issue no AIRMETs, so the middle step disappears and the aerodrome forecasts have to carry it.

The freezing level, and why it is here

Zulu carries something the other two do not: a plain measurement rather than a warning.

The freezing level section gives the height of the zero degree line across the region, and it is published whether or not ice is forecast.

That is there because the freezing level is where icing becomes possible. Below it, cloud water is warmer than zero and will not freeze onto you. Above it, the water can be supercooled.

So the number gives you a floor for the risk and a target for an escape. If you pick up ice, warm air below the freezing level is one way out, terrain permitting.

It is the most useful single number in the whole AIRMET set, and it is easy to skip past because it does not read like a warning.

Verdict on this AIRMET guide

Read an AIRMET as a statement about scale and severity: this kind of weather, over a wide area, at a level that troubles lighter aircraft.

Sierra decides whether you can fly visually. Tango tells you how rough it will be. Zulu tells you where the ice starts and where the freezing level sits. None of the three tells you about your airfield, and none of them is the worst case.

Last updated August 29, 2026