SIGWX Chart: Reading a Significant Weather Chart

A SIGWX chart puts a whole weather situation on one page. Where a SIGMET warns about one hazard in one area, the chart shows every significant hazard across a continent at a single moment.

That makes it a planning tool rather than a warning. It is the fastest way to see whether a route has a problem before you read a single line of text.

What a SIGWX chart shows

The chart carries five kinds of information, layered on one map.

Jet streams, drawn as heavy arrows with the core level and speed. The height of the tropopause, given at points across the map. Areas of significant cloud, each with a box describing it. Areas of turbulence and icing, each enclosed by its own kind of line. And surface features such as fronts and pressure centres.

High level charts run from about 25,000 to 63,000 feet. Mid level charts cover roughly 10,000 to 45,000. Which one you want depends on how high you fly.

The lines and what they enclose

Each hazard uses a different kind of boundary, and once you know the three you can read the map at a glance.

A scalloped line, drawn with rounded bumps facing outward, encloses an area of significant cloud.

A dashed line encloses turbulence, moderate or severe.

A line of small peaks, like a row of narrow triangles, encloses icing.

Inside each area sits a symbol and a number giving the levels. So a dashed region with a level band tells you where the rough air is and how far up and down it goes.

Jet streams

The jet stream arrow is the boldest thing on the chart, and it carries several facts at once.

The arrow shows the direction of flow. A box on it gives the flight level of the core and the maximum wind speed, so FL340 with 120 means the core sits at 34,000 feet and runs at 120 knots.

Double bars drawn across the stream mark where the speed changes by 20 knots. Those crossing points matter, because a rapid change in speed along the flow is where clear-air turbulence lives.

The area just below and on the cold side of a jet core is the classic place for it. If your route crosses a jet near its level, expect the dashed turbulence boundary to be nearby.

The tropopause

Tropopause heights appear as numbers in small boxes at points across the chart, sometimes with a marker for a local high or low.

That height matters for two reasons. It caps most convective cloud, so storm tops tend to flatten there. And the air above it is usually smooth and stable, which makes it a target for a quiet cruise.

A low tropopause means the smooth air starts lower. A high one, common in the tropics, means storms can build a very long way up.

Cloud boxes

Cloud areas carry a small block of text rather than a symbol alone, and it packs in a lot.

It gives the amount, the type, and the base and top levels. Bases below the chart floor are written as XXX, meaning the cloud extends below the range covered.

The type abbreviations are the familiar ones. CB is cumulonimbus and it is the one to look for first, since a storm implies severe turbulence, icing, shear and hail whether or not those are drawn separately.

ISOL, OCNL and FRQ describe how much of the area is covered, which changes whether you can pick a way through.

The trap in the valid time

A SIGWX chart is a snapshot, not a period, and this is where pilots misread it most often.

The chart is valid at one fixed time. A chart valid at 1200 describes 1200. It does not describe 0900 or 1500, and nothing on it is averaged across a window.

Charts are issued for fixed times a few hours apart. So pick the one nearest your flight, and where your flight spans two, look at both and expect the truth to sit between them.

That is the opposite of a SIGMET or an AIRMET, which are valid across a period.

Verdict on the SIGWX chart

Use a SIGWX chart to see the whole situation quickly, then go to the text products for the detail.

Learn the three boundary lines first, since they carry most of the meaning: scalloped for cloud, dashed for turbulence, small peaks for icing. Read the jet core level and look for the double bars near it. And always check the valid time, because the chart describes one moment rather than your whole flight.

Last updated August 29, 2026