A surface analysis chart is the weather at a glance: one map that shows where the pressure systems and fronts are, and so where the wind and the weather will be, before you ever open a single METAR.
This is general educational information, not operational, legal, or regulatory advice. Rules differ by authority and change over time. Always verify against current official sources and follow your operator's approved procedures.
Isobars and the pressure gradient
The lines that cover a surface chart are isobars: lines joining places of equal mean sea level pressure, normally drawn every 4 hectopascals. They are the chart's most useful feature, because their spacing is a direct read on the wind.
Where the isobars are tightly packed, the pressure gradient is steep and the wind is strong. Where they are widely spaced, the gradient is gentle and the wind is light. As the FAA Aviation Weather Handbook describes, this pressure gradient is the force that drives the wind in the first place.
Highs, lows and the wind
Two letters mark the pressure centres:
- H is a high (an anticyclone): descending air, generally settled weather, light winds near the centre.
- L is a low (a depression): rising air, generally unsettled weather with cloud and rain, and stronger winds.
The wind does not blow straight from high to low. Away from the surface it blows roughly along the isobars, as SKYbrary describes, a balance summed up by Buys Ballot's law: in the northern hemisphere, stand with your back to the wind and the low pressure is on your left. Near the ground, friction turns the wind to blow slightly across the isobars towards the lower pressure, and slows it, which is why, in the northern hemisphere, the surface wind is backed and lighter than the wind a few thousand feet up. Backing and veering with height are hemisphere-dependent, so name the hemisphere before you use them: south of the equator the same friction turns the surface wind the other way.
The fronts
A front is the boundary between two air masses, and each type has a standard symbol and its own weather, covered in the FAA Aviation Weather Handbook and the Met Office chart guidance:
- Warm front (red semicircles): warm air rising over retreating cold air, usually a long, sloping spell of layered cloud and steady rain ahead of it, then a rise in temperature.
- Cold front (blue triangles): cold air undercutting warm air, often a narrower band of heavier showers or thunderstorms, a wind shift and a drop in temperature.
- Occluded front (alternating purple): a cold front catching up with a warm front and lifting the warm air off the surface, mixing the two kinds of weather.
- Stationary front (alternating red and blue): a boundary that is not moving, which can keep similar weather over one place for a while.
On a warm, cold or occluded front, the symbols point in the direction the front is moving, and they all sit on the same side of the line. The stationary front is the exception, and it is the one that catches people out: its semicircles and triangles are drawn on opposite sides of the line, pointing away from each other, precisely because the boundary is not advancing either way. So "the symbols show you where it is going" is a rule with one deliberate exception, and the exception is telling you it is going nowhere.
Common pitfalls
- A high is not a guarantee of good flying weather. In winter, a slack high can trap fog, low cloud and poor visibility under an inversion for days.
- Analysis versus prognostic. An analysis chart shows the observed situation at a time; a prognostic (forecast) chart shows an expected one. Check the label and the valid time.
- It is a big-picture, area product. The chart tells you the synoptic situation, not the conditions at your runway, so always pair it with the latest METAR and TAF.
In Pilot EFB
Pilot EFB decodes the text products that sit alongside the chart, the METAR, TAF and SIGMET, and always keeps the raw report so you can trace each one back to source. It does not replace the official charts and is not a certified Electronic Flight Bag; it is an offline-first personal reference, so read the surface and significant-weather charts from your official provider and use the decoded text to fill in the detail.