Reading surface analysis and prognostic charts
How to interpret surface analysis charts — fronts, pressure systems and isobars — to understand the big weather picture.
A surface analysis chart shows the big-picture weather: high- and low-pressure centers, fronts, and isobars (lines of equal pressure). Reading it builds the mental model that the airport-by-airport METARs then fill in.
- Lows (L) bring rising air, clouds and precipitation; highs (H) bring sinking air and generally fair weather.
- Isobars spaced close together mean a steep pressure gradient and strong winds; widely spaced isobars mean light winds. In the Northern Hemisphere, wind flows roughly counterclockwise and inward around a low, clockwise and outward around a high.
- Fronts are boundaries between air masses:
- Cold front (blue triangles) — advances briskly, often a narrow band of showers/thunderstorms, then clearing and cooler, gusty winds. - Warm front (red half-circles) — advances slowly with widespread layered cloud, steady precipitation and lowering ceilings ahead of it. - Stationary (alternating) and occluded (purple) fronts combine these traits.
Prognostic (prog) charts project this picture forward 12, 24, 48 hours and beyond, showing where systems and fronts will move and where IFR, turbulence or precipitation is expected.
For flight planning, the surface and prog charts answer the strategic questions: *Is a front going to cross my route? Which way is the system moving? Will conditions improve or deteriorate by my arrival?* Decide the go/no-go and routing here, then confirm with TAFs, radar and PIREPs closer to departure.
*Reference and training only. Use official briefings for flight planning.*