cabbagetree.blog / learn
Reading a Synoptic Chart
A step-by-step visual guide to weather maps · NZ context
A synoptic chart is one of the most powerful tools for understanding what’s happening in the atmosphere right now, and what’s coming. Once you know the symbols, you can read the weather before it arrives. Hover or tap elements in the chart below to learn what each one means.
Interactive Sample Chart — Tasman / NZ Region
Pressure Systems
HHigh (anticyclone) — settled, fine, light winds. Clockwise in Southern Hemisphere.
LLow (depression) — unsettled, wind, rain. Anti-clockwise. Tight isobars = gale.
Isobars — equal pressure lines. Spacing = wind strength. Close = strong, wide = light.
Fronts
Warm front — warm air advancing. Steady rain ahead. Temperature rises as it passes.
Cold front — cold air undercutting. Heavy squalls, then clearing and cold conditions.
Trough — elongated low. Cloud and showers; no defined front.
How to read a chart — step by step
1Find the pressure centresLocate the H (high) and L (low) labels. A high to the west usually means fine weather approaching; a low to the southwest often means rain incoming.Toko Mouth: A high over the Tasman = settled northerly or light NW flow. A low south of the island = southerly swell and rain.
2Read the isobarsWidely spaced = light winds; tightly packed = strong or gale-force winds. Wind flows roughly parallel to isobars, angled slightly inward toward low pressure.
3Identify the frontsA warm front arrives with gradual cloud thickening and rain; a cold front arrives fast with squalls and a sudden wind shift.Toko Mouth: Cold fronts typically arrive as a sharp wind shift from NW to SW or S within minutes.
4Compare charts over timeA single chart is a snapshot. Compare two charts 12 hours apart to see how fast systems are moving. Most fronts cross NZ in 12–36 hours.
5Apply to your locationYour position relative to the high and low determines your weather. MetService publishes synoptic charts twice daily.MetService synoptic charts →