Occluded front symbol
An occluded front: the front that forms where a cold front has caught up with a warm front. It is drawn as a purple line with triangles and half circles, alternating, on the same side.
Easily confused with
How to read it
NOAA’s Weather Prediction Center defines an occluded front as a composite of two fronts, formed as a cold front overtakes a warm or quasi-stationary front, and draws its purple triangles and semicircles on the side towards which it is moving. The Met Office describes the same purple line with both triangles and semicircles, and says the cold front lifts the warm air off the ground and “hides” it. The WMO’s manual on weather charts gives occluded fronts purple when a chart is in colour.
NOAA’s JetStream school notes that there are two kinds of occlusion, cold and warm, and that they are drawn the same way on a weather map.
Also searched as: occluded front, occlusion, purple front, purple line on a weather map.
These pages describe what each symbol on a weather chart means. They are not forecasts or warnings: for those, the national weather service for the area is the source.
Look-alikes
A Stationary front symbol also carries both triangles and half circles, but in alternating red and blue, with the two kinds on opposite sides of the line. On an occluded front they are all purple and on one side. A purple line with bumps and spikes is the occlusion on WPC’s charts; the Met Office’s teaching pack describes it as a bold line with “sets of triangles and semi-circles”.
Read next
- Next in Weather map symbolsTrough symbolA trough: a long, narrow area of relatively low pressure. On NOAA’s US charts it is a wide orange dashed line; on Met Office charts it is a plain black line.
- The same shape in African flagsFlag of the ComorosThe Comoros flag is four horizontal stripes, yellow, white, red and blue from the top, with a green triangle along the pole holding a white crescent and four white stars in a line between its tips. Norway’s Store norske leksikon explains the crescent as the symbol of Islam and the four stars as the four islands of the Comoros; Lex says the four stripes are said to stand for the islands too.
More weather map symbols
The same shape elsewhere
Browse symbols drawn with curves and waves, lines or triangles.
Sources
- WPC Product Legends: Surface Fronts and Boundaries, Product legend, NOAA National Weather Service, Weather Prediction Center, Surface Fronts and Boundaries: Occluded Front
- How to read Surface Weather Maps, JetStream, the NWS online weather school, NOAA National Weather Service, Fronts: Occluded Front
- Unified Surface Analysis Manual, Version 1, NOAA National Weather Service, Weather Prediction Center (with OPC, NHC and HFO), Chapter 3, Definitions: Occluded Front
- Weather fronts, Learn about weather, Met Office, 3. Occluded fronts
- Understanding weather charts, Key Stage 4 lesson plan, Crown Copyright, Met Office (hosted by MetLink, Royal Meteorological Society), Part B: Occluded fronts
- Manual on the Global Data-processing and Forecasting System, Appendix II-4: Graphical representation of data, analyses and forecasts, WMO-No. 485, Appendix II-4 (as circulated to IPET-DRMM-III, Beijing 2015), World Meteorological Organization, 3.2 Symbols
Researched, edited and approved by Rory Hansen before publishing. Last reviewed October 9, 2026. How we research
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