High and low pressure (H and L)
H marks the centre of an area of high pressure and L the centre of an area of low pressure. On NOAA’s US charts the H is blue and the L red, each with its central pressure in millibars underlined beside it.
How to read it
NOAA’s Weather Prediction Center defines a high as a relative maximum in the pressure pattern and a low as a relative minimum, each usually with at least one closed isobar around it. Its Unified Surface Analysis Manual draws the high as a blue H and the low as a red L with an x at its centre, each with its central pressure underlined nearby. The Met Office’s surface pressure charts mark highs and lows with H and L and their central pressure. The WMO’s manual on weather charts says a pressure centre may be marked with a cross and its pressure written in whole hectopascals below it.
A Met Office teaching pack calls a high an anticyclone and a low a depression. In the Northern Hemisphere, it says, winds blow clockwise around a high and anticlockwise around a low.
Also searched as: anticyclone, blue H, depression, high pressure symbol, H on a weather map, L on a weather map, low pressure symbol, red L.
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.
Letters in other languages
The WMO’s manual asks for a capital letter “appropriate to the language of the country”, so charts outside the English-speaking world can use other letters. On English-language charts the letters are H and L.
Reading the number
The central pressure is in millibars or hectopascals, which the Met Office treats as the same unit. It gives standard sea-level pressure as 1013 hPa, and defines highs and lows relative to their surroundings, not by a fixed number. The Met Office links high pressure with settled weather and low pressure with unsettled weather.
Read next
- Next in Weather map symbolsIsobarsIsobars are the lines on a weather map that join places with the same air pressure. They are usually drawn every 4 millibars, and the closer together they are, the stronger the wind.
- The same shape in Statistics symbolsConditional probability (P(A|B))P(A|B) means “the probability of A given B”: how likely A is once you know that B has happened. The vertical bar is read “given”.
More weather map symbols
The same shape elsewhere
Browse symbols drawn with letters and numbers.
Sources
- Unified Surface Analysis Manual, Version 1, NOAA National Weather Service, Weather Prediction Center (with OPC, NHC and HFO), Chapter 3, Definitions: High Pressure System; Low Pressure System
- 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, 4.1.3 Pressure centres
- Surface pressure charts, Maps and charts, Met Office, Surface pressure charts
- High and low pressure, Learn about weather, Met Office, High and low pressure
- Understanding weather charts, Key Stage 4 lesson plan, Crown Copyright, Met Office (hosted by MetLink, Royal Meteorological Society), Part B: Anticyclones; Depressions
- Surface Map Help Page, PALS severe weather forecasting help pages, Iowa State University, Department of Agronomy, 2) High Pressure (H), Low Pressure (L), and Pressure Lines
Researched, edited and approved by Rory Hansen before publishing. Last reviewed October 9, 2026. How we research
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