Lamb weather types

The Lamb weather type is a method of classifying synoptic weather devised by H.H. Lamb (1972). It is based on the variation in surface pressure values around the British Isles. The Lamb system splits the prevailing synoptic conditions into one of ten categories: eight directional categories (the cardinal and ordinal compass points), and two vorticity categories (cyclonic or anti-cyclonic).
More recently, an objective system was devised by Jenkinson and Collinson (1977) and compared with the original Lamb data by Jones et al. (1993). It is the Jenkinson and Collinson system that is used for the data provided here.
Chen (2000) created a similar system for a region centred on Sweden; data using this method are provided here.

About these data

This page continues the online record of weather types found at the Climatic Research Unit (CRU) at the University of East Anglia. The data on this site are derived from NCEP daily average sea-level pressures rather than the source provided by the Hadley Centre and used in the CRU version. As a consequence, this version of the weather types are not exactly the same as those orginally provided by CRU. Nevertheless, there is considerable correlation between the datasets.

In addition a second series of data is computed using the 6 hourly NCEP re-analysis sea-level pressure data. Comparison with the CRU equivalents shows the LWT to be identical. The numerical processing for the daily mean and 6 hourly sample data is identical which gives another degree of verification.

File format

The Jenkinson and Collinson system extended the original Lamb weather types to 26 by adding types that are hybrids of the pure direction and anti/cyclonic types e.g. anti-cylonic and north-westerly which is abbreviated to ANW. The data files provided here generally follow the conventions used in the CRU version and use a numerical coding of the types as shown in the table below. Each file contains leading comment lines preceded with a # character in the first column, this currently contains the name and checksum of the pressure data source file for traceability. Following the comment lines, the daily circulation values (coded according to the table) are given on a regular 31x12 grid i.e. one month per line with non-existent days included, and shown as no data (-9).

0 Anti-cyclonic 20 Cyclonic
1 ANE 11 NE 21 CNE
2 AE 12 E 22 CE
3 ASE 13 SE 23 CSE
4 AS 14 S 24 CS
5 ASW 15 SW 25 CSW
6 AW 16 W 26 CW
7 ANW 17 NW 27 CNW
8 AN 18 N 28 CN
-1 Unclassifiable-9 No data

For example the synoptic surface level pressure for the St. Jude's day storm on 28th October 2013 looked like the figure below. As you might expect the classification for this event was cyclonic - category 20. The red dots in this figure indicate the locations of the sample points used in the Lamb weather categorisation process.

The data are provided year-by-year in the grid below and as a single archive in this file (intermittently updated). These will be updated roughly once per month (around the 15th). Maps of the more recent synoptic situations can be found here..

The similar circulation types of Trigo and DaCamara (2000) for Portugal could be added in the future if anybody requests them.

A word of caution: You are welcome to use the data, with the appropriate attribution, but you do so at your own risk. I have run a further series of tests on the processing which appears to be correctly implementing my interpretation of the formulae given by Jones et al. (1993). I do have some metrics for the correlation between these data and those that overlap in time with the CRU version. These show that 56% of the days are an exact match and 33% fall within one category (e.g. misses by one compass point (45°)). My conclusion is that the discrepancies between these data and the CRU version are caused by differences in the analysed surface level pressure fields.

You may need to right click and select 'Save linked content as' to download these files

UK - Daily mean LWT

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UK - 6 hourly LWT

The NCEP re-analysis actually uses a 6 hrly timestep. The files below are based on these sea-level pressure data rather than the daily mean used above. With the exception of the input data the processing is the same. The files are CSV and the format is self explanatory.

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Sweden - Daily mean LWT

These are similar to the daily LWTs for the UK, but are processed using the coordinates and coefficients given in Chen (2000). Caution: These are still in testing. If you use them please check a few values manually and let me know if there any problems.

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Produced in conjunction with the Lancaster Environment Centre, Lancaster University, UK. Scripts and web page © Andy Horseman 2013.