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Synoptic weather-typing and the SSC

Synoptic weather-typing - the classification of ambient weather conditions into categories - is a useful tool for numerous climate impact applications. On this website, the classifications produced by one such scheme, the Spatial Synoptic Classification (SSC), are presented.

The SSC is a hybrid classification scheme, based on both manual and automated processes. Initially, weather type (see right) identification was made manually for each of the weather types, based on climatological knowledge. As the character associated with these weather types changes from season to season, typical days in each type- "seed days"- were picked for each station for different times of the year. Algorithms then develop hypothetical seed days for each of the 365 days of the year.

Once this process is complete, actual conditions on each day were compared to the seed days, and the day ends up being classified as the one it most closely resembles. Hence, when the process is complete, a weather type 'calendar' is available, whereby each day in a station's period of record is classified into one of the weather types.

In order to increase spatial cohesion, seed days were transferred to geographically neighboring stations, thus creating a true spatial grid of weather types.

Weather types

DP (dry polar) is synonymous with the traditional cP air mass classification. This air mass is generally advected from polar regions around a cold-core anticyclone, and is usually associated with the lowest temperatures observed in a region for a particular time of year, as well as clear, dry conditions.

DM (dry moderate) air is mild and dry. It has no traditional analog, but is often found with zonal flow in the middle latitudes, especially in the lee of mountain ranges. It also arises when a traditional air mass such as cP or mT has been advected far from its source region and has thus modified considerably.

The DT (dry tropical) weather type is similar to the cT air mass; it represents the hottest and driest conditions found at any location. There are two primary sources of DT: either it is advected from the desert regions, such as the Sonoran or Sahara Desert, or it is produced by rapidly descending air, whether via orography (such as the chinook) or strong subsidence.

MP (moist polar) air is a large subset of the mP air mass; weather conditions are typically cloudy, humid, and cool. MP air appears either by inland transport from a cool ocean, or as a result of frontal overrunning well to the south of the region. In can also arise in situ as a modified cP air mass, especially downwind of the Great Lakes.

MM (moist moderate) is considerably warmer and more humid than MP. The MM air mass typically appears in a zone south of MP air, still in an area of overrunning but with the responsible front much nearer. It can also arise within an mT air mass on days when high cloud cover suppresses the temperature.

MT (moist tropical), analogous to the traditional mT air mass, is warm and very humid. It is typically found in warm sectors of mid-latitude cyclones or in a return flow on the western side of an anticyclone; as one approaches the tropics this weather type dominates. MT+ (moist tropical plus) is a subset of MT that was derived after the initial classification, to account for the lack of utility of a weather-type scheme in the warm subtropics when one weather type dominates most of the year. It is defined as an MT day where both morning and afternoon apparent temperatures are above seed day means, and thus captures the most "oppressive" subset of MT days. MT++ (moist tropical double plus) is an occasionally used subset of MT+, in which morning and afternoon apparent temperatures average out to being at least one standard deviation above seed day means.

TR (transitional) days are defined as days in which one weather type yields to another, based on large shifts in pressure, dew point, and wind over the course of the day.

Development of the SSC

Larry Kalkstein and Scott Greene are two principal developers of the original SSC, created in the mid-1990s for all stations east of the Rockies within the US. Calendars were originally available for only winter and summer. Kalkstein et al. (1996) contains a detailed discussion of this original system.

The SSC was then later redeveloped to be able to classify days year-round; an expansion was also done geographically to include more than 300 stations across the US and Canada. The "SSC2" - which is what is featured on this site - is written up in significant detail in Sheridan (2002). As this version is now the only available version of the SSC, it is usually just written as "SSC" and not "SSC2".

Later expansions of the SSC have taken the SSC global. Donna Bower, as part of her dissertation, worked with Glenn McGregor and Scott Sheridan to expand the SSC to Western Europe. Whereas the US and Canada classifications all began from one origin, the European classifications began from multiple origins in station 'clusters'. More detail on this methodology can be found in Bower al. (2007). Other extensions of the SSC include stations in South Korea, Brazil, and Russia.

What the SSC is

The SSC is based solely on surface based observations at an individual station. Four-times daily observations of temperature, dew point, wind, pressure, and cloud cover are incorporated into the model. It does not take upper-level conditions into account, and does not concern itself with the origin of the air above a station, though there are obvious correlations. Hence, the SSC is most properly called a weather type classification and not an air mass classification system.

Within the SSC scheme, weather-type characteristics change from station to station and day to day. Thus, a Moist Tropical weather type is hotter and more humid in the southeastern US, nearer its source region, than in the northeastern US, after it has modified somewhat. Similarly, MT is warmer at all locations in July than in January. You can look up an air mass climatology for a particular station on the left.

Available data sets

On this website, SSC "calendars" are available for nearly 400 stations across the US, Canada, and parts of Europe. The European calendars are limited to 1974-2000, but the US and Canadian calendars generally cover a station's total period of record. A total of over 8,000,000 days have been classified across all of these stations. The SSC is also being continually updated - you can see yesterday's (preliminary) classifications and today's and tomorrow's forecast classifications as well. The "official" SSC calendar for a year will be added to this site within a couple of months of the calendar year's end.

These data are free to be used in research, as long as proper citation is given. The SSC is not free to use for any commercial purpose, and the classification program is not publicly available.

If you have any questions, comments, or notice any errors, please e-mail me. Thanks!


Below is a list of known articles that have incorporated the SSC into their research design. An overview of the use of the SSC in climate-health relationships is nicely detailed in Hondula et al. 2013 and Dixon et al. 2016. If any SSC-related publications are missing, please let me know!