Air mass classification using automated procedures has become increasingly popular over recent years as a tool in applied climatology. One frequent limitation is that many classification systems cannot identify the same air mass type throughout the course of a year, because of seasonal character changes. Classifications are therefore either developed for a typical middle-month of a season (i.e. January, April, July, October), or lump an entire season together (e.g. June, July, and August) as if weather conditions were homogeneous throughout.
The original Spatial Synoptic Classification (SSC) system only produced calendars for winter and summer, and assumed all air masses would possess the same characteristics for the entire season. In contrast, the redeveloped system described here, SSC2, attempts to combat this problem by using "sliding seed days". As before, seed days, which represent the typical character of each air mass at each place, are picked for the six air masses recognized by the system. However, the SSC2 requires that typical days be picked from two-week windows throughout the year. Functions are then fit to the characteristics of the days to produce typical air mass characteristics for each day of the year.
Another improvement implemented by the SSC2 system involves "seed day transfer". With the origin al SSC, seed days needed to be chosen for each station individually. The SSC2, through a variety of comparative procedures, selects many of the same seed days for adjacent locales, accounting for local climatic differences. This method produces a much larger agreement of air mass calendars between neighboring cities. With the SSC2, Wilmington, Delaware and Philadelphia, Pennsylvania possess the same air mass on over 87 percent of January days over a 50-year period. The original SSC produced matches on under 70 percent of days.
Many important applications can benefit from the redeveloped system, especially those which require intra-seasonal evaluation. For example, it has been shown that human mortality totals vary within air
masses through the summer season. Since SSC2 accounts for intra-seasonal air mass character change, we can now determine if this variation is due to changes in the air mass through the season, or to human acclimatization.