In the calendar files, next to each date is a numerical representation of the weather type for that day. The number codes are as follows:
10 = Dry Moderate
20 = Dry Polar
30 = Dry Tropical
40 = Moist Moderate
50 = Moist Polar
60 = Moist Tropical
70 = Transition
80 or 00 = Day is missing
If you've used the SSC before, you'll notice the change that all weather types are now two digits. That is because the use of the 'plus' types is now expanded. The most common plus type, and the only one included by default before, was Moist Tropical Plus, which originally was developed for use in Heat Warning Systems, in which for many cities Moist Tropical happened far too often to be useful on its own. The expanded types are now all based on being one standard deviation or more away from the mean for the type based on thermal characteristics. For Dry and Moist Polar, this is one standard deviation below the mean apparent temperature; for Dry and Moist Tropical, one above. There is also an entirely new type, Supertransition, which is based on exceeding one standard deviation above Transition seed criteria. All of these are incremental, thus 61 is Moist Tropical Plus (one standard deviation), and 62 Moist Tropical Double Plus (two standard deviations). So:
Dry Polar: DP+ (21), DP++ (22)...
Dry Tropical: DT+ (31), DT++ (32)...
Moist Polar: MP+ (51), MP++ (52)...
Moist Tropical: DT+ (61), DT++ (62)...
Transition: TR+ (71), TR++ (72)... These do not have upward limits though above 3 standard deviations is rare.
Note that for most applications, it is appropriate to consider
these subsets as part of the main weather type... e.g. 61, 62, etc. can
all be merged into 60.
The SSC files are available for use for whatever climate or other environmental application that might be relevant. One thing to bear in mind is that these data are station data (usually airport), as archived primarily by NCEI, with some additional data coming from Environment Canada and other archivers like the Iowa Mesonet. These archives generally do minimal quality control. I have done some quality control to assess some clear issues, but given the volume of stations, it's impossible to quality-control every last observation. So if something looks amiss, please let me know.