Meet the weather types

As noted elsewhere on this site, one thing to keep in mind is that the SSC types are all spatiotemporally relative, that is the typical conditions associated with each type vary by time and space. You can see the typical conditions for a given location by looking at the climatology files linked on the left. The descriptions below are thus general characteristics of the types that apply broadly.

DP is synonymous with the traditional continental polar (cP) air mass over the mid- and upper-latitudes, however it can arise due to a number of atmospheric situations, typically when there is cold air advection and opportunities for radiational cooling. Globally is generally associated with the lowest temperatures observed at a location for a particular time of year, as well as clear, dry conditions.

DM air is mild and dry, and is typically associated with what would be considered dry, pleasant conditions for a given location. It has no traditional analogue, but is often found with zonal flow in the middle latitudes, especially in the lee of mountain ranges. In more tropical locations, it is rarer but may form when a cP air mass has been advected far from its source region and has modified considerably.

DT is analogous to the continental tropical (cT) air mass. In many cases, this type will arise in deserts or where air is advected from desert regions. Beyond this, DT can also arise under strong atmospheric subsidence, either orographically induced (such as the Chinook or Foehn), or under a ridge. It represents the hottest and driest conditions found at any location for a given time of year, and typically materializes with large diurnal temperature ranges.

MP is a large subset of the traditional maritime polar (mP) air mass; weather conditions are typically cloudy, humid, and cool. Precipitation is likely and the diurnal temperature range is minimal. MP air appears either by inland transport from a cool ocean, or as a result of frontal overrunning. In can also arise in situ as a modified cP air mass, especially downwind of water bodies. It is rarer in the lower latitudes, but can arise adjacent to cool ocean currents or on days with very high cloud cover and limited radiation.

MM (moist moderate) is considerably warmer and more humid than MP. MM typically appears in a zone equatorward of MP air, still in an area of overrunning but with the responsible front much nearer. It also arises often in marine west coast climates adjacent to mild oceans. More equatorward, it will appear when a disturbance leads to substantive cloud cover that suppresses daytime temperatures. Precipitation is also likely.

MT (moist tropical) is analogous to the traditional maritime tropical (mT) air mass. It is typically found in warm sectors of mid-latitude cyclones or in a return flow on the western side of an anticyclone; in the tropics this weather type dominates. Warm and humid, this weather type is most commonly associated with convective precipitation.

TR 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. In the mid- and high-latitudes this is often associated with a frontal passage, in the low latitudes this type is more associated with tropical disturbances. Due to the amalgamation of various sorts of transition into one category, weather conditions are generally near normal temperature-wise, with greater winds and likelihood of precipitation.

The polar and tropical types are unbounded on one end, that is for example, once conditions are cold and dry enough for a DP categorization, making it colder and drier will not change the categorization further. To this end, the plus types were created. For DP, DT, MP, MT, and TR, a 'plus' categorization occurs when the weather conditions are at least one standard deviation beyond the mean of the weather type for the location and time of year. Most frequently studied has been MT+, which is the most oppressive subset of MT days that in applied work has been most often associated with increases in human mortality. These types are useful in high- or low-latitude locations that are dominated by one type year-round. The program will identify increments of 'plus' as conditions get more extreme, such as that in rare instances, double or triple plus (e.g. MT+++) occurs. A direct hurricane passage can be associated with a TR+++++ or even greater.

You can't win 'em all. Sometimes weather data are just missing. If there are odd random gaps it's typically an issue with NCEI archiving the data, since that is the primary source of weather data for these calendars.