Understand what each SSC type means, explore where it occurs, and learn how to read the daily classification files.
The SSC uses temperature, dew point, pressure, wind, and cloud cover to place each day into one of seven weather types.
It is relative to place and time of year. Dry Tropical represents unusually hot and dry conditions for that station and season—not one universal temperature threshold everywhere.
| Code | Weather type | What it generally feels like |
|---|---|---|
| 10 | ● Dry Moderate (DM) | Mild, dry, and often pleasant for the location and season. |
| 20 | ● Dry Polar (DP) | Relatively cold, dry, and frequently clear. |
| 30 | ● Dry Tropical (DT) | Relatively hot and dry, often with a large day–night temperature range. |
| 40 | ● Moist Moderate (MM) | Mild and humid, often cloudy, with precipitation possible. |
| 50 | ● Moist Polar (MP) | Cool, humid, and cloudy, commonly with limited daytime warming. |
| 60 | ● Moist Tropical (MT) | Warm and humid, commonly associated with convective precipitation. |
| 70 | ● Transition (TR) | Rapidly changing weather, often associated with a front or disturbance. |
| 00 or 80 | ● Missing | Insufficient observations for classification. |
Usually the coldest conditions observed at a location for that time of year, with low humidity and frequent opportunities for radiational cooling.
Often analogous to continental polar air. It may occur with cold-air advection or strong overnight cooling.
Dry conditions without the relative thermal extremes of DP or DT.
Common with zonal flow in the middle latitudes, especially downwind of mountains, or when a formerly cold air mass has substantially modified.
Usually the hottest and driest conditions at a location for that time of year.
Often originates in deserts or develops with subsidence, ridging, Chinook, or Foehn flow.
Cool, damp weather with a small daily temperature range; precipitation is often likely.
Can result from inland transport from a cool ocean, frontal overrunning, or modification of cold air downwind of water.
Warmer than MP, but still frequently associated with cloudiness and precipitation.
Often occurs near frontal zones, in marine west-coast climates, or where clouds suppress daytime warming.
Warm, moisture-rich air that dominates many tropical locations and can feel oppressive.
Common in the warm sector of cyclones and in return flow around the western side of an anticyclone.
A day when one weather type gives way to another, identified through substantial changes in pressure, dew point, and wind.
Usually associated with fronts in middle and high latitudes and tropical disturbances at lower latitudes.
A plus type is part of its parent weather type, not an additional category. Each plus indicates another increment beyond the type's usual thermal character.
21, 22…Dry Polar+, Dry Polar++…31, 32…Dry Tropical+, Dry Tropical++…51, 52…Moist Polar+, Moist Polar++…61, 62…Moist Tropical+, Moist Tropical++…71, 72…Transition+, Transition++…Select a weather type and month or season. Each circle represents an observing station; color indicates the percentage of classified days assigned to the selected type.
The common fixed scale supports comparison among maps. Circles show station observations; values are not interpolated between stations.
Each record contains a date and its numerical SSC classification. All current weather-type codes use two digits.
20260715 31 │ └─ Dry Tropical Plus (DT+) └────────── Date: July 15, 2026
SSC classifications are derived from station observations, usually at airports. Observations come primarily from the National Centers for Environmental Information, with additional data from Environment and Climate Change Canada and archives such as the Iowa Environmental Mesonet.
Source archives receive varying levels of quality control. Check station completeness and period of record before drawing conclusions, especially for trends or comparisons between stations.
Map frequencies exclude missing classifications from the denominator. Month-years below the completeness threshold configured in the generator are excluded.