Weather types can be thought of as patterns of wet and dry regions, much as one would see on a daily weather map.
We found that 12 weather types are sufficient to represent the variety of day-to-day weather in the Southwest. Among those, only three of these weather types bring large amounts of moisture while the other nine are predominantly dry.
In a non-changing climate, the frequency of different weather types occurring varies from year to year (e.g., some years have more sunny days than others) but does not change over a period as long as 35 years.
To our surprise, we found in looking at historical weather data that the frequencies of the three weather types mainly responsible for rain and snow in the Southwest significantly decreased at a rate of 10 percent each decade. And they were replaced by weather types that have predominantly sunny weather.–by Andreas Franz Prein for The Conversation.
NewTerrain February 15, 2016.