Climate Twins

Every station here has two climates: the climate of its latest 15 complete years, and the climate of 1961 to 1990. A station's climate twins are the three other stations, at least 100 km away, whose 1961 to 1990 climate is closest to this station's climate now. In other words, they are the places that had, a few decades ago, the climate this station has today.

A station's climate is described by 13 numbers: its mean temperature in each of the 12 months, and its rain in a year. A year is complete when all 12 months have at least 20 days of both daily maximum and daily minimum readings.

The climate difference between two stations is the square root of two parts added together: the mean of the 12 squared differences between their monthly mean temperatures (in degrees C), and the square of the difference between their annual rain (in mm) divided by 250. A difference of 1 therefore means monthly temperatures that are 1 C apart on average, or rain that is 250 mm apart a year. Smaller is closer.

Twins are stations, not cities. A twin is one of the three stations at least 100 km away whose earlier climate is closest to this station's recent one, so a close match does not mean the two places look alike in other ways. A station move or a gap in a record can change that station's averages. The list describes past climate; it is not a forecast and does not say what caused any change.

A station is one place, often an airport, and a city can have several stations. Climate here means monthly mean temperature and annual rain only: wind, humidity, snow, extremes and the look of a place are not compared.

A station that moved, changed instruments or has long gaps may have averages that reflect the change and not only the weather. Where this site has found a suspected change of that kind, it is listed on the station's own page and on the station changes page.