Behind the Climate

What is a climate normal?
A climate normal is an average of 30 years of data.  The
Climate Prediction Center defines the current 30 year normal from a period of 1971 to 2000.  Every ten years the National Weather Service recalculates the normals using the next interval of 30 years.  In 2010, the new normals will be recalculated using the period of 1981 to 2010.

Due to the ever increasing computer power and availibilty of real-time waether data, starting in 2010 the climate normal will be recalculated each year, using the previous 30 years, to represent a more accurate trend.

Example of a cliamte normal:
The normal temperature for a specific day (ex. July 4th) is calculated by taking the average temperature during the day of every July 4th for all 30 years between 1971 and 2000. The find the average of those 30 dates.

Why does Duluth have two sets of Climate Records?
Duluth, MN was one of the nation's first weather observing sites.  Weather observations from 1871 to 1959 were taken at the origonal National Weather Service (NWS) office located in downtown Duluth, on the shore of the western tip Lake Superior.

In 1940, the NWS office was moved 6 miles inland and up 600 ft in elevation to the Duluth International Airport.  Between the years of 1941 and 1959, weather observations were taken at both locations; downtown and at the airport.  Since 1959, the official weather observations for Duluth have been taken at the National Weather Service Office located near the Duluth International Airport.

Although weather records exist from the original observing site in downtown Duluth for the period of 1941 to 1959, the official maximum and minimum temperature, as well as the daily record precipitation amount are from the threaded data which only takes into account the airport record after 1941.

Snowfall was not part of the daily climate record until 1875. Thus, the All-Time snowfall records are from 1875 to present, whereas the Airport only snowfall records are from 1943 to present. The snowfall records also have an overlap from 1943-1959 where snowfall was measured at both locations.

What is Threaded data?
, which stands for Threaded Extremes, takes the maximum and minimum temperature, and the daily total precipitation recorded at National Weather Service Automated Surface Observing Stations (ASOS) and pieces it together with other historical data nearby to create a single, long-term set of daily weather information dating back well over a hundred years. Learn More. is the U.S. government's official web portal to all federal, state and local government web resources and services.