The U.S. National Weather Service, along with other members of the World Meteorological Organization, will begin using updated climate normals on August 1, 2011. Climate normals are determined based on a 30-year average which is updated every 10 years. The familiar 30-year period from 1971-2000 will be used through the end of July 2011. All climate products issued for the month of July 2011 will still be based on the 1971-2000 period, even if the products are issued in early August. On Monday, August 1st, the National Weather Service's database of climate normals will be updated to be based on the 30-year period from 1981-2010. Once the update is completed, the 1970s will have been effectively replaced by the 2000s, the warmest decade since reliable records began in 1880.
One of the greatest differences between the old and new datasets will show up in the temperature statistics for the winter months. For example, high temperatures in January are an average of 0.9F warmer nationally than they were previously. Normal low temperatures in January are an average of 1.7F warmer nationally. By contrast, high temperatures during the summer average only slightly warmer over the western US, and showed little change in the Great Lakes.
For more on what the new climate normals mean, please see the article below:
You can also learn more by visiting NOAA Climate Services: