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Fresh Climate Normals Recently Updated

UPDATED July 24, 2011

By WeatherBug Meteorologist, Chad Merrill

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Temperatures and precipitation vary from one day to the next, but average temperatures, rain and snowfall readings provide a day-to-day reference point for what is to be expected. Changes in these "normals" have been released just this month by the National Oceanic and Atmospheric Administration (NOAA).

To understand why these numbers will be changing, it`s important to understand what a climate average or normal represents. It is simply a 30-year average for a high or low temperature or precipitation for any given day or month.

The World Meteorological Organization requires climate normals to be calculated every 30 years and recommends them to be updated every 10 years for weather and climate stations across the globe. In the U.S., these climate statistics are updated as each decade concludes.

The U.S.`s normals going forth for the next ten years are based on weather data collected from all years 1981 through 2010. Climate normals first came into existence in 1956 after the numbers were crunched from the 1921-1950 period. They have been updated every decade since.

Daily normals for high and low temperatures and precipitation are computed simply by averaging the appropriate 30 values for a particular day from the 30-year period. Monthly average normals are determined by using the 30 monthly average values for a given month. The annual normals are figured out by using the 12 monthly normals.

The fresh 1981 to 2010 normals show a distinct warming trend. Statewide averages of normals for high and low temperatures are warmer. Normal temperatures are warmer for every month with the largest monthly change being January where temperatures are 1.7 degrees warmer than the current 1971 to 2000 normals.

During the 1981 to 2010 period, the United States had its fourth warmest summer on record in 2010 while the summer of 2006 was the second warmest on record. The U.S. experienced its fourteenth warmest April on record in 2010 while the Southeast and Northeast had their warmest May through July period on record in 2010 as well. Meanwhile, January 2009 was the fifth driest for the U.S. More recently, October 2009 was the third coolest and wettest ever for the U.S.

According to NOAA, a weather station needed at least 2 years of complete data with no erroneous data and must have reported at least one value in 2010 to be considered for the 1981 to 2010 climate normals. Temperatures from a station that changes location and gets new instrumentation are compared to nearby stations for glaring differences. A statistical method is then used to smooth out the numbers from that station to keep climate normals consistent. In some cases, a 1 to 3 degree difference in the monthly high temperatures could result from these weather station changes.

While the weather has the most dramatic affect on temperatures and precipitation, certain other factors impact individual temperature sensor readings. Urban heat island effect, which is the warmth that builds in major cities due to heat-collecting pavement, buildings and factories, can be a cause of some temperature increases. Many stations change location within the course of 30 years. In addition, upgrades to weather instrumentation also influence temperature and precipitation records at a location.

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