So, take the average November temperature at an airport that routinely reports data such as Philadelphia International Airport. The average temperature is first calculated by adding up the high temperatures for each day of the month and dividing that sum by the number of days in the month (30, in this case). The average low temperature is calculated in the same way. Then, these two results (the average high and average low temperature) are divided by two to produce the average November temperature for any particular month. This method was done for each November between 1981 to 2010 to come up with the average November temperature for Philadelphia, which is 47.6 degrees Fahrenheit.
The above paragraph defines how the averages are produced but why are they calculated over a 30-year period and are they updated by the government? This process was implemented by the World Meteorological Organization about 100 years ago. The first set of 30-year climate averages were implemented in the early 1950s. So, the first climate normal consisted of the averages covering the period from 1921-1950. Statisticians indicate at least 30 numbers are needed to get a reliable estimate of their average, so that is why the World Meteorological Organization chose the 30-year period for climate normals.
Since they are updated every 10 years, the succession of climate normals over the years have been as follows: 1921-1950, 1931-1960, 1941-1970, 1951-1980, 1961-1990, 1971-2000 and the current set of climate normal are 1981-2010. So, the average precipitation (units are inches) you see for each of the forecast zones for each month of the year in the Hagerstown Almanack are derived from the most recent averages of 1981-2010.
The year, 2020, is the final year’s dataset in what will be the next set of climate normals. Since the climate normal are updated once every 10 years, we should see the latest averages released from the government during the summer of 2021. This is based on when the government released the last climate normal, which happened 7 months after 2010 was finished, so July of 2011. So, later in 2021 or early in 2022 if there is any delay in distributing the climate normal, we will see the new average temperatures and precipitation for cities, states and the U.S. These numbers will be crunched using the daily high and low temperatures and precipitation from each of the 9,800 stations across the U.S. from the years 1991-2020.
Hagerstown Town and Country Almanack
National Weather Association
Weathercasters Seakl of Approval