1. What Does "Normal" Mean When the News Says It Is Warmer Than Normal?
You might hear news reports say, "This August is warmer than normal." This "normal" is not the weather from when you were born, nor is it a fixed value set long ago. The JMA defines the "normal value" as the average over 30 years, starting from years ending in 1, and updates it every 10 years. The current normal values are the averages from 1991 to 2020, and they began being used on May 19, 2021. [1]
These normal values serve as a benchmark for evaluating weather at a given time, such as a cool summer or a warm winter. At the same time, they represent the climate of that specific location. The JMA explains that the average from 1991 to 2020 is called the "2020 Normal Values" and will be used from 2021 to 2030. [2] [3]
However, this yardstick has a rule: it ends its role after 10 years. First, let’s look at why the period is 30 years.
2. Why Is the Normal Value an Average of 30 Years?
The method for creating normal values is to look at a 30-year window, shifting it every 10 years. Imagine having 30 years of diaries and taking the average only from the part visible through a window. In reality, the items lined up are not diaries but observed numerical data.
It is introduced that the 30-year period was first decided in 1935, and the first normal values were the average from 1901 to 1930. It seems that at the time, available observation data was limited, making it difficult to extend the period further. However, this is an introduction from an explanatory article, and it is not said that there was a decisive reason for choosing exactly 30 years. [5]
Currently, the method follows the rules of the World Meteorological Organization (WMO). The JMA explains that, under the WMO definition, a normal value is the average over 30 consecutive years starting from a year ending in 1, updated every 10 years. Japan began using the values from 1981 to 2010 starting in 2011, and the values from 1991 to 2020 from May 19, 2021. [4]
We have seen how the 30-year period came to be used. So, why is it replaced every 10 years?
3. How Much Do Normal Values Change When the 30-Year Period Is Updated Every 10 Years?
The WMO recommends updating the 30-year reference period every 10 years to better reflect the changing climate and its impact on daily weather. [8]
When the update actually happened, the values moved. The new normal values (1991 to 2020) are about 0.1 to 0.5°C (about 0.2 to 0.9°F) higher in annual average temperature nationwide compared to the previous ones (1981 to 2010). This is the range of difference by location; it did not rise uniformly across the whole country. For precipitation, it increased by about 10% at many locations depending on the season. [1]
Because the yardstick moves, the same temperature might be called "high" under the 10-year-old standard but "normal" under the current standard.
So, what range does "normal" refer to?
4. How Are "Low," "Normal," and "High" Ranges Decided from 30 Years of Data?
At the JMA, the 30 years of observed values are arranged in ascending order. The 10 smallest values are classified as "low (little)," the 11th to 20th are "normal," and the rest are "high (much)." Each group contains 10 of the 30 values. [6]
For example, if you write the average August temperatures for 30 years on cards and arrange them from lowest to highest, the first 10 cards are "low," the 11th to 20th are "normal," and the remaining are "high."
The important point here is that "normal" does not refer to the area right near the exact average value. "Normal" is the range that falls into the middle 10 years when the 30 years are arranged in order. The values for this classification are also updated every 10 years and are currently made from observation values from 1991 to 2020. [6]
By the way, over 30 years, the location of observation stations or the instruments may change. What happens then?
5. How Are Normal Values Adjusted When an Observation Station Moves?
An observation station may be moved to a different place.
When creating normal values, if relocation or another change has caused inhomogeneity, a corrected "homogeneous cumulative value" is used. As a result, if you simply average the original statistical values for 30 years, they may not match the normal value. [4]
In corrections for relocation, it is assumed that a step-like shift occurred in the time series data due to the move. The temperature correction page explains that if the period after relocation is less than 5 years, no correction value is made. If it is 5 years or more but less than 8 years, a correction value is made using the data for the number of years that can be handled. These years are conditions for the correction method explained on that page, not conditions for the entire normal value. Also, that page states that corrections are not made for AMeDAS locations. [7]
Besides relocation, the way measurements are taken can also change.
6. How Are Normal Values Handled When Measurement Methods Change or Data Is Missing?
For AMeDAS daily maximum and minimum temperatures, values were derived from hourly observations until 2002. From 2003 onwards, they were derived from observations every 10 minutes. Furthermore, between 2008 and 2010, the method changed to deriving values from observations every 10 seconds at each location.
If the measurement interval differs, systematic differences appear in the maximum and minimum temperatures. Therefore, in normal values, correction values were added to create daily cumulative values. [4]
There are also rules if there are years with no observations. If the number of years with data is 8 or more, and missing data is 20% or less of the statistical period, normal values are calculated. For a 30-year period, missing data can be up to 6 years maximum. For locations that started observations after 1991, values are calculated from the first observation year to 2020. [4]
In this way, relocations, changes in measurement methods, and missing years are adjusted as much as possible to create the 30-year yardstick.
7. How Can You Check What "Normal" Means in a Forecast Yourself?
What you can do today: If you see the words "normal" or "normal range" in weather forecasts or news, try saying what they are comparing against. The current standard is the 30-year period from 1991 to 2020. [1]
Look it up: On the JMA’s Normal Values page, look up the monthly values for your location and compare them with the 1981–2010 values. You can check for yourself whether the annual average temperature changed within the 0.1 to 0.5°C range. [3]
If the rule of updating every 10 years is followed, the average from 2001 to 2030 should be used starting in 2031. However, this is an estimate based on the rule, and the JMA’s announcement has not been confirmed. [3]