NORFOLK, Va. — It goes without saying that July is the hottest month in Hampton Roads, and looking at the average, or normal, high temperatures, we are entering the hottest two weeks of the year.
Between July 6 and July 22, our normal high temperature is 90°. But what are these normals? And how are they calculated?
Climate normals are 30-year averages of the temperature and precipitation. But they are not just simply 30-year averages of daily observations. They are a smoothed average, so there is not a lot of variation from day to day. This ensures they represent the full 30-year period and are consistent.
Our current set of climate normals use data from 1991 through 2020, even though temperature records for Norfolk go back to 1874 while precipitation records date to 1871.
Why do we use a 30-year average? The governing body of international meteorology decided in the 1930s to use a 30-year average because that was the longest period of time most countries had reliable climate records. So the first set of climate normals was for 1901-1930.
In 2015, the World Meteorological Organization decided that climate normals would be updated every 10 years.
Normals serve two purposes. They are a reference for monitoring current weather and climate and are a basis for determining whether today’s weather is warmer or colder, wetter or drier.
For more information on climate change and the climate normal, check out climate.gov.