What is a Climate Normal?

In this investigation, you’ve heard me talk a lot about past trends and future trends. Both are important perspectives in understanding how climate has changed (past), how it will possibility change (future), and the confidence we have in those future projections (by comparing the observed past to the predicted future).

But what about the present?

Establishing a baseline of present climate extreme conditions can be just as important! This allows us to understand how this are changing from what is “average”.

What is normal anyway?

Let’s use a weather example. Have you ever heard a meteorologist say something like “today’s high is 10°F above the average for today” to emphasize a really hot day? In order for this statement to exist, that meteorologist has to know what that average temperature is.

Figure 1: The month climate normals for Baltimore Maryalnd are the mean temperature and precipitation from 1981 to 2010.

Figure 1: The monthly climate normals for Baltimore, Maryland are the mean temperature and precipitation from 1981 to 2010. Credit: US Climate Data

Averages in weather often use a climate normal, which are 30 year means. By averaging 3 decades worth of data, scientists can have a pretty good idea of the “mean” condition. But mean does not translate to expected! For example, a mean of 50°F could be the result of a 25°F day and a 75°F day.

Figure 1 shows the monthly average precipitation and high and low temperatures for Baltimore, MD using the 30 year monthly mean of weather data from 1981 to 2010. The high temperature in Baltimore yesterday (May 15) was ~60°F, so we could say it was ~15°F cooler than normal.

Climate normals are updated every 10 years, which allows natural variability (such as El Nino effects) as well as climate changes to be included in these means.

Climate Normals from our study!

Now that you know that calculating climate normals is gets us a baseline, what does the climate extremes in the near-shore Chesapeake Bay region look like? Check out Table 1 to find out!

Table 1:

Table 1: The climate normals for all annual-based climate extreme indices in this study. Shaded cells mean there was a significant difference from the current (1981 to 2010) climate normal from the old (1951 to 1980).

Data like this can be used to assess the future climate extreme trends as they unfold as well as tell us if a year was wetter or drier than “normal.”

Kari Pohl

About Kari Pohl

I am a post-doctoral researcher at NOAA and the University of Maryland (Center for Environmental Science at Horn Point Laboratory). My work investigates how climate variability and extremes affect the diverse ecosystems in Chesapeake Bay. I received a Ph.D. in oceanography from the University of Rhode Island (2014) and received a B.S. in Environmental Science and a B.A. in Chemistry from Roger Williams University (2009). When I am not busy being a scientist, my hobbies include running, watching (and often yelling at) the Boston Bruins, and taking photos of my cat.
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