What Exactly is Normal Weather?

Posted: October 13, 2015, 10:46 am by cspeciale


Record Flooding in Hackettstown, NJ after Tropical Storm Irene Aug 2011


During extreme weather events spanning from heat waves to blizzards to floods, meteorologists often compare impressive temperature and precipitation values to “normal” to help put unusual weather events into perspective. But how exactly do scientists know the “normal” or “average” weather for any given location? Read on to find out.

First off, the climate community uses data measured over a 30 year period at weather stations monitored by the National Oceanic and Atmospheric Administration (NOAA) to find out the typical temperature and precipitation trends that occur on a daily, monthly and even yearly time frame. Most stations have been active for at least 30 years with some for than more than 75 years. Unfortunately, not every spot on the map houses a weather station so we do not have data for every location; however, temperature is measured at 7500 weather stations across all 50 states and U.S territories while there are 9300 and 6400 stations that measure precipitation and snowfall, respectively.

Currently, meteorologists use the weather data observed over the 30 year period between 1981 and 2010 to define "normal" or "average" weather which scientists call climate normals. These normals are recalculated every ten years so expect NOAA to release a new set of climate normals in 2021 which will include data measured between 1991 and 2020.

But once we define normal temperature and precipitation amounts, how are extreme weather events identified? Let's think back to one month that sticks out in many minds...August 2011. Tropical Storm Irene as well as other major rain events soaked the Northeast and led to catastrophic flooding. By comparing the August 2011 monthly rainfall total to the 30 year average, meteorologists learned the extent of the extreme rainfall anomaly that occurred during the month. As the graph below shows, New York City's normal August rainfall or its 30 year rainfall average is 4.44 inches. However, in August 2011 the city measured nearly 4 times that amount at 18.95 inches! Even Hartford, CT and Baltimore, MD observed incredible rainfall at 11.67 inches and 10.38 inches, respectively, or nearly 3 times the average!



Normal August rainfall as defined over the 30 yr period 1981-2010
Compared to August 2011, one of the wettest months on record!


Therefore, climate normal prove to be useful tools when deciding if a region is in the midst of an extreme event or if such events typically occur in that region. Of course, some weather parameters, namely snow, can be calculated on a smaller time scale (5 or 10 years) and updated every year. It's important to note these averages do not undergo the same quality control measures that are performed on the 30 year averages. For more information, see our blog.

So, next time your local TV meteorologist uses the word “normal” to illustrate how unusual a weather event is, you have perspective on what she means!

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