What can we learn about the weather in North Carolina triangle area the over the last decade? Let’s look at 30-year normals, record amounts, and actual amounts. Since Raleigh Durham International Airport (RDU) is in the middle of NC’s technology triangle, and we have excellent, consistent weather records taken at the airport, we can use this data as a good proxy to answer these questions for Raleigh, Durham, and Chapel Hill–in fact, for the whole triangle region.

First, what are “normals” anyway? From NOAA’S 1981–2010 U.S. CLIMATE NORMALS: An Overview

Climate normals are typically defined as 30-yr averages of meteorological conditions, such as air temperature, precipitation, etc. They are arguably the most fundamental attributes of the climate of a given locale. In fact, the terms normal and climatology are often used interchangeably. As a measure of central tendency, climate normals characterize the background state about which anomalous conditions and even extremes are allowed to operate. They can be used to determine what crops to plant, what clothes to pack for an extended trip, the rates a power company can charge its customers, where and when to schedule an outdoor wedding, and countless other applications.


Temperature

The dark gray bands are the 30-year normals. The outer edges of the light gray bands indicate record highs and lows. The colored lines are daily data. Colored dots indicate new record highs and lows.



Normals smooth out the variation that is common in our weather. Over the last decode there have been many more days above average temperature than below average. The dotted line indicates half the year.



We see high and low records spread apparently randomly throughout the year. There was an unusually large number of record lows from the 1960s to early 1980s. Since then there has been an unusually large number of record highs.



When ordered temperature instead of day of year on the Y axis, we again see the spread of highs and lows, a record high in winter is a lot colder than a record high in summer).



We see the same decadal dynamic if we count current records by decade.



Precipitation


It’s been much wetter than normal this last decade, and we see a pattern in which the first couple of months are extra-dry, then by the end of spring the the area is above normal precipitation. Only 2011 and 2012 experienced extended periods that were dryer than normal. In 2018 Hurricane Florence pushed the yearly precipitation above the 99th percentile.



Rainfall histogram, considering only days when there was precipitation.


Density plots provide a smoothed view. The most common weekly rainfall is quite small, and very large rain events are very rare. In the following right-hand plot the X axis is on the \(log_{10}\) scale, which makes visible the density curve between zero and one inch of rain–the most common amounts.



Counting the number of days at least so much rain fell at RDU. We see a trend over the decade: an increase in the number of days wth rain.



Since the late 1980s there have a been an unusual number of days with 4+ inches that set records.



However if we count the number of current records by decade, recent decades do not seem extraordinary in this respect.



Notes and sources

Inspiration, patterns and a little code reuse:


Data sources



By Daniel Moul (heydanielmoul via gmail)

CC-BY This work is licensed under a Creative Commons Attribution 4.0 International License