Climate vs. Weather

Climate and weather both describe phenomena in the Earth’s atmosphere, but the basic difference between them is time.

Weather describes the atmospheric conditions at a specific point in time. It can be measured hourly, daily, weekly, monthly, or yearly.

Climate is the average or typical weather over a long period of time – a few years or more. Understanding climate can help us see patterns in the weather, such as what time of year or what locations different types of storm systems occur or what seasonal changes are typical. Unlike weather, climate changes slowly over years, centuries, or millenia.

Daily weather reports will often compare the daily observed maximum and minimum temperatures with the average for that day or week. These averages are the "climate normals".

The National Centers for Environmental Information (NCEI, formally NCDC) explains, "the climatic normal is simply the arithmetic average of the values over a 30-year period (generally, three consecutive decades)."

In other words, for a given day at a given location, weather values, including temperature, wind, rainfall, etc., are measured over a fixed number of years and averaged to determine the climatic normal weather for that day at that location.

The current set of climate normals is based upon observed weather from the years 1991 to 2020. In 2031, a new set of climate normals will be generated based upon the observed weather between 2001 and 2030.

These climatic normals help provide context for predictions such as "this winter will be wetter (or drier, or colder, or warmer, etc.) than normal” or "unseasonably warm (or cool)." These phrases are comparisons of current weather conditions to the climatic normal for that time.

A significant example is the all-time record rainfall for the Dallas/Fort Worth Airport in 2015.

Climate normal for rainfall in that area is 36.14" (918 mm); the actual rainfall for the year was 62.61" (1,590 mm). In only three months – May, October, and November – nearly a normal year’s worth of rain fell.

The annual rainfall for 2015 broke the old all-time record by over 9" (229 mm). This was truly an extraordinary rain record that will stand for tens of decades if not centuries.

2015 was also a roller coaster year for the occurrence of rainfall in the DFW area, experiencing a stretch of 41 consecutive days with NO precipitation – the third longest number of rain-free days on record.

Not all deviations from the climate normals are as obvious as the above example.

During that same record-breaking year at the Dallas/Fort Worth Airport, on November 27, the average of the maximum and minimum temperature was 55°F (13°C). The normal for that day is 52°F (11°C). At first glance the temperature would appeare to be a "near normal" day.

However, while the maximum temperature for the day was 70°F (21°C), it occurred around 3 AM that morning, a very unusual time for a daily high temperature. Then, a strong cold front moved through and the temperature began to fall, dropping to a minimum temperature of 39°F (4°C) just prior to midnight.

The average temperature was near normal climate-wise, but that day was quite different weather-wise.

The important thing to understand is that large swings in weather day-to-day, month-to month, and even year-to-year do not necessarily imply large, rapid changes in climate. Weather, over time, will become part of the 30-year normal.