Use Your (Correct) Words

This is weather.
weather observations, portland oregon

This is climate.
Gross Reservoir Colorado monthly climate data

They are not the same thing. The American Meteorological Society’s Glossary of Weather and Climate (AMS, 1996) defines weather as the state of the atmosphere at a particular time. Weather consists of the short-term variations of the atmosphere, on timescales of minutes to weeks. Climate, by contrast, is the total of all statistical weather information for a given place over a specified interval of time. Climate is a “synthesis” of weather, averaged over time periods of months to decades.

In the first figure above, air and dewpoint temperatures are plotted using hourly observations over a 72-hour period. In the second figure, the monthly-averaged maximum and minimum temperatures are shown based on data from 1978-2005. Individual weather events do not and cannot give information about the climate of the area, because weather is short term and climate is long term. People (even, unfortunately, some in the scientific community) often use the terms interchangeably, and in discussions of climate change, this carelessness with words provides a disservice.

A particularly warm or cold week in your hometown does not mean that climate change is or is not happening. It means that the weather that week was anomalously warm or anomalously cool compared to the long-term climatological average. And all averages also have a variability associated with them. We’ll go back to the AMS Glossary, in which climate variability “denotes deviations of climate statistics over a given period of time (e.g., a month, season, or year) from the long-term climate statistics relating to the corresponding calendar interval.” Variability is an inherent characteristic of the climate system: we know that even on average, no month or year is likely to behave exactly like the previous month or year.

Climate change, on the other hand, is “a significant change in the climatic state of a locale or large area, typically evident with a significant change in the mean (or average) values of a weather element.” A cold week or large precipitation event does not indicate climate change: we know from the definitions above that those are weather events. However, if the average temperature for July is significantly warmer than the average for all previous Julys on record, and this pattern happens for say three (or eight, or fifteen) Julys in a row, then by the definition what has been observed is a *change* in climate.

Most often, scientists refer to climate change on a global scale. But people care most about their locality, so here are temperature records for two continental U.S. locations. On the first figure, the temperature data themselves are plotted. The second figure shows the anomalies from the long-term average, along with global temperature anomalies and linear fits to both. Do the plots correspond to weather or climate? Do you see climate variability? What about climate change?

Minneapolis St Paul temperature record
Click image to view original data plot. Source: C. Fisk; Minnesota Climatology Working Group

North Carolina, globe temperature trends
Click image to view original data plot and summary. Source: State Climate Office of North Carolina


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