Weather vs. Climate: What's the Difference?
People often talk about climate and weather as the same thing. However, they are quite different and these differences have important implications for how we predict changes in weather and climate.
Weather describes the atmospheric conditions at a specific place at a specific point in time. For example, the observed weather in Seattle, Washington, on Saturday, October 16, 2010 was sunny with a high of 57°F.
Weather is generally described in short time frames - minutes, hours, days, and weeks. Conditions associated with weather include (but are not limited to) sunshine, rain, cloud cover, winds, hail, snow, sleet, freezing rain, flooding, blizzards, ice storms, and thunderstorms (NASA 2010).
Climate refers to the statistics of weather. In other words, the average pattern for weather over a period of months, years, decades, or longer in a specific place. Going back to our earlier example from the weather discussion, the average high temperature for Seattle, Washington, on October 16 for 1971-2000 is 60°F. This value is determined by taking the average of all high temperatures recorded for the 30 October 16ths that have occurred between 1971-2000.
These examples of actual versus average conditions point to an important difference between weather and climate: while climate is what you expect, weather is what you get. Another easy way to remember the difference between weather and climate is the following statement:
"You pick your vacation destination based on the climate but pack your suitcase based on the weather."
Climate can vary seasonally or annually (e.g., because of El Niño or La Niña), over decades (e.g., because of the Pacific Decadal Oscillation), or over much long time scales (centuries or more). Climate has always varied because of natural causes. Increasingly, however, human increases in greenhouse gas emissions are beginning to cause changes in climate as well.
The following table summarizes some of the important differences between weather and climate. The difference between weather and climate is also illustrated in a short Norwegian animation about a man walking his dog. Watch the YouTube video clip.
|Definition||Describes the atmospheric conditions at a specific place at a specific point in time.||Describes the average conditions expected at a specific place at a given time.|
Short term: Minutes, hours, days, or weeks
|Long term: Months, years, decades, or longer|
|Determined by:||Real-time measurements of atmospheric pressure, temperature, wind speed and direction, humidity, precipitation, could cover, and other variables||Aggregating weather statistics over periods of 30 years ("climate normals").|
Predicting Weather and Climate
People often wonder how future changes in climate can be predicted with any reliability when weather cannot be predicted more than a week in advance.
As noted previously, weather describes conditions at specific points in time and place. Predicting weather involves trying to specifically predict how, when, and where certain weather conditions will occur. Will the storm occur in Seattle or will it move south to Tacoma? Will the storm occur on Friday or Saturday? Will it occur in the day or the evening, during rush hour? Will the temperature when the storm hits be 40°F (making it a rain storm) or 32°F (making it a snow storm)? Answers to these specific questions have important implications for how people potentially affected by the storm prepare for and react to the storm. These answers require a high level of specificity in weather modeling that contribute to the challenges of reliably forecasting weather more than a week in advance.
Climate projections do not look to answer such specific questions. Climate projections provide insight into possible future changes in average conditions (e.g., temperature and precipitation) for a given location (e.g., the Pacific Northwest) for a given period of time (e.g., the decade of the 2020s, 2040s, or 2080s). Changes in the seasonality of certain events, such as changes in peak streamflow, or changes in extreme events, such as heat, precipitation, or flooding, are also important questions when projecting changes in climate.
While both weather and climate forecasting are challenging, projecting changes in climate does not require the same level of specificity as weather forecasting. This makes it possible to project how climate conditions may change in the future with some reliability.
More on projected changes in Pacific Northwest climate is available here.