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In brief
Weather is the atmospheric condition at a specific place and moment — rain, wind, or temperature right now. Climate is the average of those weather patterns over a long period, usually 30 years or more, according to NASA and NOAA. The difference is essentially one of timescale, not of the phenomena themselves.
Weather and climate describe the same underlying atmospheric system, but on different timescales. According to NASA's Science division, weather refers to local, short-term atmospheric changes that occur over minutes, hours, days or weeks — phenomena such as rain, snow, clouds, wind, thunderstorms, sleet and hail.
Climate, by contrast, is the same kind of weather data averaged over several decades, as NASA describes it, and can be assessed for a region or for the whole planet. The U.S. National Oceanic and Atmospheric Administration (NOAA) draws a similar line: weather is the atmospheric state a person experiences on any given day, while climate is the average of a location's weather patterns over a longer period, usually 30 years or more.
Why 30 years is the benchmark
NOAA states that climate scientists generally regard a 30-year span as long enough to capture the typical range of atmospheric conditions across all seasons for a given location, which smooths out the short-term variability that defines individual weather events.
NASA notes that the effects of climate change are harder for people to notice day to day than weather, because consequences such as sea-level rise, melting glaciers and ice sheets, and shifting seasons unfold over extended timescales rather than appearing suddenly.
NOAA adds that weather can shift abruptly within minutes and vary sharply between nearby locations, whereas climate stays comparatively steady across a given geographic area over time, with mountainous terrain being an exception.
Origin of a famous explanation
A widely repeated line summarizing the distinction between weather and climate is often attributed to Mark Twain. A 2012 investigation by the fact-checking site Quote Investigator found no evidence that Twain ever said or wrote it.
According to that investigation, the earliest documented version of the idea traces to British geographer Andrew John Herbertson's 1901 textbook, Outlines of Physiography (quoted below). A phrasing closer to the modern form later appeared in science-fiction author Robert Heinlein's 1973 novel, Time Enough for Love, which Quote Investigator credits with popularizing it. Twain's actual, separate connection to the subject is a 1887 essay in which he cited a student's exam answer describing climate as lasting all the time while weather lasts only a few days.
Climate is what on an average we may expect, weather is what we actually get.
— Andrew John Herbertson, "Outlines of Physiography" (1901), as cited by Quote Investigator
Questions
What is the simplest way to tell weather and climate apart?
Weather is the atmospheric condition at a specific place and time — what you see outside right now. Climate is the long-term average of those conditions, typically measured over 30 years or more, according to NASA and NOAA.
Why do scientists use a 30-year period to define climate?
NOAA says climate scientists generally regard 30 years as long enough to capture the typical range of weather conditions for a location across all seasons, smoothing out the short-term variability of individual weather events.
Did Mark Twain really say "climate is what you expect, weather is what you get"?
No. Quote Investigator found no evidence Twain said this. The earliest known version appeared in an 1901 geography textbook by Andrew John Herbertson, and a phrasing close to the modern version was popularized later by novelist Robert Heinlein in 1973.
Can a single cold or snowy day disprove climate trends?
No single weather event — hot, cold or stormy — represents climate, which NASA defines as an average over decades; short-term weather and long-term climate operate on different timescales and are not directly comparable.
What are examples of climate-scale changes versus weather-scale events?
NASA lists sea-level rise, melting ice sheets and glaciers, and shifting seasons as climate-scale changes, as opposed to weather events like rain, thunderstorms, sleet or hail, which occur over minutes to weeks.
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