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Weather is what's happening in the atmosphere on any given day, in a specific place. Local or regional weather forecasts include temperature, humidity, winds, cloudiness, and prospects for storms or other changes over the next few days. (Learn more about how NCAR studies weather.)
Climate is the average of these weather ingredients over many years. Some meteorologists like the saying that "climate is what you expect; weather is what you get," memorable words variously attributed to Mark Twain, Robert Heinlein, and others.
In practical terms, the climate for a particular city, state, or region tells you whether to pack short-sleeved shirts and shorts or parkas and mittens before you visit, while the local weather forecast tells you if you'll want to wear the parka by itself or with an extra sweater today.
Climate varies across space and time, so climate is studied on a variety of spatial and time scales.
To interpret today's atmospheric conditions, we need a reference period of average, or "normal," climate to compare it against. How long is long enough to define the average climate for a city, state, or region? The National Oceanic and Atmospheric Administration's National Weather Service calculates a 30-year average once a decade. The current "normals" (issued July 1, 2011) are based on data from 1981 to 2010. NOAA's FAQ helps put this metric in context. For example, it notes that "Normals were not designed to be metrics of climate change."
When it comes to climate on a global scale, the "normal" reference period depends on which climate components scientists want to study. For example, many scientists compare average global temperatures, precipitation, and other variables for the 20th and 21st centuries with the 30-year averages for 1870 to 1899, before major industrialization produced large quantities of greenhouse gas.
You can see how recent observations and future projections of warming and cooling compare to conditions at the end of the 19th century by watching a visualization of data from the NCAR-based Community Climate System Model in our Climate Change Multimedia Gallery.
To understand how climate varies across time, scientists examine three kinds of climate data: observations, historical accounts, and environmental evidence locked up in fossils, ice cores, and other "proxy climate records."
Observations of temperature at Earth's surface date back as far as 350 years for some locations in England, but only about 100 to 150 years in most of the developed world. But even before the thermometer was invented, ancient civilizations kept records of droughts, floods, unusual hot or cold weather, and other climate indicators, including planting and harvest times.
While human accounts can take us back hundreds or thousands of years, we need other tools to understand how Earth's climate has varied during its much longer lifetime of about 4.5 billion years.
Paleoclimatology delves into the deep history of past climate variation through what are called "proxy records." Air bubbles trapped in ice cores, the composition of lake sediments, changes in tree rings, pollen fossils, and other parts of Earth's ancient environment have given scientists many clues to past temperature, precipitation, wind patterns, and the chemical composition of the atmosphere through time.
Observations, historic accounts, and paleoclimate data are used to test the reliability of computer models that simulate Earth's climate on time scales from decades, to centuries, to millennia. Studying prehistoric variations can also provide important clues about what to expect in a warmer world.
In Depth: Weather on Steroids (AtmosNews: NCAR & UCAR news, views, analysis)
Paleoclimate Research in the Climate Change Research Section, Climate and Global Dynamics Division, NCAR
For teachers and students
Learning Resources: Climate (Spark: UCAR Science Education)
What is Climate? (Living in the Greenhouse - Kids' Crossing, UCAR)
How Do We Investigate Climates of the Past? (Windows to the Universe)