Weather vs. Climate

There’s an old saying that “climate is what you expect, weather is what you get.” Weather is the state of the atmosphere in terms of hot or cold, wet or dry, windy or calm, cloudy or clear. Instantaneous, or synoptic, measurements of meteorological variables—namely temperature, precipitation, humidity, pressure, winds, and cloudiness—are used to quantify the weather. These variables are often shown on a map or chart at a given time for a particular region.

Climate is the statistical collection of average weather conditions at a given place, typically defined over a 30-year time interval (or “normal”). At present, “normal” refers to the 1971-2000 average for a particular variable. Note that the climate defined using different periods of time may be different (e.g., the normals defined by the 1931-1960 average are different from those of 1961-1990); spatial scale also affects the definition of normals. Long-term climate is usually defined as a time average of a century or more.

Maine’s instrumental record of meteorological variables has been systematically kept for about 130 years, although measurement stations are not distributed uniformly in time or space. It is from this instrumental record that climate variables can be calculated and examined in terms of any systematic climatic change that may have occurred. For the purposes of this discussion, we restrict ourselves to temperature and precipitation as diagnostics of climate and climate change.