Weather vs Climate

About this topic

Weather is the state of the atmosphere at a given place and time: temperature, rain, wind, cloud and pressure. It changes hour to hour. Climate is the average weather of a region over a long period, usually 30 years or more. "Weather is what you get, climate is what you expect."

A cold day doesn't disprove global warming, just as one cold snap doesn't change the climate trend. Meteorologists forecast weather; climatologists study long-term patterns and changes.

Imagine a day of 35 degrees C and sunshine in October in a place that usually gets 15 degrees. That is a weather event, but a decade of Octobers that are consistently warmer shows a climate trend. Weather forecasts are reliable for about a week because the atmosphere is chaotic, in the sense that tiny differences grow, an idea known as the butterfly effect. Climate predictions work for decades because averages and long-term forcing, such as greenhouse gases, are more predictable than day-to-day detail.

Climate is described by averages and ranges of temperature and rainfall, and includes extremes and variability. Climate scientists use records from thermometers, satellites, tree rings, ice cores and ocean sediments. Farmers, insurers, planners and governments use climate data to decide what to grow and where to build. A typical misunderstanding is to use a cold winter to argue that global warming is false; individual events are weather. Another is to think that because forecasts are sometimes wrong, climate models cannot be trusted; they answer different questions and have been shown to predict warming trends well.