Weather vs climate: what the difference actually is

Weather is the state of the atmosphere at a particular place and time: 72°F, wind from the southwest at 12 mph, a 40 percent chance of rain this afternoon. Climate is the statistical description of weather over a long period, conventionally 30 years: the average high in Chicago in July is 84°F, Seattle gets about 37 inches of rain a year, Phoenix records 110°F or higher on a typical 20 days each summer. The line usually credited to Mark Twain, and actually written by Robert Heinlein, puts it in one sentence: climate is what you expect, weather is what you get. The confusion between the two shows up in the same place every year: a bitter cold snap in January is offered as evidence against a warming climate, and a hot week in May as evidence for it. Both are weather.

Time scale is the whole difference

• Weather: minutes to about two weeks. It is measured by thermometers, radar and satellites, and forecast by physics-based models that lose skill quickly after day seven. Our guide to how accurate a 10-day forecast really is shows where the skill runs out.
• Climate: decades to centuries. It is measured by averaging weather records, and described with normals, extremes and frequencies rather than a single value. A climate forecast does not say it will rain on July 14; it says July gets four inches on average, spread over about ten rainy days.

In between sit seasonal outlooks, the "warmer than average winter" forecasts issued months ahead. Those are climate-scale statements shaded by known patterns such as El Niño and La Niña, which is why they speak in probabilities; El Niño vs La Niña and US weather covers how much they actually shift the odds.

The 30-year normals

When a forecast says "five degrees above normal", the normal is a specific number: the average for that date over the most recent 30-year period, currently 1991 to 2020, published by NOAA's National Centers for Environmental Information and updated every ten years. Thirty years is long enough to smooth out individual hot and cold years and short enough to describe the climate people actually live in. The update matters: the 1991 to 2020 normals are warmer than the 1981 to 2010 set for most of the country, which means "normal" itself has moved. Those normals are what our monthly pages are built from, for example Chicago in January or Phoenix in July, and they are the right tool for packing for a trip a month away, when no weather forecast exists yet.

Why one cold week proves nothing either way

Climate is a distribution, and weather is a single draw from it. A record cold morning in Dallas in February is entirely consistent with a warming climate; the distribution has shifted, but its cold tail still exists and will still be sampled occasionally. What the climate record shows is the change in the odds: over the past few decades, the United States has set roughly twice as many daily record highs as record lows, where a stable climate would produce about equal numbers. That ratio is a climate statement. Any single record is weather.

Which one to use when

• Today through about 7 days: the weather forecast, and the closer the day, the more you should trust it. On our city pages the NWS blend, the American GFS and the European model sit side by side, and our accuracy scoreboard shows which one has been closest in your city.
• Days 8 to 14: a blend. The forecast still carries some skill on whether the period will be warmer or colder than normal, but not on which day the rain falls. Read it as a lean, and read the normals alongside it.
• Beyond two weeks: climate. Use the normals and the historical range, plus a seasonal outlook if one exists. A wedding in October in New York is planned on climate; the rain plan is made on weather, the week before.

Two words that get swapped for climate

Climate change is a change in the climate statistics themselves over decades, the shift in the distribution. Global warming is the specific rise in the global average temperature, one component of climate change. Neither term describes any single storm, heat wave or cold snap, although climate change alters how often each occurs; heat domes and US heat waves covers the event that has shifted most. And for the mechanics of the forecasts on the weather side, how weather forecasting works is the place to start.