Weather is not climate
Weather is what happens this week; climate is the pattern over decades. Learn to tell them apart, so a cold snap or a hot day never fools you again.
TWO TIMESCALES
Weather is today; climate is the pattern
Weather is the state of the air right now and over the next few days: temperature, rain, wind, cloud. Climate is the long-run pattern of that weather in a place, usually summed up over 30 years: the typical temperatures, how much rain falls in which season, how often heat waves or frosts come.
"It will rain in Tabriz tomorrow" is weather. "Tabriz has cold, snowy winters and warm, fairly dry summers" is climate.
Two different questions
Weather
Asks: what will it be like on Friday? Changes hour to hour. Forecast a few days ahead, and it gets unreliable after about a week or two.
Climate
Asks: what is it usually like here, and how often do extremes happen? Changes over decades. Measured by averaging many years of weather.
Check yourself
In late March, Tehran gets a week of snow and freezing nights. Reza's uncle says, "So much for global warming." What is the best reply?
- He's right: if the climate were warming, a snowy week in March couldn't happen
- It's one week of weather in one city; climate is the pattern over decades
- Snow is actually caused by global warming, so it proves the opposite
- Weather and climate are the same thing, measured by different instruments
Show the answer
It's one week of weather in one city; climate is the pattern over decades
Exactly. A cold week is weather. Warming shows up as a shift in averages and in how often extremes happen, which one week in one city can't tell you.
CLIMATE IS A STATISTIC
Why scientists use 30 years
Any single year can be unusually hot, cold, wet or dry. So meteorologists describe a place's climate with climate normals: averages and ranges over 30 years. That is long enough for the lucky and unlucky years to cancel out, and short enough to notice when the pattern itself starts to move.
Rasht, on the Caspian coast, typically gets well over 1,000 mm of rain a year. Yazd, in the central desert, gets well under 100 mm. No single rainy week in Yazd changes that: it takes decades of different weather to change a climate.
Weather is the roll, climate is the dice
Each day's weather is like one roll of a die: you can't predict it far ahead. Climate is the die itself, which numbers come up how often. Warming is like slowly loading the die so high numbers come up more and low numbers less. You still roll a 1 now and then; you just roll 6s more often. Where the analogy breaks: real weather isn't pure chance. Tomorrow depends on today, which is why forecasts work for a few days.
Check yourself
Weather or climate?
- A dust storm closes schools in Ahvaz on Tuesday
- Bandar Abbas has hot, humid summers
- Heavy rain is forecast for Saturday
- Frosts in a mountain town now come later in autumn than they did 40 years ago
- Last night was the coldest night of the month
- Most of Iran's rain falls between autumn and spring
Show the answer
Weather: A dust storm closes schools in Ahvaz on Tuesday, Heavy rain is forecast for Saturday, Last night was the coldest night of the month
Climate: Bandar Abbas has hot, humid summers, Frosts in a mountain town now come later in autumn than they did 40 years ago, Most of Iran's rain falls between autumn and spring
SIGNAL AND NOISE
A slow trend under loud wiggles
Year-to-year weather is noise: big, fast ups and downs. A change in climate is a signal: a small, steady drift underneath. To see the signal you need many years and, ideally, many places. That's why scientists talk about the global average temperature, averaged over the whole planet and the whole year: local and seasonal noise mostly cancels out.
A city can swing 15 °C between two days in spring. The whole planet's yearly average usually moves by only a tenth of a degree or two from one year to the next. That calm number is where a slow trend shows up clearly.
Check yourself
Mina's city had a noticeably cooler summer this year than last year. That is good evidence that her city's climate is not warming.
Show the answer
False
False. Two summers are two rolls of the die. A cooler summer after a hot one is normal noise, even in a warming climate. To judge the trend, Mina would need to compare averages over decades, for example 1991–2020 against 1961–1990.
Check yourself
Match each question to the kind of answer it needs
Show the answer
- Should I take an umbrella tomorrow? → A weather forecast
- Is my city getting hotter? → Decades of local records
- Is the whole planet warming? → The global average temperature over many years
- Which crops suit this region? → The region's climate normals
What this means for you
- When someone points at one day, one week or one season, ask: is that weather or climate?
- Climate change shows up as shifted averages and changed odds of extremes, not as the end of cold days.
- Local records need decades; the global average shows the trend most clearly.
- The rest of this course uses the same habit: look for the long, wide pattern before drawing a conclusion.
Check yourself
Kian wants to know whether summers in his hometown have really got hotter. Which check would actually answer his question?
- Look up the hottest day ever recorded there
- Ask his grandparents whether summers used to feel milder when they were young
- Compare the station's 1991–2020 summer average with 1961–1990
- Check this week's forecast against the same week last year
Show the answer
Compare the station's 1991–2020 summer average with 1961–1990
Yes. Two 30-year averages from the same station compare climate with climate, so one odd year can't fool him.
Lesson recap
- Weather is the state of the air over hours and days; climate is its pattern over decades, usually 30 years.
- Climate is a statistic: averages, ranges and how often extremes happen.
- A warming climate still has cold days; the odds shift, like a slowly loaded die.
- To see a trend, look past the noise of single days and years to long averages, ideally global.