This is one of those questions that seems simple on the surface, but when you scratch the surface a bit, it opens up a world of understanding about how we think about weather and climate.
It’s something I often find myself explaining to visitors on the Durham Weather website, especially when we’ve just had a spell of odd or extreme weather and someone asks, “Is this climate change?”
Right, let’s start with the basics.
Weather and climate are not the same thing.
They’re related, of course – a bit like how your daily moods don’t necessarily reflect your long-term personality. You might feel grumpy on a Tuesday because you didn’t sleep well, but that doesn’t mean you’re a permanently grumpy person. Weather is that grumpy Tuesday. Climate is your general nature over time.
Everyday weather is what you experience on a short-term basis.
It’s the stuff we talk about every morning. Will it rain today? Is it going to be windy? Is there a frost expected overnight? Weather forecasts usually look a few days ahead, and they can be accurate (ish) up to about a week or so.
Beyond that, things get wobbly. Weather is influenced by a lot of constantly shifting variables – temperature, air pressure, humidity, wind speed and direction, cloud cover, and so on. It’s chaotic and it changes rapidly.
Now, climate is the average of all that weather, taken over a longer period.
The standard used by the World Meteorological Organisation is a 30-year period. These 30-year averages are called “climate normals,” and they’re used as a baseline to work out what’s typical for a region. So when we say, “August is usually warm and settled in Durham,” we’re referring to the *climate* – the average pattern of August weather over three decades.
Why 30 years?
Well, it’s long enough to smooth out most of the short-term blips. A single hot summer might be a fluke. A couple of dry winters could just be natural variation. But if you start seeing a consistent trend over 30 years – say, winters getting wetter, or average temperatures creeping up – then you can reasonably start to suspect that something more fundamental is happening. It’s not just noise. It’s a signal.
Meteorologists and climatologists use these 30-year periods to spot changes over time. The current standard period is 1991–2020, which replaced 1981–2010 a few years ago.
Every decade, the baseline shifts forward.
This lets us compare the current climate against the previous normal and see if anything’s drifting.
See the 1991-2020 Averages For Durham Here.
For example, when I run temperature charts on the Durham UK Weather blog, I often overlay the 1991–2020 averages to show how the current month’s temperatures are comparing.
If we’re consistently above average, month after month, that starts to tell a story. It might not prove anything on its own, but it adds weight when we look at longer-term patterns.
And that’s where the importance comes in. Without these 30-year baselines, it would be hard to say what’s “normal” and what’s unusual. People have short memories when it comes to weather – we tend to remember the really hot days, or the snowiest winters, but we forget the drizzly average days in between.
Climate normals help cut through that fog. They give us a fixed yardstick to measure against, and that’s vital when it comes to understanding whether our climate is actually changing.
It’s especially important when people try to dismiss climate trends by pointing at day-to-day weather.
You’ll sometimes hear things like “How can global warming be real if it’s snowing in April?”
But that’s like saying your local pub is failing just because it had a quiet Tuesday lunchtime. One cold snap doesn’t disprove a warming trend – especially if the long-term averages are climbing.
The 30-year period isn’t perfect, of course. It can lag behind rapid changes. Some researchers have started looking at shorter reference periods, especially as climate shifts seem to be accelerating. But for now, the 30-year norm remains the gold standard.
Here’s something to ponder – if we’ve now got several 30-year periods to compare (say, 1961–1990, 1981–2010, 1991–2020), and each one is warmer than the last, how long can we keep shifting the goalposts before our idea of “normal” is completely out of date? Do we risk underestimating the scale of change by constantly updating what we call average?
Would love to hear your thoughts.