Whatever the weather: A guide to climate and weather by Sylvia Knight

Learn about weather and climate with Sylvia Knight, Head of Education at the Royal Meteorological Society.

In England, as I write this, we’ve had an unusually dry, warm and sunny start to the year so far.  

The weather has an impact on almost everyone. Bright, sunny weather tends to make people feel good. But, as the ground dries and reservoir levels fall, there are negative impacts too – on vegetation and wildlife, water availability, hydroelectricity, and wildfires. 

In the UK, and probably in most places where the weather is highly variable, the weather forms a staple part of conversation. But what is the weather, and what controls how it varies from day to day and location to location?

What do we mean by ‘climate’?

“Climate is what you expect but weather is what you get”, a phrase frequently attributed to Mark Twain, neatly sums up the difference between weather and climate. 

Climate is the average weather – for example the mean annual rainfall in Glasgow, the average January temperature in Paris, or how many hurricanes usually happen in the North Atlantic each year. However, on any particular day, it might be wetter or drier than usual in Glasgow, colder or warmer than usual in Paris or there may be more or fewer North Atlantic hurricanes in a particular year, but this won’t necessarily affect the average, the climate, unless there is a string of years with the same weather. 

The climate is ultimately controlled by long-term, large-scale factors in the climate system, from the balance between the incoming and outgoing energy at the top of the atmosphere to the colour and slope of the Earth’s surface in a particular location or the current position of the continents. 

How is this different from ‘weather’?

Weather and climate are inexorably linked – the climate determines the range of possible or probable weather types you might experience at a given place at a given time of year, as well as what is most likely. Extreme weather is usually defined as the weather which occurs less than 10% of the time – so, for example the 5% of the warmest or 5% of the coldest temperatures for a specific location at a given time of year. 

The weather is chaotic, not random, in the way that it varies from day to day and year to year. If the weather were random, it would mean that there would be no possible way of knowing what it was going to do next. However, the weather obeys the laws of physics and every change in the weather has a cause. The problem is that since there are so many possible causes, we can’t know about them all.

Weather forecasting

The butterfly effect (first proposed by Ed Lorenz in the 1960s) explains that a butterfly flapping its wings in the Amazon rainforest might, through a long line of unlikely but possible consequences, cause a storm over Texas. So, if we don’t know what’s going on in the atmosphere and on the Earth’s surface down to the detail of a butterfly flapping its wings now, we can’t hope to know how that’ll affect the weather in a fortnight’s time. The possible range of consequences grows with time into the future – and the ability to accurately forecast the weather falls. 

Modern forecasting techniques try to capture the range of possible future weather by making an ‘ensemble’ of weather forecasts – rather than making one forecast with one set of starting conditions (the weather now) they make many forecasts, each with tiny differences in the weather now – trying to take into account the effects of all the possible ‘butterflies’ or other tiny details of the climate system that we can’t possibly measure. The ensemble of forecasts gives forecasters a range of possible weather forecasts, with some indication of what’s most likely, and what might happen.

Climate change and weather

The climate, unlike the weather, is not chaotic. If climate is ‘average weather’, then if the large-scale factors which control the climate are known – the composition of the atmosphere, the location of the continents, the Earth’s position in relation to the Sun etc. then it’s possible to predict the climate.

If the weather and climate are inexorably linked, then climate change is too. As global warming changes the climate, the range of possible or probable weather types you might experience at a given place and at a given time of year changes too. 

As the climate isn’t chaotic, then projections of how it may change in the future simply (!) rely on knowing or speculating how the large-scale factors which control the climate will change. Some things – such as the Earth’s future position relative to the Sun, are easy to know very precisely. Others, such as the composition of the atmosphere, which in turn depends on the emission and/ or removal of gases, can only be speculated. This is why projections of future climate are based on a wide range of socio-economic scenarios. 

Myles Allen (University of Oxford) summed up the impact of climate change with an adaptation of the quotation we started with: 

“Climate is what we affect, weather is what gets you.”

There are few ways in which say a 1.5°C change in temperature, global climate, will directly affect people. Sea level rise would be one, driven by the expansion of sea water as it warms as well as the melting of land-based ice, as well as the shifting of biomes. However, the impact of this 1.5°C climate change on the range of possible or probable weather types arguably has wider consequences, from coral bleaching events to droughts and more intense tropical cyclones and mid-latitude rainfall. 

Attribution and seasonal forecasts

The opposite to forecasting is attribution – using computer models to identify the cause of a particular extreme weather such as extreme heat or precipitation. Our ability to attribute (or not) an extreme weather event to global warming is improving rapidly.

In between weather forecasts and climate projections come seasonal forecasts – the ‘what will the weather be next winter’ type questions. As the weather is chaotic, this is very hard to do, but there is some skill to be found in looking at the large-scale influences on the weather – for example, the El Niño/ La Niña seasonal oscillation or the North Atlantic Oscillation all make certain weather patterns more likely.

Final thoughts

The weather can be awe-inspiring, life-giving or simply enjoyable, as well as destructive or frustrating. It impacts on us all in direct and indirect ways, making weather (and climate) literacy key in empowering individuals to understand and respond to weather-related risks and impacts, ultimately improving their ability to navigate daily life and make informed decisions as individuals and local and global citizens.

The Royal Meteorological Society is the leading independent expert in weather and climate, offering, amongst other things, MetMatters, a blog for all those interested in weather and climate and the annual Weather Photographer of The Year competition.

Sylvia Knight is Head of Education at the Royal Meteorological Society and a visiting Professor at the University of Reading.