What Is the Difference Between Weather and Climate?
By the BrainSnail editorial team. How these articles are written and checked, and how to tell us when one is wrong.
A snowstorm in April does not disprove global warming and a hot week in June does not prove it, and the reason is the difference between weather and climate. Weather is the state of the atmosphere at a place and time: the temperature, wind, cloud and rain outside right now. Climate is the pattern those states make over decades. One is a day's mood and the other is a personality, and confusing them is the commonest mistake in public arguments about the atmosphere.
Two timescales
Weather changes by the hour and is driven by the movement of air masses, fronts and pressure systems across the surface. It is chaotic in the technical sense: a small difference in conditions today grows into a large difference in a week, which is why forecasts are good for a few days and useless beyond about two weeks. Nobody can say whether it will rain in London on a given day next August.
Climate is the statistics of weather: the average, the range and the frequency of extremes at a place, conventionally calculated over thirty years. It is driven by things that change slowly, the amount of sunlight a latitude receives, the distance from the sea, the height above it, the ocean currents offshore and the composition of the atmosphere. Nobody can say what next 15 August will bring in London, but anyone can say it will be warmer than 15 February, and that is a statement about climate.
Why a cold snap proves nothing
A warming climate is a shift in the whole distribution of weather, not a ban on cold days. If the average temperature rises by one degree, cold days still happen; they become somewhat rarer and hot days become somewhat more common, and the record highs start to outnumber the record lows. In the United States, record daily highs now beat record lows by about two to one, where in a stable climate the ratio would be even. A single cold week is a sample of one from a distribution that has moved, and it cannot show the movement.
The same logic runs the other way. A heatwave is not proof of climate change either, though scientists can now calculate, using models run with and without the extra greenhouse gases, how much more likely a particular heatwave was made by warming. The 2021 heat in the Pacific Northwest of North America, they concluded, would have been virtually impossible without it. Attribution of single events is a statement about probability, which is a statement about climate.
How climate is measured
The thermometer record from weather stations goes back to the mid-nineteenth century, with ships' logs filling in the oceans, and it is checked by satellites since 1979 and by measurements of ocean heat, sea level, glaciers and the dates of spring. Before instruments, climate is read from proxies: tree rings, whose width records each summer's growth; ice cores, whose layers trap air bubbles and record temperature in the ratio of oxygen isotopes; ocean sediments; and corals. Together they reconstruct the last several hundred thousand years and show that the current rate of warming is faster than anything in that record. The differences in one table:
- •Weather: hours to days; climate: decades to millennia
- •Weather: a measurement; climate: a statistic of many measurements
- •Weather: predictable for about a week; climate: predictable for decades, because it depends on slow causes
- •Weather: what to wear today; climate: what to build, plant and insure for
Why the distinction matters
The two questions need different tools and different experts. Meteorologists forecast weather by tracking the atmosphere's current state forward; climatologists project climate by working out how the energy balance of the planet responds to changes in the sun, the oceans and the atmosphere, and the second kind of prediction is in some ways easier, in the way that predicting a hot summer in the Sahara is easier than predicting rain next Tuesday. It also matters for decisions. A farmer plants for the climate and harvests in the weather; a city builds its drains for the climate it expects and floods in the weather it gets; and the gap between the two is where the damage of a changing climate arrives, in weather that the infrastructure was never designed for.
The takeaway
Weather is the atmosphere's condition at a place and moment, chaotic and forecastable only for days; climate is the long-run pattern of that weather, driven by slow causes and predictable for decades. A warming climate shifts the whole distribution of weather rather than abolishing cold days, so no single storm or heatwave proves or disproves it, and the difference is the difference between what to wear and what to build.