"It snowed last week, so how can the planet be warming?" "The forecast was wrong yesterday, so how can scientists predict the climate decades ahead?" These common questions come from mixing up two related but very different ideas: weather and climate. Weather is what is happening in the atmosphere right now or over the next few days. Climate is the pattern of weather over many years. Understanding the difference helps make sense of daily forecasts, seasonal patterns and the long-term changes scientists are measuring around the world. This explainer describes what weather and climate are, how they are studied and how they are connected.
Weather: the atmosphere right now
Weather is the state of the atmosphere at a particular place and time. It includes:
- Temperature: how hot or cold the air is.
- Precipitation: rain, snow, sleet or hail.
- Humidity: how much water vapor is in the air.
- Wind: its speed and direction.
- Air pressure: the weight of the air above, which influences weather systems.
- Cloud cover and visibility.
Weather can change from minute to minute and from one town to the next. A sunny morning can turn into a stormy afternoon. It is driven by the uneven heating of Earth's surface by the Sun, which creates differences in temperature and pressure. Air moves from high-pressure to low-pressure areas, creating wind and carrying heat and moisture around the globe.
Climate: the long-term pattern
Climate is the average weather, and its typical range of variation, in a region over a long period, usually at least 30 years. It describes what conditions are normal for a place and season, and how often extremes occur.
A simple way to remember the difference: weather is what you get; climate is what you expect. Another popular saying: climate is your personality, weather is your mood.
For example, the climate of Phoenix, Arizona, is hot and dry, with mild winters and very hot summers. On any particular day, though, the weather in Phoenix might be cool and rainy. The occasional rainy day does not change the fact that the climate is dry.
Climate includes not only averages but also variability and extremes: how often heat waves, droughts, floods and cold spells occur, and how severe they tend to be.
Why a cold day does not disprove warming
Because weather varies so much, a single cold day, a snowy winter or a cool summer does not say much about the climate. Climate trends are measured over decades and across large regions or the whole planet. Even in a warming world, cold weather still happens, but over time, record heat becomes more common than record cold.
Scientists measure global climate using thousands of weather stations on land, ships, buoys and floats in the oceans, weather balloons and satellites. By averaging these measurements over many years, they can detect long-term trends that are hidden by day-to-day variation.
How weather forecasts work
Modern weather forecasting is one of the great achievements of science. It works through several steps:
- Observation: Weather stations, radar, satellites, balloons, aircraft and ocean buoys collect millions of measurements of temperature, pressure, humidity and wind around the globe every day.
- Data assimilation: These observations are combined into a detailed three-dimensional picture of the current state of the atmosphere.
- Numerical models: Supercomputers solve equations of physics that describe how air moves, heats, cools and condenses. The atmosphere is divided into a grid of millions of boxes, and the model calculates how conditions in each box change over time.
- Ensembles: Forecasters run the model many times with slightly different starting conditions. If most runs agree, the forecast is more confident. If they diverge, the forecast is less certain.
- Human interpretation: Meteorologists interpret the results, using their knowledge of local conditions.
Why forecasts lose accuracy
Today, a five-day forecast is roughly as accurate as a one-day forecast was in the 1980s. Still, forecasts become less reliable the further ahead they look. Detailed forecasts are generally useful up to about one to two weeks.
The reason is that the atmosphere is a chaotic system. Tiny differences in starting conditions, too small to measure, grow over time into big differences in outcome. The meteorologist Edward Lorenz discovered this in the 1960s and popularized it with the idea of the "butterfly effect": the notion that the flap of a butterfly's wings could, in theory, influence the development of a storm weeks later.
Then how can climate be predicted?
If weather cannot be predicted beyond a couple of weeks, how can scientists project climate decades ahead? Because they are predicting different things.
Climate projections do not try to forecast the weather on a specific day in 2050. They estimate long-term averages and probabilities, such as how much warmer summers will be on average or how often heavy rainfall will occur. These depend on factors that change slowly and predictably, such as the amount of energy arriving from the Sun and the concentration of greenhouse gases in the atmosphere.
An analogy: you cannot predict the result of a single roll of a die, but you can predict that if you roll it thousands of times, each number will appear about one-sixth of the time. If someone loads the die, you can predict that the average will shift, even though individual rolls remain unpredictable.
What controls climate?
Different places have different climates because of several factors:
- Latitude: Regions near the equator receive more direct sunlight and are warmer; regions near the poles receive less and are colder.
- Altitude: Temperature generally falls with height, by roughly 3.5 °F per 1,000 feet (6.5 °C per kilometer), so mountains are cooler than lowlands.
- Distance from the ocean: Oceans heat and cool slowly, so coastal areas have milder climates. Inland areas have more extreme temperature swings, a pattern called continentality.
- Ocean currents: Warm currents like the Gulf Stream make Western Europe milder than other places at the same latitude. Cold currents cool coastlines like those of California and Peru.
- Prevailing winds and mountains: Mountains can block moist air, creating wet conditions on one side and dry "rain shadows" on the other.
- Large-scale circulation: Global patterns of rising and sinking air create wet tropical regions and dry subtropical deserts.
Climate zones
Scientists often classify climates using systems like the Köppen classification, which groups them into categories such as tropical, arid, temperate, continental and polar, based on temperature and precipitation patterns.
Natural climate variability
Climate is not perfectly constant even without human influence. It varies naturally on many time scales:
- El Niño and La Niña: Every few years, changes in sea surface temperatures in the tropical Pacific Ocean shift weather patterns worldwide, influencing rainfall, droughts and temperatures on several continents.
- Volcanic eruptions: Large eruptions can inject particles into the upper atmosphere, reflecting sunlight and cooling the planet for a year or two.
- Solar variations: Small changes in the Sun's output have minor effects on climate.
- Orbital cycles: Over tens of thousands of years, changes in Earth's orbit and tilt have driven ice ages and warm periods.
Climate change
Today, scientists are observing a rapid change in Earth's climate. Global average surface temperatures have risen by about 1.2 °C (about 2.2 °F) or more since the late nineteenth century. According to the Intergovernmental Panel on Climate Change, it is unequivocal that human activities, mainly the burning of fossil fuels, which adds carbon dioxide and other greenhouse gases to the atmosphere, have caused this warming.
Effects include more frequent and intense heat waves, changes in rainfall patterns, rising sea levels from melting ice and warming oceans, shrinking glaciers and Arctic sea ice, and shifts in ecosystems. The connection between weather and climate is important here: as the climate warms, the odds of certain kinds of extreme weather events, especially heat extremes and heavy rainfall, increase.
Scientists can now often estimate how much climate change has influenced the likelihood or intensity of a specific extreme weather event, a field called attribution science.
Questions readers ask
What is the main difference between weather and climate? Weather describes atmospheric conditions over hours or days. Climate describes the average conditions and their variability over decades.
How long is needed to define a climate? Climate is typically defined using averages over at least 30 years.
Why are weather forecasts sometimes wrong? The atmosphere is chaotic, and small uncertainties in measurements grow over time. Forecasts become less accurate further into the future.
Can a cold winter happen in a warming climate? Yes. Weather varies naturally. Climate trends are measured over long periods and large areas.
What is El Niño? A natural climate pattern in which the tropical Pacific Ocean warms, shifting weather patterns around the world for months at a time.
In short
Weather is the condition of the atmosphere at a given time and place, changing from hour to hour; climate is the long-term pattern of weather over decades. Weather forecasts rely on observations and computer models but become uncertain after a week or two because the atmosphere is chaotic. Climate projections focus on averages and probabilities influenced by slowly changing factors like greenhouse gases. Latitude, altitude, oceans and winds shape the world's climates, and today human activities are warming the climate, shifting the odds of the weather we experience. Knowing the difference helps you read both forecasts and climate news with clarity.