Weather and climate

Look out of the window. Is it sunny? Is it raining? That is the weather.

Weather can change fast. Climate is the weather a place usually has. Some places are usually hot. Some places are usually cold.

Weather and climate are about the same things: temperature, rain and snow, wind and clouds. The difference is time.

Weather is what is happening in the air right now, or today, or this week. It might be sunny in the morning and stormy by afternoon. Climate is the usual pattern of weather in a place over many years. For example, a place might usually have snowy winters and warm summers. That is its climate, even if one winter day turns out warm and rainy. A handy way to remember it: weather is what you get today; climate is what you can expect.

The WMO defines weather as the state of the atmosphere at a particular time, as described by meteorological elements including temperature, precipitation, atmospheric pressure, wind and humidity. Weather describes short-term events, hour by hour and day by day.

Climate, in the WMO's words, describes the average weather conditions for a particular location over long periods of time, usually years or decades. To capture typical conditions, meteorologists compute a climatological standard normal from 30 years of data. Climate is more than a single average, though: normals also describe how much conditions usually vary, such as how many rainy days a month brings. The WMO describes the climate system as five linked parts, the atmosphere, the hydrosphere (oceans, lakes and rivers), the cryosphere (ice and snow), the land surface and the biosphere (living things), whose exchanges of energy, water and carbon dioxide set each region's climate.

Weather can be hot or cold. It can be wet or dry. It can be windy or still.

There is sunshine, rain, snow, hail and mist. There are storms too. Weather forecasters tell us what the weather will do next.

Scientists who study weather, called meteorologists, describe it with a few main measurements: temperature (how hot or cold the air is), precipitation (rain, snow, sleet or hail), air pressure, wind and humidity (how much water vapor is in the air).

Weather can be sunshine, rain, snow, hail, sleet, mist, blizzards or storms. Because it changes so quickly, people use weather forecasts to decide what to wear and to plan their day. Forecasts also warn people when dangerous weather, such as a big storm or a flood, is on the way, so they can get ready and stay safe.

Weather is measured through a handful of variables: air temperature, precipitation, atmospheric pressure, wind speed and direction, and humidity. The WMO lists everyday weather as sunshine, rain, snow, hail, sleet, mist, blizzards and storms, and describes extreme weather as events that are rare for a particular place and time of year, such as heatwaves, cold waves, heavy precipitation, drought, tornadoes and tropical cyclones.

Forecasts depend on international cooperation. The WMO coordinates a global network of Earth-system observations and the free and open exchange of data among national weather services, which feed numerical weather prediction. Forecasts support everyday choices and lifesaving decisions: the WMO co-leads the United Nations initiative Early Warnings for All by 2027, which works to strengthen observation, monitoring, prediction and warning in every region of the world.

To find a place's climate, scientists look at its weather for a long time. They look at about 30 years!

They work out what is usual. Is it usually hot in summer? Does it usually rain a lot?

To describe a place's climate, scientists collect weather measurements for 30 years and work out the averages. These averages are called climate normals. They show the usual high and low temperatures for each month, how much rain or snow usually falls and much more.

In the United States, NOAA works out new normals every 10 years, using measurements from thousands of weather stations. The newest ones cover 1991 to 2020. Normals help people in lots of ways: travelers pack the right clothes, farmers choose the best crops to plant, and power companies plan how much energy people will need for heating and cooling.

Normals are the working definition of climate for a place. NOAA's National Centers for Environmental Information (NCEI) calculates official U.S. Climate Normals over a uniform 30-year period, as WMO members are required to do every ten years. The 1991–2020 set, released in May 2021, draws on more than 15,000 stations reporting precipitation and more than 7,300 reporting temperature, and includes annual, seasonal, monthly, daily and hourly averages and statistics for temperature, precipitation and more; supplemental 15-year normals (2006–2020) were released alongside them.

Normals are not simply raw averages: NCEI checks 30 years of observations for quality, compares them with nearby stations and fills gaps before calculating them. NOAA describes normals as both a ruler for comparing today's weather and a predictor of near-future conditions, and notes that they reflect the effect of a changing climate on day-to-day weather rather than measuring long-term trends, which are assessed separately.

In Chicago, winter is usually cold. Summer is usually warm.

That is Chicago's climate. But one winter day can be warmer than usual. That is just the weather that day.

Here is an example. WMO figures for Chicago, in the United States, show that in January the usual daytime high is about −0.6 °C, just below freezing, and the usual nighttime low is about −8.6 °C. In July the usual high is about 28.9 °C. These numbers are averages for 1981 to 2010. They describe Chicago's climate.

But no single day has to match them. A January day in Chicago can be milder or much colder than the normal, and a July day can be cool and rainy. When you hear a weather report say it is warmer than normal today, it is comparing today's weather with the climate.

The WMO's World Weather Information Service lists Chicago's 1981–2010 normals: a January mean daily maximum of −0.6 °C and minimum of −8.6 °C, a July mean daily maximum of 28.9 °C and minimum of 17.7 °C, and monthly precipitation from about 44 mm in January to about 125 mm in August, with roughly 8 to 12 days of precipitation each month.

These figures are the climate: what a Chicago resident can reasonably plan for. The weather on a given day is one draw from that range, and individual days often land well above or below the mean. A forecast tells you what today's draw is likely to be; normals tell you the shape of the whole range. When forecasters say a day is "10 degrees above normal", they are measuring weather against climate, which is exactly what NOAA describes normals as being for.

Places in the world have different climates.

In Singapore, it is warm all year and it rains a lot. In Cairo, it hardly ever rains. In London, it is mild and rains a little in every month.

Climates are very different around the world. WMO figures show that Singapore, near the Equator, is warm all year: its usual daytime highs stay between about 30 and 32 °C in every month, and about 2,100 millimeters of rain falls in a year. Cairo, in Egypt, gets only about 25 millimeters of rain in a whole year, and in summer its usual highs reach about 35 °C. London has mild weather, with usual highs from about 8 °C in January to about 23 °C in July and some rain in every month.

Why are they so different? Many things work together to shape a climate, and the climate zones page explores them.

WMO normals make the contrasts concrete. Singapore (1991–2020) has mean daily maxima between 30.5 and 32.4 °C in every month and about 2,113 mm of precipitation a year. Cairo (1981–2010) receives about 25 mm a year, with July and August maxima near 35 °C. London (1981–2010) ranges from about 8 °C mean daily maxima in January to about 23 °C in July, with about 558 mm of rain spread across every month.

Each place's climate emerges from the exchanges of energy, water and carbon dioxide that the WMO describes among the atmosphere, oceans, ice, land and living things. Geographers group the resulting patterns into climate zones, from tropical to polar, which the climate zones page explores.

Climate stays much the same for a long, long time. But it can change slowly.

Scientists measure the weather every day, all over the world. This helps them see if a climate is changing.

Weather changes every day, but climate changes much more slowly. The WMO explains that when the long-term average of weather shifts, that is called climate change.

Climate can change for natural reasons, such as big volcanic eruptions or changes in the Sun. Scientists have found that people are also changing the climate today, mostly by adding greenhouse gases to the air, which is warming the planet. Because weather is measured carefully every day at thousands of places, scientists can compare the averages over time and see how climates are changing.

The WMO defines climate change as shifts in the long-term average of weather variables and patterns. The climate system changes under its own internal dynamics and because of external forcings, including volcanic eruptions, solar variations, changes in Earth's orbit and human influences such as changing the composition of the atmosphere and land use. The WMO states that human influence contributes to increased warming of the atmosphere, ocean and land, largely through activities that increase greenhouse gas emissions.

This is why careful, standardized observation matters. The WMO sets technical standards so that measurements from different countries are quality-controlled and comparable, and has published its State of the Global Climate reports for more than three decades. A single unusual day says little about climate; long, consistent records reveal how the averages themselves are moving.