Weather vs Climate: Core Definitions and Elements
Weather is the state of the atmosphere at a particular place and time (temperature, pressure, humidity, precipitation, wind and cloud), while climate is the average pattern of weather in a place over a long period, usually at least 30 years. In brief: weather changes from hour to hour and day to day, whereas climate describes what is normal for a region and changes slowly. Weather is what you get, climate is what you expect. The six main elements used to describe both are temperature, air pressure, humidity, precipitation, wind and cloud cover.
Mixing up weather and climate is one of the most common errors in geography and in public discussions of climate change. This guide defines weather and climate, explains the elements of weather and how each is measured, compares weather and climate in a table, works through calculations of averages and anomalies, outlines the history of the two ideas and corrects common mistakes.
What are weather and climate?
Weather is the condition of the atmosphere at a specific place and time, described over minutes to days: “It is raining and 12 °C in Seattle this afternoon.” It results from the movement of air masses, fronts and pressure systems, and it can change rapidly. Climate is the long-term pattern of weather, summarized by averages, ranges and the frequency of extremes. The World Meteorological Organization uses 30-year periods (called climate normals, such as 1991–2020), which are long enough to smooth out year-to-year variations. Climate describes, for example, that Seattle has mild, wet winters and dry, warm summers. Climate variability is the natural year-to-year change around the average (for example, El Niño), whereas climate change is a lasting shift in the average and the extremes. Weather forecasts apply for days, while climate projections apply for decades.
What are the elements of weather?
- Temperature: how hot or cold the air is, measured with a thermometer in degrees Celsius (°C), in the shade about 1.5 m above the ground.
- Air pressure: the weight of the air above, measured with a barometer in hectopascals (hPa) or millibars; the average sea-level pressure is about 1,013 hPa. Falling pressure usually means unsettled weather; rising pressure means fair weather.
- Humidity: the amount of water vapor in the air, often given as relative humidity (the percentage of the maximum that the air can hold at that temperature), measured with a hygrometer.
- Precipitation: water falling from clouds as rain, drizzle, snow, sleet or hail, measured with a rain gauge in millimeters (1 mm over a square meter is 1 liter).
- Wind: moving air, described by its speed (anemometer, in km/h, m/s or knots) and direction (wind vane); the direction is the one from which the wind blows.
- Cloud cover: the fraction of the sky covered by cloud, estimated in oktas (eighths) or percent.
Other elements are visibility, sunshine hours and, for the climate, the seasonal pattern of these variables.
Weather and climate compared
| Feature | Weather | Climate |
|---|---|---|
| Time scale | Minutes to days | 30 years or more |
| What it describes | Current state of the atmosphere | Average and typical conditions |
| Changes | Quickly | Slowly |
| Predicted by | Forecast models | Climate statistics and models |
| Example | Thunderstorm this afternoon | Tropical wet and dry season pattern |
Worked example: averages and anomalies
- Weather data: the daily maximum temperatures during a week were 18, 20, 19, 22, 17, 16 and 21 °C. The sum is 133, and the mean is 133 ÷ 7 = 19.0 °C. The range is 22 − 16 = 6 °C.
- Climate normal and anomaly (hypothetical): the 30-year average July temperature in the town is 21.0 °C. This July had an average of 23.4 °C. The anomaly is 23.4 − 21.0 = +2.4 °C. A single warm month is a weather event; it does not show a climate change by itself.
- Annual rainfall: the monthly rainfall totals (mm) are 62, 48, 55, 50, 58, 52, 45, 50, 53, 70, 72 and 68. The total is 683 mm, and the monthly mean is 683 ÷ 12 = 56.9 mm. The wettest month (72 mm) is 72 − 45 = 27 mm wetter than the driest (45 mm).
- Rainfall volume: a rainfall of 20 mm over a field of 1 hectare (10,000 m2) gives 0.020 m × 10,000 m2 = 200 m3, which is 200,000 liters.
- Pressure: a barometer reading falls from 1,018 hPa to 1,004 hPa in 12 hours, a drop of 14 hPa, or about 1.2 hPa per hour, which usually signals an approaching low-pressure system and worsening weather.
How did the study of weather and climate develop?
The word “climate” comes from the Greek klima, meaning slope or inclination, referring to the angle of the Sun at different latitudes. The invention of the thermometer and the barometer in the 17th century (Torricelli’s barometer was made in 1643) allowed measurements. The telegraph in the 19th century made it possible to collect observations quickly and to draw weather maps, and national weather services were founded. Numerical weather forecasting using computers started in the 1950s, and weather satellites have been used since 1960. The World Meteorological Organization was formed in 1950 and coordinates the observations; the 30-year normal is its standard period for climate averages.
Which mistakes are common?
Mistake 1: Treating one cold or hot day as evidence for or against climate change
Weather varies from day to day. Climate is judged from long-term averages and trends.
Mistake 2: Saying “the climate is rainy today”
Today’s conditions are weather. Climate describes the typical pattern over many years.
Mistake 3: Confusing relative humidity with the amount of water vapor
Relative humidity depends on temperature: warm air can hold more vapor, so the same amount of vapor gives a lower relative humidity in warm air.
Mistake 4: Giving the wind direction as the way it is blowing toward
A westerly wind blows from the west toward the east.