Weather vs. climate: Related but different

The weather often comes up in conversation. We bring it up during  small talk, and we discuss it to make plans for the hours, days and weeks ahead. But how does weather differ from climate? Are they two words for the same thing?

Hoar frost at Lockport Prairie Nature Preserve. (Photo courtesy of Michelle Wendling)

Weather and climate are related, but they are two different things. One simple way to explain the difference is with a well-known saying from American writer Mark Twain: "Climate is what we expect, weather is what we get."

Let’s think about that. Climate is what we expect. Here in northern Illinois, we expect winter to be cold and summer to be hot. Weather is what we get. That means the weather is the conditions we experience on a given day. So while yesterday may have been sunny and clear, today is cloudy and windy. That's the weather of those days.

Weather is determined by the state of Earth's atmosphere at a particular time and place using variables including temperature, wind speed, wind direction, moisture and barometric pressure. Climate is the expected frequency of specific states of atmosphere based on variables such as temperature, moisture and wind speed and direction.

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Words to know

Atmosphere: The envelope of gases that surround Earth or another planet.

Continental: Belonging to a continent.

Economy: The wealth and resources of a country or region in terms of production and consumption of goods and services.

Forecast: To predict or estimate a future event or trend.

Variable: Able to be changed or adapted.

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Essentially, climate is a description of what weather is like over a long period in a specific place or region. In many places, including here in Illinois, climate varies by season.

The weather is determined by the condition's in Earth's lower atmosphere, called the troposphere. We describe the weather based on conditions such as temperature, wind, humidity and precipitation. It can be a warm night or a snowy morning, for example.

The sun affects many of these conditions. As the sun heats air in the lower atmosphere at varying rates, it affects wind and also water vapor, which impacts precipitation.

The recorded weather conditions over time contribute to the climate information for a place or region. Information about climate is typically determined by looking at averages of temperature, humidity, precipitation, wind, sunshine and other measures over a long period of time. The 30-year averages of weather observations for a place are called climate normals.

In Illinois, we experience a continental climate with warm summers and cold winters with frequent fluctuations in temperature, humidity, wind conditions and cloudiness. Compare our climate to that of Phoenix, Arizona. There they have long, extremely hot summers and short, mild winters. Knowing our local climate helps us understand how to prepare for the seasonal changes that occur throughout the year.

Within each season, we rely on the weather forecast to plan for each day. In the winter, we watch for snow in the forecast so we are prepared to wear boots and snow gear. In the spring and summer, we check to see if we need to pack an umbrella or a raincoat.

Climate forecasts exist too, but they don't include predicted weather events like weather forecasts do. Instead, climate forecasts look at the probability of conditions, such as a colder than usual winter or a wetter than usual spring. Climate forecasts are long term, looking at periods of weeks, months or even seasons.

Understanding climate isn't just important because it helps us prepare for seasonal changes. It also helps scientists take note of changes or trends and examine how they affect people all over the world.

Climate change will impact weather patterns and shift climate conditions worldwide, and having years of data available contributes to the understanding of how it will affect human health and the global economy.