Climate vs. Weather

What is the difference between weather and climate?

This ‘cloud tsunami’ – a wedge of cloud which forms at the leading edge of a thunderstorm – rolled across Sydney in November 2015. —Credit: Club Marine.

Weather is the day-to-day state of the atmosphere, and its short-term variation in minutes to weeks. People generally think of weather as the combination of temperature, humidity, precipitation, cloudiness, visibility, and wind. We talk about changes in weather in terms of the near future: "How hot is it right now?" "What will it be like today?" and "Will we get a snowstorm this week?"

Climate is the weather of a place averaged over a period of time, often 30 years. Climate information includes the statistical weather information that tells us about the normal weather, as well as the range of weather extremes for a location.

We talk about climate change in terms of years, decades, and centuries. Scientists study climate to look for trends or cycles of variability, such as the changes in wind patterns, ocean surface temperatures and precipitation over the equatorial Pacific that result in El Niño and La Niña, and also to place cycles or other phenomena into the bigger picture of possible longer term or more permanent climate changes.

Example of a synoptic chart for 28 August 1980. —Credit: M. Serreze and R. Barry (1988).

Forecasting weather and predicting climate

Weather forecasters try to answer questions like: What will the temperature be tomorrow? Will it rain? How much rain will we have? Will there be thunderstorms? Today, most weather forecasts are based on models, which incorporate observations of air pressure, temperature, humidity and winds to produce the best estimate of current and future conditions in the atmosphere. A weather forecaster then looks at the model output to figure out the most likely scenario.  The accuracy of weather forecasts depend on both the model and on the forecaster's skill. Short-term weather forecasts are accurate for up to a week. Long-term forecasts, for example seasonal forecasts, tend to use statistical relationships between large-scale climate signals such as El Niño and La Niña and precipitation and temperature to predict what the weather will be like in one to six months time.

Climate predictions take a much longer-term view.  These predictions try to answer questions like how much warmer will the Earth be 50 to 100 years from now? How much more precipitation will there be? How much will sea level rise? Climate predictions are made using global climate models. Unlike weather forecast models, climate models cannot use observations because there are no observations in the future.

Arctic climate

Like other places on Earth, the weather in the Arctic varies from day to day, from month to month, and from place to place. But the Arctic is a unique place for weather and climate, because of the special factors that influence it. Sunlight is perhaps the most important of those factors. Above the Arctic Circle, the sun disappears in the winter, leaving the region dark and cold. What light does reach the region in the winter comes in at a low angle. In summers, the sun shines around the clock, bringing warmth and light. The Arctic also experiences frequent inversions. Inversions occur when cold air settles close to the ground, with warm air on top of it. Inversions separate the air into two layers, like oil and water: this tends to slow down the winds close to the surface. Over cities, inversions can trap pollutants, creating smoggy conditions that last until the inversion clears.

Photo of an inversion layer. —Credit: Willie Angus.

Scientists separate the Arctic into two major climate types. Near the ocean, a maritime climate prevails. In Alaska, Iceland, and northern Russia and Scandinavia, the winters are stormy and wet, with snow and rainfall reaching 60 cm (24 inches) to 125 cm (49 inches) each year. Summers in the coastal regions tend to be cool and cloudy; average temperatures hover around 10 degrees Celsius (50 degrees Fahrenheit).

Away from the coasts, the interior regions of the Arctic lands have a continental climate. The weather is dryer, with less snow in the winter and sunny summer days. Winter weather can be severe, with frigid temperatures well below freezing. In some regions of Siberia, average January temperatures are lower than -40 degrees Celsius (-40 degrees Fahrenheit). In the summer, the long days of sunshine thaw the top layer of permafrost and bring average temperatures above 10 degrees Celsius (50 degrees Fahrenheit). At some weather stations in the interior, summer temperatures are warmer than 30 degrees Celsius (86 degrees Fahrenheit).