The difference between climate and weather

The distinction has often been made intuitively. For example this quote from Robert Heinlein “Climate is what you expect; weather is what you get”. This makes sense; we know that winter is colder than summer, and that the UK has a terrible climate.

From this we can draw the following relatively simple definition for weather.

Weather is the here and now; it is the current state of the atmosphere. Is it raining? Is it windy? Weather varies on timescales of minutes to days. Storms, rainbows and sunny days are all examples of weather.

Climate is a bit harder to nail down. We speak of one summer being particularly warm. This suggests that our notion of climate is based on more than a single year’s weather. The exact length of time for climate is unhelpfully fuzzy, but a working definition would be:

Climate is the long-term average of the weather for a particular location and time of year.

Variants on these definitions have been used extensively to define climate and weather. For example, this poster by the [US] National Weather Service, and this page by the [US] National Center for Atmospheric Research.

But, a recent paper by Lovejoy and Schertzer has questioned the validity of these distinctions. The authors have analysed historical weather data from a number of sources. They looked at how much variation there was in temperature from day to day, week to week, month to month, year to year, millennia to millennia.

They found that the largest temperature variations occurred over 10 day periods, which is about how long a weather pattern lasts in the atmosphere. The variation then reduced to a minimum for time scales of 30 years – that is, average temperatures were fairly consistent within each 30 year period. The variation in average temperatures then rose again when considering very long time periods – for example, as ice ages came and went. This is an important point, suggesting that on very long time scales climate (as defined above) shows significant variability, in much the same way that weather varies from day to day.

The authors coined a new term “macroweather” which takes the place of climate in our previous definitions. Thus leading to the following three terms:

Weather is the short term variations.

Macroweather is the long term (10 – 30 year) average of weather.

Climate is the variation of macroweather on very long time scales (30 – 100,000 years).

In this framework macro-weather takes the place of climate as we generally understand it.

This article is still very recent, so it’s unclear whether this new definition will catch on. However, I feel pretty confident in saying that the general public will not start talking about “macroweather change” as the great moral challenge of our generation. Perhaps they should have coined the term “macroclimate” instead.

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