Definition of Climate: Average versus Accumulation

Definition of Climate: Average versus Accumulation:

Climate is often defined as the “average weather.” More complete definitions include not only the average, but statistical representations of variability. Because weather is not a measurable parameter, the definition of climate ultimately relies upon a set of more basic measures of, traditionally, the state of the atmosphere – temperature, wind, and moisture. Furthermore, the quantification of the climate relies upon the definition of the averaging period.

Climate, however, encompasses much more than the weather and the atmosphere. Therefore, the definition of climate is extended beyond measures of the atmosphere to include other components of the Earth system: cryosphere, ocean, land, and their chemical state. In addition, measures of the state of the Sun and the geological state of the Earth are potentially important. The specification of the averaging period is often anchored in the quality and completeness of the observational record. The World Meteorological Organization recognizes a 30 year averaging time as a standard. The study of weather is implicit in this choice of an averaging period because of the availability of global measurements which reliably sample the spatial and temporal scales of weather. As our data record is extended, we discover important modes of variability that are longer than 30 years. Also, the time scales in the non-atmospheric components of the Earth system are longer than those in the atmosphere; hence, the average states of these components are not represented by the same average that might yield an “average weather.”

The two extensions to the definition of climate listed above are scientific in nature and any deficiencies in the definition can be accounted for with quantitative rigor. A third challenge to the definition of climate follows from the exposure of scientific investigation of the climate to society as a whole. Climate science and predicted climate change become not solely the purview of the science community. If the knowledge of climate predictions is to become the foundation for decisions in energy policy, infrastructure investments, and adaptation strategies, then the definition of climate and climate variability needs to accommodate the requirements of these communities. This will nuance and extend the definition of climate.

The scientific definition of climate as spatial and temporal averages and deviations from that average impacts the way that information from the science community is provided to other communities. It impacts the way scientists analyze the results of climate models, and the strategies to develop more accurate and robust climate models. While a powerful and useful way to organize and quantify both observational and model data, the representation of the physical state of the climate as averages weakens the ability to investigate cause and effect in climate models. For example, how the change of a model component such as the convective parameterization impacts the average spatial pattern in the inter-tropical convergence zone is difficult to isolate.

This paper explores a nuance on the definition of climate. That is, rather than climate being defined as the average weather, weather is viewed as one of the elements of the climate system. In the climate system the role of weather is, fundamentally, to carry heat from the equator to the pole; it is a transport process. Weather is not the only dynamical transport mechanism. The ocean circulation serves the same purpose – the transport of heat from low to high latitudes. Also, in the atmosphere there are other scales of motion, planetary waves and gravity waves for instance, that are important dynamical features of the climate, but they are not the core focus of the study of weather and weather forecasting.

The transport of heat from the equator to the pole is the fundamental role of weather in the climate system. This role is complicated by the importance of water and the energy associated with the phase changes of water. Indeed, our notion of climate stability is anchored in the notion of a global-scale balance of the ice, liquid, and gas phases of water. The differential heating between the equation and the pole drives a circulation to reduce the temperature gradient. The dynamical systems that develop also transport water. The range of temperatures that are common to the atmosphere cause phase changes of water, which in turn significantly impact the spatial and temporal distribution of energy.

Weather is a subset of the dynamical systems that develop to transport energy and that respond to that transport of energy. The subset of dynamical systems that comprises the weather is especially important because weather is the most immediate and important way the climate impacts people and ecosystems. The accumulation of the transport associated with weather systems is the contribution of weather to climate. This nuanced definition of the role of weather in the climate system as an accumulated transport mechanism will be investigated in this paper.


(This is the draft of introduction of a paper I’m writing.)

Here is a list of links to basic definitions used in climate.

IPCC Glossary


Arctic Climatology and Meteorology Glossary

Wikipedia Climate Definition

World Meteorological Organizations Climate Theme Page





Figure 1: A picture I took of the Tower of the Winds in the Roman Agora in Athens, Greece, 2004. The carvings show representations of the winds from different directions. This was a meteorological observatory a couple of millennia ago. Just thought it was a nice picture - and not irrelevant.