What’s the difference between ‘weather’ and ‘climate’?

The World Meteorological Organization (WMO) defines climate as the statistical description of weather averaged over a period of time – the period taken being long enough to be able to identify statistical characteristics. Whilst 'weather' describes the physical state of the atmosphere at a particular place at a particular time, 'climate' can be defined as the probability of deviations from average values, including the probability of extreme values. A period of 30 years is usually used to assess the climate. The standard classification of the Earth's climatic zones is mainly based on the annual cycles of temperature and rainfall.

The word 'climate' is derived from the Greek word ‘klinein’, which means ‘to slope’. This is because the Earth's seasonal differences are a consequence of the inclination of the Earth's rotation axis relative to its orbit around the sun - otherwise known as the ‘ecliptic’. At present our inclination is 23.5º, which means that each hemisphere receives more solar radiation during its summer than its winter.

The amount of solar radiation received at a given point on the Earth’s surface depends on the location of the point (more is received at the equator, less at the poles) and the time of year (more is received in summer, less in winter). This causes spatial variations in the temperature of the surface, and thus horizontal temperature gradients in the lower atmosphere. These in turn bring about differences in air pressure between regions, which cause the movement of the air. However, the atmosphere is not an isolated system, but is part of a complex, interacting system that includes the hydrosphere (the oceans and also the water cycle, both on the continents and in the atmosphere), the cryosphere (ice and snow), the biosphere (animals and plants), the pedosphere (the soils) and the lithosphere (the Earth’s crust and upper mantle). These constituent parts of the climate system (see figure) move at vastly differing speeds and have drastically different heat conductances and capacities. The various timescales of these components shape the dynamics of the climate system, and thus also the statistics we use to describe the climate. For instance, the lower atmosphere adjusts to surface conditions in hours, whilst hundreds of years are needed for the circulation of the deep ocean to come to a new equilibrium with the changed atmospheric state, and adjustments in a large inland ice sheet like on Antarctica take thousands of years.

Schematic of the Earth’s climate system: