Introduction

The title of this book implies two questions— what do we mean by 'weather' and 'climate', and how do we account for them? As regards definitions, 'weather' concerns the conditions prevailing during a particular few hours over a specified area which is perhaps 30 km across, whilst 'climate' is the atmospheric character of such an area, shown by records over thirty years or so. We are especially interested in the weather and climate near the surface, i.e. temperature, radiation, humidity, rainfall and wind. The difference between weather and climate is that the latter is the aggregate or composite of the weather. It involves the extremes and variability of the weather as well as average values: floods and droughts at a place do not create a good climate overall. However, any brief definition of climate needs to be greatly expanded to be satisfactory.

The short answer to the second question is that various atmospheric processes and surface features explain the weather, and that in turn explains the climate. Surface features include the latitude, elevation, landform, orientation to the Sun and distance from the sea. Whether the surface is ocean, vegetated, ice, desert or densely populated is also important. The meteorological processes are numerous and interrelated, but can be grouped for convenience into four categories—(i) those concerning the air's composition and structure, (ii) those involving the Sun's energy, (iii) processes related to the transformations of water from liquid to vapour, cloud, rain and snow, and (iv) winds. These are dealt with respectively in Parts I to IV of the book.

Processes that affect the weather operate on various scales, from the microscopic scale of the formation of cloud droplets (discussed in Chapter 8) to the size of the Earth, e.g. the winds discussed in Chapter 12. This range of spatial scales applies to climates too (Table 1.1). For example, we can talk about changes of the Earth's climate, or the climate of Australia, or differences between the climates of Wellington and Santiago, or those of the front and back gardens of a house. The climate of each these domains is influenced by the climate of the next largest, shown in Table 1.1, and also by the conditions actually within the domain. So, for

Table 1.1 Scales of climate

Scale

Domain

Typical extent*

Relevant features of the atmosphere

Typical time f

Relevant chapters

Global

Earth

20,000 km

Solar radiation, ocean gyre,

1 year

2, 11, 12

general circulation

Synoptic

Continent

1,000 km

Frontal weather, weather

4 days

13, 14, 16

forecasting tropical cyclone

Mesoclimate

Region

50 km

Thunderstorm, sea-breeze,

4 hours

9, 14

weather

Topoclimate

Locality

5 km

A thermal, cumulus cloud,

1 hour

7, 8, 10

rainfall

Microclimate

Site

10 m

Irradiance, evaporation,

1 minute

2, 4, 6, 14

humidity, gusts

*The representative horizontal extent

|A time characteristic of the features in the previous column

*The representative horizontal extent

|A time characteristic of the features in the previous column instance, a garden's microclimate is affected, firstly, by the suburb's topoclimate (i.e. the overall climate of the locality, determined by its elevation, nearness to the coast, etc.), and, secondly, by the site, e.g. the garden's wetness, shelter and slope towards the Sun.

In what follows, we shall consider examples of various scales of time and distance, with especial attention to the southern hemisphere, which consists largely of water (Note 1.A). We begin with the largest space, the atmosphere as a whole, and the span of time since the Earth began.

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