THE ATMOSPHERE AND ITS LAYERS The atmosphere is usually recognized as layered; some of its main features are illustrated in Fig. 3, in which altitude is drawn on a logarith mic, instead of a linear, scale in order to allow the various layers to be represented together on one page and to illustrate vividly how the proper ties of the atmosphere change most sharply near the ground.
Barometric pressure, density of the air, and (as a rule) temperature, decrease with increasing height above the Earth's surface. These changes are all quantitatively important in aviation, and calculations are based on a table of an agreed `International Standard Atmosphere'. Changes with altitude in temperature, humidity, density, and viscosity will be complex in their effect on a suspended spore, but are not likely greatly to affect its terminal velocity.
The three vertical panels of the diagram represent conditions in contrasting weather types. The central panel represents a dull, windy day, with a cloud layer shielding the ground from direct sunlight (conditions on a cloudy night are not very different). The right-hand panel represents a sunny day, and the left-hand panel a still, cloudless night. The thickness of each individual layer of air varies according to conditions; the boun daries between them vary in definiteness: sometimes transitions are imper ceptible, but there is sometimes even a visible interface between layers. The layers are variously named in the literature and this may be confusing unless the following approximate equivalents are borne in mind (see p. 23).
It is convenient to describe these layers in the reverse order, from ground-level upwards, beginning in the troposphere with the laminar boundary layer.