Troposphere

Lower down again is the tropopause, where there is generally a temperature minimum of around -50°C and a change of the lapse rate. It was first detected by Teisserenc de Bort in 1898. There is no agreed definition of the tropopause, but it does separate the stratosphere's ozone and temperature conditions from the positive lapse rates of the troposphere, the lowest layer. This contains all the Earth's mountains, about 80 per cent of the atmosphere and virtually all the water vapour. Note 1.L gives characteristic values of the temperature, pressure and density at various levels of the troposphere.

Within the troposphere but close to the ground is the planetary boundary layer (PBL), also called the 'Ekman layer' or 'mixed layer', in which the air is well mixed by heating (Chapter 3) and surface roughness (Chapter 14). Typically, the PBL is about 1 km thick, according to wind speed and solar radiation (Chapter 2), but may be less than 100 m on a still, cold, night. It is a feature of the topoclimate (Table 1.1).

Towards the bottom of the PBL is the surface layer (or 'friction layer'), where winds vary rapidly in speed and direction with height, in response to the friction of the ground (Chapter 14). In the case of a city, which has an irregular profile, there is a roughness layer where turbulence is caused by the buildings, and between them is the canopy layer of air (Figure 1.11).

The lowest tenth or so of the surface layer is the interfacial layer or 'constant-flux layer', constituting the main barrier to vertical exchanges between the surface and the air, e.g.

planetary boundary layer

mixed air

surface layer

roughness layer

canopy layer

TT

Qrr

Figure 1.11 Schematic diagram of the layers of air over a city. Level a is at roof top height, b is the limit of stirring caused by the city's individual buildings, c is the extent of the effect of the city as a whole, and d is the base of the inversion layer (Chapter 7) which defines the planetary boundary layer.

Figure 1.11 Schematic diagram of the layers of air over a city. Level a is at roof top height, b is the limit of stirring caused by the city's individual buildings, c is the extent of the effect of the city as a whole, and d is the base of the inversion layer (Chapter 7) which defines the planetary boundary layer.

evaporation from the ocean and the transfer of carbon dioxide to vegetation. It is a feature of the 'microclimate'. There is negligible horizontal advection within this layer, i.e. little carrying of heat or moisture sideways in winds.

Lastly, there is the laminar (viscous) sublayer, which consists of air held almost stationary around all solid and liquid surfaces by molecular forces. This layer is only millimetres thick, depending on the wind speed, but it provides important thermal insulation.

So we have outlined the entire atmosphere. From now on we shall concentrate on the troposphere, because that is the arena of processes which produce weather and determine our climatic environment (Table 1.1).

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