Dr Xiaobing Huang
An atmosphere is a layer of gases that may surround a material body of sufficient mass. The gases are attracted by the gravity of the body, and are retained for a longer duration. Depth of the atmosphere of the Earth is 100 to 500 km.
Earth's atmosphere protects living organismsfrom ultraviolet rays.
The Earth's atmosphere consists, from the ground up, of the troposphere (which includes the planetary boundary layer or peplosphere as lowest layer), stratosphere, mesosphere, ionosphere (or thermosphere), exosphere and the magnetosphere as shown in figure 1.
Figure 1 Approximate thickness of layers
Each of the layers has a different lapse rate, defining the rate ofchange in temperature with height. Figure 2 shows the temperature change with altitude in the atmosphere.
The lapse rate is defined as the negative of the rate of change in an atmospheric variable, usually temperature, with height observed while moving upwards through an atmosphere.
Figure 2 Temperature change with altitude in the atmosphere
The layer you live in. Three quartersof the atmosphere mass and 99% of water vapour and aerosols lie within the troposphere, and the depth of this layer varies between 20 km at the equator, 7 km (in summer and indistinct in winter) at the poles and 17 km in the middle of latitudes. It is characterized by a fairly regular decrease of temperature and pressure with height.
This region is above the troposphere with athickness of 12 – 50 km. Within this layer the temperature remains more or less constant initially first and then increasing.
At approximately 50 – 80 km height the layer is called mesosphere. In this region the temperature falls to its lowest value (about -900).
Finally we have the thermosphere, in which the temperature rises rapidly, reaching 6200C at about 170 km.Physical properties of air
Air is a compressible fluid. The composition of air is listed below:
Element By volume % By weight %
Nitrogen 78.1 75.5
Oxygen 20.9 23.1
Argon 0.93 1.3
Carbon dioxide, Methane, Rare (inert) gases 0.1 0.1
Note: for all practical purposes the atmosphere can be regarded as consisting of 21 % oxygen and 79 % nitrogen by volume.
Three states of matter:solid, liquid and gas.
What is a Fluid?
- Capable of flowing
- Conform to the shape of containing vessels
- Offer little resistance to change of form
- When being in equilibrium, fluids cannot sustain tangential or shear forces
- All fluids have some degree of compressibility
Definition: A fluid is a substance, which deforms continuously under the action of shearing forces, however smallthey may be.
Liquid and gas
Liquid: • difficult to be compressed incompressible
• have a fixed volume and a free surface if the volume of the container is greater than that of the liquid
Gas: • easy to be compressed compressible
• volume change largely with pressure and temperature
• have not a fixed volume and fill full of the container no free surface
Differences between solidand fluid
• For a solid, the strain is a function of the applied stress, providing that the elastic limit is not exceeded. For a fluid, the rate of strain is proportional to the applied stress.
• The strain in a solid is independent of the time over which the force is applied and, if the elastic limit is not exceeded, the deformation disappears when the force is removed. A fluid continues to flowonce the force has been exerted and will not recover its original form when the force is removed
Properties of Fluids
Density (): the mass (m) per unit volume (V).
Density varies with temperature and pressure. In gases, the variation is considerable but in liquids it is considered constant.
Density of some fluids
Fluids Mass Density
Water at 4 0C 1000
Water at 20...