Lower Atmospheric Physics, Physics tutorial


Atmosphere comprises of the mixture of ideal gases: though molecular nitrogen and molecular oxygen predominate by volume, minor elements carbon dioxide, water vapor and ozone play vital roles. Forcing of atmosphere is mainly from Sun, although interactions with land and ocean are also significant. The atmosphere is constantly bombarded by solar photons at infra-red, visible and ultra-violet wavelengths. Few solar photons are scattered back to space by atmospheric gases or reflected back to space by clouds or Earth's surface; few are absorbed by atmospheric molecules (particularly water vapor and ozone) or clouds, leading to heating of parts of atmosphere; and some reach Earth's surface and heat it.

Atmospheric physicists study Earth's atmosphere, analyzing weather systems, electrical phenomena, and characteristics of middle and upper atmospheric layers.

Atmospheric physicists try to model Earth's atmosphere and atmospheres of other planets by using fluid flow equations, radiation balancing, chemical models, and energy transfer procedures in atmosphere (and how these tie in other systems like the oceans). To model weather systems, atmospheric physicists use elements of scattering theory, wave propagation models, cloud physics, statistical mechanics and spatial statistics that are extremely mathematical and associated to physics.

Inside Atmospheric Physics:

Atmospheric physics is the branch of meteorology and is associated to climatology. Atmospheric physicists utilize mathematical and physical models to study and know Earth's atmosphere and its weather systems. For instance, they apply theory of fluid dynamics to atmospheric tides. They also employ data collected by satellites, meteorological radar, and research aircraft to explore layers of atmosphere, weather systems, and climatic occurrences such as thunderstorms. Atmospheric physicists are generally used as faculty at universities or as research scientists at national labs.

Descriptions of atmospheric behavior:

Mobility of fluid system makes its description complex. Atmospheric motion redistributes mass and comprises into the variety of complex configurations.  Like any fluid system, atmosphere is administered by laws of continuum mechanics. They can be derived from laws of mechanics and thermodynamics which govern the discrete fluid body by generalizing those laws to continuum of such systems. In atmosphere, discrete system to which such laws apply is the infinitesimal fluid element, or air parcel, that is stated by fixed collection of matter.

Mechanisms influencing atmospheric behavior:

Of the factors affecting atmospheric behavior, gravity is single most significant. Although it has no upper boundary, atmosphere is contained by gravitational field of Earth that prevents atmospheric mass from escaping to space. As it is such a strong body force, gravity determines several atmospheric properties. Most instant is geometry of atmosphere. Atmospheric mass is concentrated in lowest 10 km - less than 1% of the Earth's radius. Gravitational attraction has compressed atmosphere in shallow layer above Earth's surface in which mass and constituents are stratified vertically: They are layered.

Through stratification of mass, gravity forces a strong kinematic constraint on atmospheric motion. Circulations with dimensions greater than few tens of kilometers are quasi-horizontal. Vertical displacements of air are then much smaller than horizontal displacements.

Composition and structure:

Earth's atmosphere comprises of mixture of gases, mainly molecular nitrogen (78% by volume) and molecular oxygen (21% by volume). Water vapor, carbon dioxide, and ozone, along with other slight constituents, include remaining 1% of atmosphere. Though present in very small abundances, trace species like water vapor and ozone play main role in energy balance of Earth through their involvement in radiative procedures. As they are made and destroyed in specific regions and are associated to circulation through transport, these and other minor species are extremely variable. For this explanation, trace species are cared separately from primary atmospheric constituents that are referred to just as dry air.

Thermal and dynamical structure:

Atmosphere is classified according to thermal structure that determines dynamical properties of individual regions. Simplest picture of atmospheric thermal structure is vertical profile of global-mean temperature. From surface up to approx 10 km, temperature decreases upward at nearly constant lapse rate that is defined as rate at which temperature decreases with altitude. This layer immediately above Earth's surface is called as troposphere that means turning sphere.


Relation between the circulation and transfers of energy from Earth's surface is thermodynamics. Thermodynamics deal with internal transformations of the energy of the system and exchanges of energy between that system and its environment.

A thermodynamic system refers to the specified collection of matter. Such a system is termed as closed if no mass is exchanged with surroundings. Or else it is open. Air parcel which will serve as system is, in principle, closed. In practice, though, mass can be exchanged with surroundings through entrainment and mixing across system's boundary that is referred to as control surface. Additionally, trace constituents like water vapor can be absorbed by diffusion across control surface.

Thermodynamic state of system is stated by the different properties characterizing it. All these properties should be specified to state system's thermodynamic state. Though, that requirement is simplified for several applications.

Thermodynamic properties:

Two kinds of properties characterize state of a system. Property which doesn't depend on mass of the system is said to be intensive. Or else it is extensive. Intensive  and  extensive properties are generally  denoted with lower and upper case symbols, respectively. Pressure and temperature are examples of intensive properties. Volume is the extensive property.  Intensive property z may be stated from extensive property Z, by referencing latter to mass m of system.

Intensive property is then referred to as specific property. Specific volume v=V/m is example. If system's properties don't differ in space, it is termed as homogeneous. Or else it is heterogeneous. As air parcel is of infinitesimal dimension, it is by definition homogeneous (so long as it engages only gas phase). Stratification of density and pressure make atmosphere as a whole a heterogeneous system. The system can exchange energy with surroundings through two fundamental mechanisms. It can carry out work on its surroundings that represents mechanical exchange of energy with environment. Additionally, heat can be transferred across control surface that represents thermal exchange of energy with environment.

Heat transfer:

Energy can also be exchanged thermally, through heat transfer Q across system's control surface. If air parcel moves in the environment of higher temperature, it will absorb heat from surroundings (like through diffusion or thermal conduction). If system is open, a related procedure can take place through absorption of water vapor from surroundings, followed by condensation and release of latent heat. Heat can also be exchanged by radiative transfer. If it interacts radiatively with surroundings at higher temperature, air parcel will absorb more radiant energy than it produces.

If no heat is exchanged between the system and environment, control surface is termed as adiabatic. Or else, it is diabatic. Since, for several applications, heat transfer is slow compared with other procedures which influence a parcel, adiabatic behavior is good approximation.

First law:

Internal energy:

The First Law of Thermodynamics is inspired by the empirical finding: Work done on adiabatic system is independent of procedure. That is, under adiabatic conditions, work is independent of path in state space followed by system. For the adiabatic procedure, expansion work depends only on initial and final states of system. It thus behaves as the state variable. Internal energy u is stated as that state variable whose change, under adiabatic conditions, equals work done on system or minus work performed by the system

Δu = -wad

The Second Law of Thermodynamics manages direction of thermodynamic procedures and efficiency with which they take place. As these characteristics control how system evolves out of given state, Second Law also runs stability of thermodynamic equilibrium.

Natural and reversible processes:

The procedure for which the system can be restored to initial state without leaving the net influence on system or on its environment is termed as reversible. Reversible procedure is actually idealization:  a  procedure which is free of friction and for that changes of state take place slowly enough for system  to  remain in thermodynamic equilibrium.

Limited forms of the second law:

The entropy of the system can increase or decrease, depending on heat transfer require attaining same change of state under reversible conditions. For certain procedures, change of entropy implied by Second Law is simplified. So entropy can then only increase. It follows that irreversible work increases system's entropy. Letting control surface of hypothetical system pass to infinity eradicates heat  transfer  to  environment. 

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