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Information about Troposphere

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Atmosphere diagram showing the troposphere and other layers. The layers are not to scale.
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Temperature and pressure against altitude from the NRLMSISE-00 standard atmosphere model


The troposphere is the lowest portion of Earth's atmosphere. It contains approximately 75% of the atmosphere's mass and almost all of its water vapor and aerosols.

The average depth of the troposphere is about 11 km in the middle latitudes. It is deeper in the tropical regions (up to 20 km) and shallower near the poles (about 7 km in summer, indistinct in winter). The lowest part of the troposphere, where friction with the Earth's surface influences air flow, is the planetary boundary layer. This layer is typically a few hundred meters to 2 km deep depending on the landform and time of day. The border between the troposphere and stratosphere, called the tropopause, is a temperature inversion.[1]

The word troposphere derives from the Greek "tropos" for "turning" or "mixing," reflecting the fact that turbulent mixing plays an important role in the troposphere's structure and behavior. Most of the phenomena we associate with day-to-day weather occur in the troposphere.[1]

Pressure and temperature structure

Composition

The chemical composition of the troposphere is essentially uniform, with the notable exception of water vapor. The source of water vapor is at the surface through the processes of evaporation and transpiration. Furthermore the temperature of the troposphere decreases with height, and saturation vapor pressure decreases strongly with temperature, so the amount of water vapor that can exist in the atmosphere decreases strongly with height. Thus the proportion of water vapor is normally greatest near the surface and decreases with height.

Pressure

The pressure of the atmosphere is maximum at the surface and decreases with higher altitude. This is because the atmosphere is very nearly in hydrostatic equilibrium, so that the pressure to equal the weight of air above a given point. The change in pressure with height therefore can be equated to the density with this hydrostatic equation:[2]



where:

*gn stands for the standard gravity
*ρ stands for density
*z stands for height
*p stands for pressure
*R stands for the gas constant
*T stands for temperature in kelvins
*m stands for the molar mass


Since temperature in principle also depends on altitude, one needs a second equation to determine the pressure as a function of height, as discussed in the next section.

Temperature

Main article: Lapse rate
The temperature of the troposphere generally decreases with altitude. The rate at which the temperature decreases, , is called the lapse rate. The reason for this decrease is as follows. When the air is stirred by convection, and a parcel of air rises, it expands, because the pressure is lower at higher altitudes. As the air parcel expands, it pushes on the air around it, doing work; but generally it does not gain heat in exchange from its environment, because its thermal conductivity is low (such a process is called adiabatic). Since the parcel does work and gains no heat, it loses energy, and so its temperature decreases. (The reverse, of course, will be true for a sinking parcel of air.) [1]

Since the heat exchanged dQ is related to the entropy change dS by dQ=T dS, the equation governing the temperature as a function of height for a thoroughly mixed atmosphere is
where S is the entropy. The rate at which temperature decreases with height under such conditions is called the adiabatic lapse rate.

For dry air, which is approximately an ideal gas, we can proceed further. The adiabatic equation for an ideal gas is [3]
where is the heat capacity ratio (=7/5, for air). Combining with the equation for the pressure, one arrives at the dry adiabatic lapse rate,[4]
If the air contains water vapor, then cooling of the air can cause the water to condense, and the behavior is no longer that of an ideal gas. If the air is at the saturated vapor pressure, then the rate at which temperature drops with height is called the saturated adiabatic lapse rate. More generally, the actual rate at which the temperature drops with altitude is called the environmental lapse rate. In the troposphere, the average environmental lapse rate is a drop of about 6.5 °C for every 1 km (1000 meters) increase in height. [1]

Depending on the weather conditions, one may find that the environmental lapse rate (the actual rate at which temperature drops with height, ) is not equal to the adiabatic lapse rate (or correspondingly, that ). If the upper air is warmer than predicted by the adiabatic lapse rate (), then when a parcel of air rises and expands, it will arrive at the new height at a lower temperature than its surroundings. In this case, the air parcel is denser than its surroundings, so it sinks back to its original height, and the air is stable against being stirred. Such a situation is called temperature inversion, and can lead to the trapping of air pollution in basins such as that of Los Angeles. If, on the contrary, the upper air is cooler than predicted by the adiabatic lapse rate, then when the air parcel rises to its new height it will have a higher temperature and a lower density than its surroundings, and will float. Such a process can happen spontaneously, and under such conditions, the air will be stirred by spontaneous convection currents.[1][2]

Temperatures decrease at middle latitudes from an average of 15°C at sea level to about -55°C at the beginning of the tropopause. At the poles, the troposphere is thinner and the temperature only decreases to -45°C, while at the equator the temperature at the top of the troposphere can reach -75°C.

Tropopause

Main article: Tropopause


The tropopause is the boundary region between the troposphere and the stratosphere.

Measuring the temperature change with height through the troposphere and the stratosphere identifies the location of the tropopause. In the troposphere, temperature decreases with altitude. In the stratosphere, however, the temperature remains constant for a while and then increases with altitude. The region of the atmosphere where the lapse rate changes from positive (in the troposphere) to negative (in the stratosphere), is defined as the tropopause.[1] Thus, the tropopause is an inversion layer, and there is little mixing between the two layers of the atmosphere.
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An idealised view of three large circulation cells.

Atmospheric circulation

Main article: Atmospheric circulation
The basic structure of large-scale circulation in the troposphere remains fairly constant. There are three convection cells in each hemisphere: the Hadley cell, the Ferrel cell, and the Polar cell, which guide the prevailing winds, thereby transporting heat from the equator to the poles.[1]

References

1. ^ Danielson, Levin, and Abrams, Meteorology, McGraw Hill, 2003
2. ^ Landau and Lifshitz, Fluid Mechanics, Pergamon, 1979
3. ^ Landau and Lifshitz, Statistical Physics Part 1, Pergamon, 1980
4. ^ Kittel and Kroemer, Thermal Physics, Freeman, 1980; chapter 6, problem 11

External links

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Earth's atmosphere
Troposphere • Stratosphere • Mesosphere • Thermosphere • Exosphere
Tropopause • Stratopause • Mesopause • Thermopause / Exobase
Ozone layer • Turbopause • Ionosphere
Earth's atmosphere is a layer of gases surrounding the planet Earth and retained by the Earth's gravity. It contains roughly (by molar content/volume) 78% nitrogen, 20.95% oxygen, 0.93% argon, 0.
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Water vapor or water vapour (see spelling differences), also aqueous vapor, is the gas phase of water. Water vapor is one state of the water cycle within the hydrosphere.
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Particulates, alternatively referred to as particulate matter (PM), aerosols or fine particles, are tiny particles of solid or liquid suspended in a gas. They range in size from less than 10 nanometres to more than 100 micrometres in diameter.
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Friction is the force of two surfaces in contact. It is not a fundamental force, as it is derived from electromagnetic forces between atoms. When contacting surfaces move relative to each other, the friction between the two objects converts kinetic energy into thermal energy, or
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The planetary boundary layer (PBL), also known as the atmospheric boundary layer (ABL) or peplosphere, is the lowest part of the atmosphere and its behavior is directly influenced by its contact with a planetary surface.
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A landform comprises a geomorphological unit, and is largely defined by its surface form and location in the landscape, as part of the terrain, and as such, is typically an element of topography.
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The tropopause is a boundary region in the atmosphere between the troposphere and the stratosphere. Here the air ceases to cool at -50°C, and the air becomes almost completely dry.
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inversion is a deviation from the normal change of an atmospheric property with altitude. It almost always refers to a temperature inversion, i.e., an increase in temperature with height, or to the layer within which such an increase occurs.
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turbulence or turbulent flow is a flow regime characterized by chaotic, stochastic property changes. This includes low momentum diffusion, high momentum convection, and rapid variation of pressure and velocity in space and time.
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weather is the set of all extant phenomena in a given atmosphere at a given time. The term usually refers to the activity of these phenomena over short periods (hours or days), as opposed to the term climate, which refers to the average atmospheric conditions over longer periods of
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Water vapor or water vapour (see spelling differences), also aqueous vapor, is the gas phase of water. Water vapor is one state of the water cycle within the hydrosphere.
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Evaporation is the process by which molecules in a liquid state (e.g. water) spontaneously become gaseous (e.g. water vapor), without being heated to boiling point. It is the opposite of condensation.
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Transpiration is the evaporation of water from the aerial parts of plants, especially leaves but also stems, flowers and roots. Leaf transpiration occurs through stomata, and can be thought of as a necessary "cost" associated with the opening of stomata to allow the diffusion of
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The saturation vapor pressure is the static pressure of a vapor when the vapor phase of some material is in equilibrium with the liquid phase of that same material. The saturation vapor pressure of any material is solely dependent on the temperature of that material.
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Hydrostatic equilibrium occurs when compression due to gravity is balanced by a pressure gradient which creates a pressure gradient force in the opposite direction. The balance of these two forces is known as the hydrostatic balance.
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Fluid statics (also called hydrostatics) is the science of fluids at rest, and is a sub-field within fluid mechanics. The term usually refers to the mathematical treatment of the subject.
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Standard gravity, usually denoted by g0 or gn, is the nominal acceleration due to gravity at the Earth's surface at sea level. By definition it is equal to exactly 9.80665  m·s−2 (approx. 32.174 ft·s−2).
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In physics, density is mass m per unit volume V—how heavy something is compared to its size. A small, heavy object, such as a rock or a lump of lead, is denser than a lighter object of the same size or a larger object of the same weight, such as pieces of
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The gas constant (also known as the universal or ideal gas constant, usually denoted by symbol R) is a physical constant used in equations of state to relate various groups of state functions to one another.
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The kelvin (symbol: K) is a unit increment of temperature and is one of the seven SI base units. The Kelvin scale is a thermodynamic (absolute) temperature scale where absolute zero — the coldest possible temperature — is zero kelvins
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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.
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trillion fold).]]

Temperature is a physical property of a system that underlies the common notions of hot and cold; something that is hotter generally has the greater temperature. Temperature is one of the principal parameters of thermodynamics.
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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.
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Work may refer to:
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thermal conductivity, k, is the property of a material that indicates its ability to conduct heat. It is used primarily in Fourier's Law for heat conduction.

It is defined as the quantity of heat, ΔQ, transmitted during time Δt
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energy (from the Greek ενεργός, energos, "active, working")[1] is a scalar physical quantity that is a property of objects and systems of objects which is conserved by nature.
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Ice melting - a classic example of entropy increasing[1] described in 1862 by Rudolf Clausius as an increase in the disgregation of the molecules of the body of ice.
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Ice melting - a classic example of entropy increasing[1] described in 1862 by Rudolf Clausius as an increase in the disgregation of the molecules of the body of ice.
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