Tropopause
This article needs additional citations for verification. (March 2012) |
The tropopause is the atmospheric boundary that demarcates the
Definition
Rising from the planetary surface of the Earth, the tropopause is the atmospheric level where the air ceases to become cool with increased altitude and becomes dry, devoid of water vapor. The tropopause is the boundary that demarcates the troposphere below from the stratosphere above, and is part of the atmosphere where there occurs an abrupt change in the environmental lapse rate (ELR) of temperature, from a positive rate (of decrease) in the troposphere to a negative rate in the stratosphere. The tropopause is defined as the lowest level at which the lapse rate decreases to 2°C/km or less, provided that the average lapse-rate, between that level and all other higher levels within 2.0 km does not exceed 2°C/km.[1] The tropopause is a first-order discontinuity surface, in which temperature as a function of height varies continuously through the atmosphere, while the temperature gradient has a discontinuity.[2]
Location
This section may be too technical for most readers to understand.(September 2018) |
The troposphere is the lowest layer of the Earth's atmosphere; it starts at the
Since the tropopause responds to the average temperature of the entire layer that lies underneath it, it is at its maximum levels over the Equator, and reaches minimum heights over the poles. On account of this, the coolest layer in the atmosphere lies at about 17 km over the equator. Due to the variation in starting height, the tropopause extremes are referred to as the equatorial tropopause and the polar tropopause.
Given that the lapse rate is not a conservative quantity when the tropopause is considered for stratosphere-troposphere exchanges studies, there exists an alternative definition named dynamic tropopause.
It is also possible to define the tropopause in terms of chemical composition.[11] For example, the lower stratosphere has much higher ozone concentrations than the upper troposphere, but much lower water vapor concentrations, so an appropriate boundary can be defined.
Tropical Tropopause Layer Cold Trap
In 1949 Alan West Brewer proposed that tropospheric air passes through the tropopause into the stratosphere near the equator, then travels through the stratosphere to temperate and polar regions, where it sinks into the troposphere. [12] This is now known as
Water vapor that is able to make it through the cold trap eventually rises to the top of the stratosphere, where it undergoes photodissociation into oxygen and hydrogen or hydroxide ions and hydrogen.[16] [17] This hydrogen is then able to escape the atmosphere. Thus, in some sense, the tropical tropopause layer cold trap is what prevents Earth from losing its water to space. James Kasting has predicted that in 1 to 2 billion years, as the Sun increases in luminosity, the temperature of the Earth will rise enough that the cold trap will no longer be effective, and so the Earth will dry out.[18]
Phenomena
The tropopause is not a fixed boundary. Vigorous thunderstorms, for example, particularly those of tropical origin, will overshoot into the lower stratosphere and undergo a brief (hour-order or less) low-frequency vertical oscillation.[19] Such oscillation results in a low-frequency atmospheric gravity wave capable of affecting both atmospheric and oceanic currents in the region.[citation needed]
Most commercial aircraft are flown in the lower stratosphere, just above the tropopause, during the cruise phase of their flights; in this region, the clouds and significant weather perturbations characteristic of the troposphere are usually absent.[20]
See also
References
- ISBN 978-92-63-02182-3.
- ^ Panchev 1985, p. 129.
- .
- .
- ^ Petty 2008, p. 112.
- ^ Andrews, Holton & Leovy 1987, p. 371.
- .
- ^ a b c Tuck, A. F.; Browell, E. V.; Danielsen, E. F.; Holton, J. R.; Hoskins, B. J.; Johnson, D. R.; Kley, D.; Krueger, A. J.; Megie, G.; Newell, R. E.; Vaughan, G. (1985). "Strat-trop exchange". Atmospheric Ozone 1985 – WMO Global Ozone Research and Monitoring Project Report No. 16. 1. World Meteorological Organization: 151–240.
- .
- .
- hdl:1912/3670.
- .
- .
- Bibcode:2017aeil.book.....C.
- S2CID 259520137.
- .
- .
- S2CID 4360963.
- .
- ^ Petty 2008, p. 21.
Bibliography
- Andrews, D. G.; Holton, J. R.; Leovy, C. B. (1987). R., Dmowska; Holton, J. R. (eds.). Middle Atmosphere Dynamics. ISBN 978-0-12-058576-2.
- Panchev, Stoǐcho (1985) [1981]. Dynamic meteorology. ISBN 978-90-277-1744-3.
- Petty, Grant W. (2008). A First Course in Atmospheric Thermodynamics. ISBN 978-0-9729033-2-5.