What Is The Troposphere?

The lower atmosphere, which scientists refer to as the troposphere, is where greenhouse gases trap heat trying to escape from the Earth's surface, thus causing global warming. The troposphere contains 99 percent of Earth's water vapor, and is where all our weather happens, including regional weather cycles like the El-Nino-Southern Oscillation. The troposphere is also where ground-level ozone forms smog.
Troposphere Explained
Photo: CC0 Public Domain

The word “troposphere” – which comes from the Greek “tropos” for “turn” or “change” – refers to the layer of air closest to the planet’s surface. It’s the densest, warmest, wettest part of the atmosphere and, as explained above, it’s where the greenhouse effect takes place.

Although the greenhouse effect is is linked to climate change, it’s worth remembering that it’s been keeping Planet Earth nice and cosy for thousands of years without any problem. It is only since humans started burning huge amounts of fossil fuels, creating huge greenhouse gas emissions in the process, that global warming has appeared.

In addition to making the planet habitable, the troposphere provides several other benefits for the biosphere and its inhabitants. It plays a vital direct role in the water cycle and carbon cycle, while also exerting a considerable influence over Earth’s climate system and radiative equilibrium. And of course, without tropospheric oxygen, no plants could respire and no animals or humans could breathe.

Where Exactly Is The Troposphere?

The troposphere is the lowest level of Earth’s atmosphere. Its average height around the globe is about 13km (8 miles), although this varies from the equator to the poles. According to the National Weather Service of the NOAA, it’s about 18-20 km (11-12.5 miles/59,000-65,600 ft) high at the equator; about 8.8 km (5½ miles/29,000 ft) high at mid-latitudes of 50°N and 50°S; and roughly (4 miles/21,000 ft) high at the poles. 1 Above the troposphere are five further atmospheric layers, the Stratosphere, Mesosphere, Thermosphere and Exosphere, whose weight compresses the troposphere causing it to be denser than its neighbors above. In fact, about half of the total mass of the atmosphere can be found in the first 18,000 feet of the troposphere.

What’s The Temperature Of The Troposphere?

The troposphere receives most of its heat from energy radiating upwards from the Earth’s surface. As a result, the troposphere gets colder, as altitude increases, with temperatures dropping from about 15°C (59°F) at the bottom, to -51°C (60°F) at the top. [1] The rate of cooling, is known as the environmental lapse rate (ELR.) Sitting on top of the troposphere is the tropopause, a mini-layer which acts as the transition between the troposphere and stratosphere. The tropopause is what’s known as an “inversion layer”, in which temperature stops decreasing with altitude and instead remains constant. 2

What Does The Troposphere Contain?

The troposphere contains four fifths of the atmosphere’s mass 3, and 99 percent of atmospheric aerosols (microscopic particles), such as dust, pollen, sea salt, tiny bits of organic matter, smoke particles or soot. 

Its chemical content is roughly 78 percent nitrogen, 21 percent oxygen, 1 percent argon, along with small amounts of other gases.

These other chemicals include greenhouse gases like carbon dioxide (CO2), methane (CH4), nitrous oxide (N2O), and tropospheric ozone (O3), as well as chlorofluorocarbons (CFCs), hydrochlorofluorocarbons (HCFCs), and perfluorocarbons (PFCs).

For an idea of how tropospheric greenhouse gases compare, as far as their heat-trapping effects are concerned, see: Global Warming Potential (GWP).

How The Troposphere Affects Climate And Global Warming

The troposphere contains 99 percent of all water vapor in the atmosphere. Concentration ranges from trace amounts in the Arctic and Antarctic to nearly 4 percent in the tropics. Due to the presence of water vapor, nearly all weather occurs in this part of the atmosphere – including ice storms, blizzards, thunderstorms, lightning, cyclones, typhoons, tornadoes and dust storms. It also contains 99 percent of all clouds in the atmosphere, chiefly cumulus and cumulonimbus (both vertically forming clouds), as well as nimbostratus, stratocumulus and stratus.

Clouds in the troposphere play a significant role in Earth’s climate although scientists are unsure of their net effect and whether this might change as temperatures continue to rise. On the one hand, clouds cover approximately 67 percent of the Earth’s surface at any one time 4, and reflect back into space about one-third of the total amount of sunlight that strikes the Earth’s atmosphere. 5 This has a cooling effect on Earth’s climate. On the other hand, tropospheric cloud cover minimizes the fall in temperature as sunlight ceases – the so-called ‘diurnal temperature variation’. Which has a smaller warming effect. For more on this topic, see: How Do Clouds Affect Climate?

Some climate models forecast that global warming in the troposphere will lead to higher levels of evaporation and therefore more clouds – hence, more cooling. But others show that rising temperatures cause more clouds to dissipate in the heat, leading to a lot more warming. At least one study shows that once CO2 levels in the air reach around 1,200 ppm (up from today’s 410 ppm), all clouds are likely to disintegrate within a very short space of time. This would have a catastrophic effect on the planet’s albedo, temperature and environment, not to say habitability. 6

The Turbulent Planetary Boundary Layer

In climate science, the planetary boundary layer (PBL), or peplosphere, is the very lowest part of the troposphere, and its behavior is greatly influenced by its contact with the planetary surface. 7 The height of the PBL varies. At night and during a cool period the PBL tends to be smaller, while during the day and during the warm season it tends to be taller. The two causal factors behind this are the wind speed and temperature. Strong wind speeds cause more convective mixing which causes the PBL to expand. At night, as rising thermals from the surface lessen, the PBL contracts. In general, because cold air is denser than warm air, the PBL will tend to be smaller in the cool season.

What are the characteristics of the planetary boundary layer? To begin with, winds inside the PBL are more turbulent. Surface friction from topography, trees and other vegetation creates turbulent wind currents and unpredictable wind patterns. Friction causes air in the PBL to spiral into low pressure since it reduces the effect of the rotational (Coriolis) force.

The PBL can undergo major changes of temperature. It typically reacts to changes in surface radiative forcing in an hour or less. Thus, it can heat up significantly during the day and cool significantly at night, even though the rest of the atmosphere may experience a fairly uniform temperature. Because of this wide range of temperatures, the PBL is an important supplier of heat and moisture to thunderstorms. 8

Wind patterns within the PBL have a strong influence over weather patterns and the movement of particulate matter (PM) between the PBL and the atmosphere above it. 9

Tropospheric Ozone And Photochemical Smog

A common feature of densely populated areas since the 1950s, is a particular type of air pollution known as photochemical smog. This restricts visibility and can be intensely irritating to the eyes and throat. It also causes a range of respiratory conditions, some of which are serious, even fatal. Vulnerable cities include Los Angeles, Mexico City, Delhi, Beijing, and Ho Chi Minh City, to name but a few. Photochemical smog appears in the lower troposphere over large urban areas, during the summer.

How is this smog produced? Through a series of chemical reactions involving (a) ground level ozone (also called tropospheric ozone); (b) a mixture of exhaust gases from gasoline-powered engines, including methane, carbon monoxide, non-methane, volatile non-organic compounds, and nitrogen oxides; and (c) sunlight. It can be severely aggravated by particulate matter (PM) from fireworks, crop or wood burning, or by the concurrent production of sulfate aerosols and organic particulates, like black carbon or agricultural smoke. 10 See also our article: What is the Effect of Wood Burning on Climate Change?

Earth’s Other Spheres

The troposphere connects directly with all the Earth’s other “spheres”, except the lithosphere. It interacts with the ocean (the hydrosphere) through cloud and wind, as well as the ocean-atmosphere exchange of gases, like carbon dioxide. It interacts with the polar cryosphere through its warming and cooling aerosols, while its clouds have a close relationship with Earth’s soil layer, known as the pedosphere. It interacts in numerous ways with the biosphere, through its supply of life-supporting carbon dioxide for photosynthesis in green plants and its supply of water for all other living creatures.

References

  1. Layers of the atmosphere. NOAA []
  2. “Meteorology.” McGraw-Hill, New York. Danielson, E.W., Levin, J. and Abrams, E. (2003) []
  3. “Troposphere”. Concise Encyclopedia of Science & Technology. McGraw-Hill. 1984. []
  4.  “Spatial and Temporal Distribution of Clouds Observed by MODIS Onboard the Terra and Aqua Satellites.” Transactions on Geoscience and Remote Sensing, 51 (7), 3826-3852. (2013) King et al. []
  5. “The Study of Earth as an Integrated System.” Earth System Science. nasa.gov []
  6.  “Possible climate transitions from breakup of stratocumulus decks under greenhouse warming.” Tapio Schneider, Colleen M. Kaul & Kyle G. Pressel. Nature Geoscience volume 12, pages 163–167. (2019) []
  7. “Boundary Layer Climates.” T. Oke. London: Methuen. 1987 []
  8. “The Planetary Boundary Layer.” Jeff Haby. theweatherprediction.com []
  9. Estimating climatological planetary boundary layer heights from radiosonde observations: Comparison of methods and uncertainty analysis.” Journal of Geophysical Research 115, D16 p.113. 27 August 2010. Seidel, D. J., C. O. Ao, and K. Li (2010). []
  10. “Tropospheric Ozone and Photochemical Smog.” Treatise on Geochemistry, Volume 9. Editor: Barbara Sherwood Lollar. pp. 612. ISBN 0-08-043751-6. Pub Date: December 2003.[]
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