# Atmospheres

So far, we've been dealing with orbital dynamics and radiation in the simulation. This has the advantage of being more or less exactly calculable, Atmospheres make an exoplanet infinitely more interesting (basically any world with exobiology on it is expected to have an atmosphere), but at the same time the simulation gets vastly more complicated.

As weather forecast codes demonstrate, the state of Earth's atmosphere cannot be reliably predicted even 14 days in advance, and such codes run on supercomputing facilities and receive tens of thousands of measured data points as input. Fluid dynamics of heat and moisture transport in the atmosphere is a very complicated problem involving phenomena such as turbulence, latent heat, interaction of solar radiation with air molecules, fluid droplets and aerosols as well as a host of other phenomena.

It should therefore be clear that the worldbuilder code cannot (and does not) provide an accurate computing framework for atmospheres, but at best a coarse approximation - which nevertheless may provide interesting insights.

The general idea pursued here is that a model is roughly adjusted to Earth parameters, and when the simulation is carried out for other worlds which are not too different from Earth standards, earth-relative values of properties like the overall transport can be used.

As it is, the scheme has missing ingredients and we will point out what they are and where their effect is seen when appropriate.

## Specifying an atmosphere

An atmosphere is specified by its relative composition in percentage by volume (the fractions don't have to be exact though, the code normalizes to 100% in the end). This is followed by the surface pressure in earth-relative units. Here's an example declaration an Earth-like atmosphere with (increased CO2 content):

 atmosphere h2o 0.5 ch4 0.00018 o3 0.00006 n2 78.08 o2 20.95 ar 0.934 co2 0.08 surface_pressure 1.0

Most common gases that make up significant fractions of atmospheres known from the solar system are recognized. The code responds by computing a number of properties from the speficied information:

 Atmosphere ---------- Surface pressure [mbar]: 1013.25 Scale height [m]: 7329.34 Column mass [kg]: 10318.4 Dry lapse rate [K/km]: 9.57451 Surface temperature [K]: 288.129 Emissive temp. [K]: 250.355 Heat capacity CP [J/K*kg]: 1.02562 Atmospheric composition ----------------------- by volume by mass O2 : 20.84 % 23.07 % CO2 : 0.07957 % 0.1211 % N2 : 77.66 % 75.22 % Ar : 0.9289 % 1.285 % H2O : 0.4973 % 0.3096 % CH4 : 0.0001790 % 9.909e-05 % O3 : 5.968e-05 % 9.909e-05 % Atmospheric IR retention: 0.4324 Atmospheric transport coeff: 1.000
The most relevant of those to put to work in the simulation are the IR retention and the transport coefficient as they directly affect temperature distribution on the planet.

## Greenhouse effect and transport

An Earth-like atmosphere allows most of the visible light to pass, however it is fairly opaque to the IR emission of the planet back into space, i.e. it retains part of the outgoing radiation. This is known as the Greenhouse effect (see the section to learn how the worldbuilder computes it). The parameter given is an estimate of what percentage of IR radiation is captured by the atmosphere based on the specified gas mixture and atmosphere pressure as well as on surface temperature.

Likewise, an atmosphere transports heat from a high-temperature location on a planet to a region with lower temperature. This transport can utilize sensible heat (actually measurable temperature) as well as latent heat (energy stored e.g. in water vapour that is released when the vapour condenses). The ability to transport heat depends on the amout of atmosphere as well as on the heat capacity (however it also depends on the prevailing global circulation weather patterns, so things are never quite as simple). If the parameter is close to unity, the code estimates that the atmosphere is about as efficient as Earth's in transporting heat, if the parameter is smaller it is estimated to be less efficient.

None of these parameters is a hard-and fast value - the IR retention depends on the gas mixture, but that may in itself depend on temperature (the water vapour content of Earth's atmosphere is temperature dependent), and so there may be feedback loops at work. For that reason, the parameters are not automatically used in the temperature evolution but have to be specified separately.

However, before we study a working model of Earth's atmosphere, we need to consider yet another factor - weather (i.e. the formation of clouds as well as precipitation).

Continue with Weather.

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