A planet’s atmosphere consists of the available atoms and molecules that are in the form of gases. Only those having a kinetic energy below a given threshold are retained by the planet’s gravity. Which atoms and molecules will be part of the atmosphere depend on the planet’s surface and atmospheric temperatures, air pressure, and the chemistry of the atoms and molecules themselves.
Gases are the least dense form of a state of matter, so they are concentrated at and above the planet's surface. They have an indefinite shape and volume, and combined with their low density, they are rather easy to make flow over the Earth's surface, creating our weather and altering climate regimes so they aren't solely a function of the Sun's radiant energy hitting the Earth's surface.
Gases are compressible, and air pressure is the weight of an air column extending vertically from the given location to the top of the atmosphere. Because air is compressible, the density of air increases toward the Earth's surface. Roughly 75% of the mass of the atmosphere is within 10 km of the Earth's surface - this is only 0.15% of the Earth's radius, but this is enough to support life on Earth, create a complex energy budget by capturing and transporting large amounts of heat, drive the water cycle, and keep liquids available on our planet. Without an atmosphere, our oceans would have quickly boiled away.
We don't need to look far to study the effects of a body in space without an atmosphere - our moon! Because the moon's gravity is not strong enough to maintain an atmosphere, its daytime surface temperatures reach 260ºF (127ºC) and night time temperatures plummet to -280ºF (-173ºC) - that is a 540ºF diurnal temperature change!
What's in Our Milky Way?
Nature builds things with the materials that are available, so to understand what Earth had to work with when it formed, look at the chemical composition of the Milky Way:
Many of these atoms readily form gas, and so were either part of our atmosphere early in its history or have accumulated over time.
Evolution of Earth's Atmosphere Over Time
Earth's atmosphere has evolved over time due to a number of factors: cooling temperatures of our planet’s surface over time, gravity, temperature and solar activity of the Sun, strength of Earth's magnetic field over time, volcanic activity caused by plate tectonics, presence and abundance of photosynthesizing life, and technologically advanced life forms.
Major Gases in Earth's Current Atmosphere
There are only four major gases found in Earth's atmosphere, yet only three are well mixed throughout the lower layers of the atmosphere. Water is concentrated in the lowest layer of the atmosphere.
Nitrogen (78.1%) gas is in the the form of two tightly bonded nitrogens, which is nearly inert to chemical reactions in this form. Other gaseous nitrogen compounds are not incorporated into many rock minerals, so nitrogen is not readily removed from the atmosphere once emitted during volcanic activity.
Oxygen (20.9%) gas is in the form of two bonded oxygens, but these bonds are not as strong as nitrogen's. Oxygen is highly reactive, so it is taken up in water, other atmospheric gases (carbon dioxide for example), living organisms, and many rock minerals. Because it is so reactive, it can be removed from the atmosphere for long periods of time; however, the vast amount of photosynthesizing plants provide a steady supply of atmospheric oxygen..
Argon (0.9%) is a noble or inert gas, and is predominantly a radioactive decay product of potassium-40, a radioactive isotope of potassium. Minimal in Earth's early atmosphere, argon has accumulated as potassium-40 has decayed over time.
Water vapor (0-3%) concentration is highly variable in the atmosphere, but it plays critical roles in energy flows, matter cycles, and life webs on Earth. Since hydrogen and oxygen are plentiful in the universe, water is also plentiful as it forms readily in the cold conditions of space.
Air with 76% nitrogen (light gray), 20% oxygen (green), 1% argon (red), and 3% water vapor (blue).
Recall from the Blackbody Radiation software that objects with a temperature above absolute zero (0K) emit a broad spectrum of electromagnetic radiation (thermal radiation) with a total energy proportional to that temperature to the fourth power - so a small change in an object's temperature has a very large impact on how much radiation it is emitting. Also, the hotter the object, the radiation shifts to shorter wavelengths. So the Sun's surface is quite hot, and its spectrum ranges mostly between ultraviolet to near infrared. Earth surfaces warmed by sunlight emit radiation in the mid to far infrared.
In the schematic of a planet without an atmosphere (above) the absorbed solar radiation warms the planet's surface, which in turn radiates more thermal radiation away from surface. The surface temperature stops changing when the energy being gained from the solar radiation is balanced by the energy being lost emitting its thermal radiation. This creates quite a hot surface, such as the moon's maximum temperatures of 260ºF (127ºC).
When a planet has an atmosphere, the solar radiation absorbed by the atmosphere warms the gases in the atmosphere. The absorbed solar radiation heats the surface, which in turn warms the air near the ground through conduction. The warmed air rises and is replaced by cooler air. The conduction also cools the planet's surface, so there is less thermal radiation emitted. The warmed atmosphere emits more thermal radiation, roughly half is directed back to the planet's surface and half is lost to space. The surface absorbs the thermal radiation emitted from the atmosphere, which adds to warming the atmosphere through conduction.
This planet's surface has three ways to cool: conduction, convection (which increases the rate of conduction), and thermal radiation. But as with the planet without an atmosphere, the amount of energy coming into the planet equals the amount of radiation leaving the planet. All other factors being equal, the surface of a planet with an atmosphere will be cooler during the day and warmer during the night than a surface without an atmosphere. The heat exchanged with the atmosphere emits thermal radiation to space but also transfers heat back to the surface.
There are five processes that transfer heat from one object to another:
When I first started teaching these processes, my students remembered it by calling it CRACL, which became an easy way to remember that all of the processes should be considered when exploring a system's energy budget. Review each of these at the Energy page.
What are They?
Greenhouse gases (GHGs) absorb the mid to far infrared emitted from the warmed Earth surface. To determine their influence on warming a planet there are three key variables to consider:
How Do They Warm a Planet?
Similar to planet with an atmosphere without GHGs, heat is stored in these gases through a variety of processes. But now, with GHGs, more heat is stored before it is ultimately emitted to space.
As GHGs absorb the mid to far infrared emitted from Earth's surfaces, more heat is stored in the atmosphere. Part of extra atmospheric heat is transferred back to Earth's surface, keeping it warmer, and part is emitted out to space. The bottom line, a warmer atmosphere that has allowed water to remain in liquid form for much of Earth's 4.5 billion years.
The two gases to the right are the critical GHGs, although the most focus is on carbon dioxide due to its long residence time and that humans are releasing large quantities for our energy and industrial needs. The gases below are also important, especially since humans have a role in releasing them into the atmosphere.
This is the most potent GHG, and it usually is the highest GHG concentration, but it has a very short residence time - about 9 days! It condenses to form clouds (so no longer a gas) and forms precipitation which removes it from the atmosphere.
The effect of water on Earth's energy budget is complex. Evaporation, condensation, cloud reflectance, and precipitation all have large roles in warming and cooling Earth's surface and atmosphere.
Moderately absorbent to mid to far infrared, carbon dioxide has a growing but minor concentration in the atmosphere. However, its residence time is one the order of a century, making it is a key GHG.
In addition, methane, also a GHG, is converted to carbon dioxide in about a decade in the atmosphere.
Both given off by natural and human sources, methane is roughly two orders of magnitude more potent than carbon dioxide as a GHG, but it chemically alters to carbon dioxide in about a decade. So ultimately it is a source of long-term atmospheric warming, but in the form of carbon dioxide.
A very potent natural and human-generated GHG that has a residence time of roughly a century, but fortunately has a low concentration in the atmosphere.
A very potent human-generated GHG that has a residence time of roughly a millennium, but fortunately has a very low concentration in the atmosphere.
Atmospheric processes are driven by the heat transfer processes: conduction, radiation, advection, convection, and latent heat (CRACL). The behavior of Earth's atmosphere is not uniform with height due to interactions with the Sun's radiant energy, the diurnal and seasonal heating patterns of the Earth's surface, and the availability of liquid water at the Earth's surface. Rather, there are layers defined by how temperature changes with height (see image above. Credit: Randy Russell, UCAR). Note that in all layers air density must always decrease upward - if not, the less dense lower air will immediately rise vertically to a level where it is the same density as its surroundings.
Since Earth’s atmosphere is mostly transparent to the Sun’s thermal radiation, the Earth’s surface receives significant proportions of its radiant energy, so it heats up during the day and cools off at night, both of which affect the temperature of the layer of air next to the Earth's surface: theTroposphere. Since the air next to the ground tends to heat during the day time when the ground is warming, it may become less dense than the surrounding air and rise. As we see in the Clouds section, rising air expands since pressure acting on the air is decreasing, which makes the rising air cool. Combining the two processes, most of the heating occurring at the Earth's surface and rising air cooling as it expands, temperature decreases with height on average at a rate of 6.5ºC/km throughout the troposphere.
The largest reservoir of water, the oceans, is located at the base of the troposphere, so the greatest concentration of water vapor is within the troposphere. Rising humid air drives our day-to-day weather events, so we will focus on the troposphere to understand a large component of the water cycle.
Interactively explore the seasonal and latitudinal effects on atmospheric variables with height using the Atmosphere, Clouds, and Precipitation software.
The atmosphere is not completely transparent to the Sun's radiation. It turns out oxygen and ozone absorb the deadly wavelengths of ultraviolet in the Stratosphere. Based on the Sun’s surface temperature, it emits a wide range of wavelengths of electromagnetic radiation, spanning predominantly from the ultraviolet to far infrared (see Energy). All photons (massless packets of radiant energy) are not created equal - the amount of energy a photon contains is inversely proportional to its wavelength. Shorter wavelength photons contain more energy. Wavelengths of visible light and shorter are intense enough to cause photodissociation (also known as photolysis and photodecomposition) in which a molecule is broken down when the photon is absorbed. Photosynthesis by plants is a common process involving photodissociation.
There are three bands of ultraviolet: UV-C with the shortest wavelengths (and most energetic photons), UV-B and UV-A. UV-C are deadly to most living organisms, UV-B cause DNA damage to living cells, and UV-A cause wrinkles and premature aging of human skin.
Oxygen (O2), ozone (O3), and life-damaging UV create self-generating system that protects life on Earth:
In addition, this warm layer creates a ceiling that stops rising air at the top of the troposphere, which helps to create our large-scale atmospheric circulation patterns.
Mesosphere : is the layer of air above the stratosphere in which the temperature decreases with height. It is the least observed layer of our atmosphere.
Thermosphere : this layer is the least dense and the disperse molecules absorb the Sun's x-rays and some UV-C so that temperature increases with height.
For the Earth Systems arc, we will focus on the weather created in the troposphere and the life-saving processes occurring in the stratosphere.
The stratosphere requires that the atmosphere must contain sufficient oxygen levels (a minimum of 10%). But oxygen in Earth's atmosphere slowly developed as photosynthesizing plants evolved and eventually thrived.
Prior to the stratosphere, the troposphere wasn't capped by this low density ceiling. As you explore the Clouds and Precipitation sections, consider what weather systems may have been like before the stratosphere formed roughly a billion years ago.