Modern Atmosphere

Main Points

  • We live in the troposphere, which contains lots of gases and trace amounts of greenhouse gases, such as carbon dioxide (CO2).
  • Differential heating of the Earth’s surface (hotter at the equator, cooler at the poles) is what drives surface wind patterns.
  • The Coriolis Effect causes the deflection of object to the right in the Northern Hemisphere, and to the left in the Southern Hemisphere. Along the equator, the Coriolis Effect is zero.
  • There are three main air cells in each hemisphere: the Hadley, Ferrel, and Polar cells.
  • Where warm air from these cells rises, areas of low pressure are created. Where cooler air falls, they create areas of high pressure. These pressure differences result in wind.

The Troposphere and Stratosphere

The first step to understanding how and why our climate is changing is to understand some basic principles about Earth’s atmosphere.

The atmosphere is defined as all the gases that surround a planet, including the Earth, that are held in place by gravity. There are different layers to Earth’s atmosphere: the troposphere (6-20 kilometers above the Earth’s surface), the stratosphere (20-50 km), the mesosphere (50-85 km), the thermosphere (85-690 km), and the exosphere (690-10,000 km). The layers we will be most concerned with are the troposphere and stratosphere.

The troposphere is the layer of the atmosphere that contains the most gases, and is the layer in which we live. The troposphere is made up of 78% nitrogen, 21% oxygen, 0.9% argon, and traces of other gases such as carbon dioxide and water vapor. The troposphere is also the layer of the atmosphere where weather occurs. The layer above the troposphere, the stratosphere, contains the ozone layer. The ozone layer absorbs most of the sun’s radiation.

Atmospheric Gases

We are able to inhabit the Earth because it’s warm, there’s oxygen, and we’re protected from the Sun’s UV rays by the gases in our atmosphere. Some of the Sun’s energy that reaches the Earth is reflected back into space, but some is reflected back into the atmosphere, where it is trapped by greenhouse gases. Greenhouse gases include water vapor (H2O), carbon dioxide (CO2), ozone (O3), methane (CH4), and nitrous oxide (N2O). Recall from the above paragraph that these gases occur in small percentages in our atmosphere.

The Greenhouse Effect

Left image: naturally occurring greenhouse gases in the atmosphere trap some of the Sun’s heat while most if reflected into space. Right: Human activities are increasing the amount of greenhouses gases in the atmosphere, which causes more heat to be reflected back to Earth’s surface instead of being reflected back into space.

The Greenhouse Effect is caused by the small percentage of greenhouse gases in our atmosphere that reduce the rate at which Earth reflects energy, or heat, back into space. Earth and all surfaces have some ability to reflect or absorb energy. The reflectance of the planet is referred to as albedo. The higher albedo, the more energy that is being reflected; conversely, a low albedo means that less energy is being reflected. For example, if you wear a black shirt outside on a sunny day, you tend to be much warmer than if you wore a white or light-colored shirt. This is because the darker colors and surfaces absorb more of the energy emitted from the sun than light colors. Areas that are covered in snow reflect more energy back into the atmosphere than parking lots.

Energy from the sun that makes it into the atmosphere or onto Earth’s surface is eventually re-radiated back into space. Some gases in the atmosphere can absorb or trap that energy. The selective absorption is what results in the warming of the atmosphere and what we call the Greenhouse Effect. This effect is a natural phenomenon, and without it, the global temperatures on Earth would be far too cold for us to survive!

You may be asking yourself, if the Greenhouse Effect is a natural phenomenon, then why is all of this such a big deal? The problem with the current warming crisis is that the amount of greenhouse gases being emitted by human activity has increased dramatically in the past several centuries. This increase in greenhouse gases in our atmosphere is causing warming at a faster pace than what scientists have ever observed in the rock record.

Heating of the Earth’s Surface

Hypothetical wind circulation patterns on the Earth. In the Northern and Southern Hemispheres, there is only one main wind circulation cell, which transports warm air from the equator to the poles.

The Earth is tilted and shaped like a sphere. The combination of tilt and shape cause unequal heating of the Earth’s surface. The rays of the sun directly hit the equatorial region (largest portion of the sphere) and less heat from the sun reaches the poles. This differential heating of the Earth’s surface is what powers global wind patterns. In general, the bigger the temperature difference between the equator and poles, the stronger the winds. This is due to warm air being transferred from the equator to the poles as air parcels move from areas of high to low pressures. The figure on the right demonstrates how differential heating of the Earth’s surface leads to two main air circulation paths, or cells, on an Earth that was stationary. Air that is warmed at the equator is less dense than the cold air at the poles, so this less dense air rises and moves to higher latitudes. As the warmed air moves towards the poles, it cools, condenses, and sinks. As the more dense air becomes super cooled, it flows back to equatorial regions and the process begins again. But, our planet isn’t stationary in space. Instead, it is spinning about its axis and around the Sun, and this movement causes deflection, or bending, of winds and ocean currents, which is called the Coriolis Effect.

If you are interested in differential heating click here to watch a video on it!

Coriolis Effect

Hypothetical planet rotating about its axis showing the effect of the Coriolis Effect on bullets fired from the equator to the north and south, towards the poles.

The Coriolis Effect influences the surface winds as well as ocean currents. The Coriolis Effect is an invisible force that appears to deflect objects as they move over the surface of the Earth without actually being in contact with the Earth itself.

The Earth is a rotating mass, and due to its spherical shape, a point at the equator rotates more quickly than one at the poles. This is because the Earth is wider at its equator, and thus an object at the equator has to travel farther in a day than an object located at the poles. As an object moves from the equator to the poles, it will be deflected to the right in the Northern Hemisphere, and to the left in the Southern Hemisphere. An object traveling about the equator has no apparent deflection. The figure to the right illustrates the Coriolis Effect on bullets fired from the equator to the north and south poles. In the Northern Hemisphere, the bullet is deflected to the right. In the Southern Hemisphere, the bullet is deflected to the right.

The Coriolis Effect has to be taken into account by airplane pilots when flying long distances. This is why when flying from city X to city Y, you travel over city B, even though it’s not a straight line. Although the Coriolis Effect is used in mathematical equations, it is purely caused by the equator rotating faster than the poles, and is really not a force at all. It does, however, play a huge role in the Earth’s winds, and well as surface currents, which will be discussed in subsequent pages.

Earth’s Wind Patterns

The figure to the right represents the ideal wind patterns (in other words, the pattern of the winds at different latitudinal bands) for an Earth without continents that is spinning. Geoscientists do this to give ourselves a simplified view of the wind patterns. Even with continents, the average wind patterns still follow the pattern shown in the figure at left.

The ideal wind patterns of the Earth if there were no continents and their associated names. Notice that the wind bands are separated by the 0, 30, and 60 degree latitude lines. The major wind cells on the Earth, indicated by the pink lines. Arrows indicate the direction of rising and falling winds. The rotational axis indicates which way the Earth is spinning.

As mentioned above, the Earth is unequally heated at the equator relative to the poles due to a surplus of heat at the equator relative to the poles. We also discussed how air cells move on an Earth that is stationary. Notice that on a spinning Earth, there are now three main air cells in each hemisphere.  Warm, less dense air at the equator rises, and as it moves towards the poles, it cools, condenses, and sinks at 30° North and South of the equator, creating the Hadley Cell. Other convection cells are also present between 30° and 60° North and South (Ferrel cells), and from 90° to 60° North and South (Polar cells). Where warm air rises it creates areas of low pressure on the Earth’s surface and cold air sinking creates areas of high pressure.

Areas along the equator experience more rainfall (think tropical rainforests) because of rising warm, moist air. By the time the air cools, condenses, and begins to fall again, there is very little moisture left. Thus, areas where cool, dry, air falls, such as around 30° and at polar regions, receive very little precipitation. This is the reason most deserts of the world occur between 30° and 50° in the Northern and Southern Hemispheres.

Where the air masses are rising, we call these areas of low pressure. Where air masses condense and fall, these are areas of high pressure. Winds blow across the Earth’s surface from high to low pressure areas, which is partly due to the unequal warming of Earth’s surface by the Sun as discussed above.

More information about our atmosphere can be found at the following links:

 

Proceed to ‘Ocean Layers & Mixing’

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