................................................................... The atmosphere

Introduction. When we decide to look for life on another planet or satellite, one of the most important characteristic is whether it has an atmosphere or not. The reason is that if there is no atmosphere, the temperature on surface can vary over a hundred degrees between day and night, as happens on the Moon, which is incompatible with life. Therefore, it is thanks to atmosphere that our planet is livable.

Sometimes is said, that because the atmosphere contains oxygen, there is a life on planet Earth. It is actually just the opposite. It is thanks to living beings, specifically to cyanobacteria which over 3500 million years ago began to perform photosynthesis, that there is oxygen in our atmosphere.

Currently we are endangering this layer (global warming, weakening the ozone layer, acid rain, etc.) which we have to stop doing, because it jeopardize the survival of future generations. In addition, many forms of terrestrial life could still exist with an atmosphere very different than nowadays, but we could not. Below you will find many information about this layer.

Activities to do. Read the explanations about the composition and layers of the atmosphere and do the exercise "Identify the atmosphere layers 7". Then read the text on the origin of the atmosphere, cloud formation and dynamics of the atmosphere and do the exercises "Multiple Choice Test 7" and "Crossword 7".

1 . The Earth's atmosphere.
It is the gaseous layer that covers our planet. It is consisted of a mixture of gases called air. Depending on the composition of the air, it is divided into the following layers:

Homosphere. Homosphere. It is the layer having a constant air composition. It extends up to 80kn high. It is composed of:

      • 78,1% of Nitrogen (N2)
      • 20,9% of Oxygen (O2)
      • 0,93% of Argon (A)
      • 0,035% of Carbon Dioxide (CO2)
      • 0,035% of neon, helium and other gases

Heterosphere. It is the layer that does not have a constant composition of the air. It is located above the homosphere.

2 . The atmospheric gases and environmental problems.

Nitrogen. It is a gas, which at a room temperature does not react with other substances, therefore cannot be used by plants and animals. Only a few microorganisms can capture it. This prevents a concentration of oxygen that is extensive for life and which excess fires.

Oxygen. It is a gas that allows animals and plants respiration, it is a gas that reacts with molecules derived from food and generates vital energy and CO2. From oxygen (O2) is formed an ozone (O3), which protects the organisms from cancerous mutations that can be made by ultraviolet rays. Some gases, such as chlorofluorocarbons (CFC) by being used in aerosols, refrigerators and air conditioners, slowly destroy ozone layer. This thinning is commonly known by name "the ozone hole".

Argon, neon and helium. Are gases that do not react with other substances (are noble gases), so are not affecting the life of organisms.

Carbon dioxide. It a gas that plants use to produce organic matter through the photosynthesis. This is also a gas that animals and plants release by breathing and is produced in fires and combustions. This gas allows the entry of solar radiation but not exit of heat given off by rocks and hot water. This phenomenon, called greenhouse effect, is natural and good as it helps to maintain stable temperature of a planet. Unfortunately, the excessive increase of the CO2 produced by burning oils and coals, is causing excessive heating of the planet which leads to climate changings.

Pollutant substances. Besides CO2 and CFC the human activity produces other substances that pollute the atmosphere. The main ones are:

      • Sulfur oxides (originated from combustion of carbon) and oxides of nitrogen (originated from diesel engines). Those oxides combined with vapor of water result in acid substances that fall as rain, called acid rain, that acidify ground killing plants.
      • The combustion smoke (small carbon particles being in suspension in the air) and dust from quarries cause respiratory diseases and allergies.
      • Dioxins. Substances produced in the incineration of garbage, that favor the appearance of cancers.
3 . Structure of the atmosphere. In the atmosphere can be differenced five layers, depending on their gas composition and temperature. It is recommended to start studying from the bottom layer, the troposphere, where we live, and to continue in ascending order.

5. Exosphere. It is the most external layer of the atmosphere. It starts at 500 km of height. It has only a few and very separated air molecules, which make it very difficult to know where it ends (it may reach up to 2000 km).


4. Thermosphere or ionosphere. It starts at 80 km and reaches up to 500 km. As the temperature vary from -80 ºC to 1000 ºC, hence the name thermosphere. It contains electrically charged particles (ions), hence the name ionosphere. It also reflects radio waves, and inside occur Northern Lights that can be seen from the polar regions.



3. Mesosphere. Starts at 50 km and reaches 80 km. It is a layer without ozone and vapor of water. In it, the meteorites become incandescent and produce shooting stars. The temperature vary from about 80 ºC at 50 km to about -80 ºC at 80 km.



2. Stratosphere. Starts at 13 km and reaches up to 50 km. It is dominated by the horizontal movement of the air, hence the name. Contains the ozone layer that absorbs UV radiation. This reaction releases energy and it causes an increase in temperature from about -60ºC at 13 km up to 80 ºC at 50 km.



1. Troposphere. Extends up to 13 km. Contains 80% of gases and nearly all of vapor of water. Inside it, for every 100m of high the temperature decreases of 0,65 ºC reaching -60 ºC at 13 km. The vertical movements of the air are, called convection current, predominate. In it the precipitation (rain and snow) are produced.

Identify the layers of the atmosphere (Spanish activity. Activity in English is under construction)

4. Origin of the atmosphere. When 4500 million years ago Earth was formed, the atmosphere was very hot and contained nitrogen, carbon dioxide, methane, ammonia and a lot of water steam. Then, when cooled, intensive rain occurred, and oceans were formed. 2.5 billion years ago, cyanobacteria, during the photosynthesis and generating the oxygen started to enrich the atmosphere with said oxygen, so 2000 million years ago it was very similar to the current one.

5 . The clouds formation. Clouds are masses formed by microscopic drops of water or microscopic crystals of ice, which are founded in suspension in the air. When the drops bind together and grow up to the size that allows them to fall, there is a precipitation of water called drizzle, rain or shower depending on its intensity. When the crystals of ice are binding, there is a precipitation of solids, as happens with snow (when fluffy mass formed by microscopic crystals of ice, called "snowflakes" fall) and hailstorms (ice grains fall called "hail").

Clouds are formed when a mass of humid air, that is with a lot of steam of water (gas) is being cooled until condensation occurs in water (liquid) or in ice (solid). This can happen because:

  1. This air rises up to areas of the atmosphere where the temperature is lower, resulting in high clouds,
  2. This air comes in contact with the ground that has cooled at night, thereby giving rise to low clouds (fog) and when they condense on the ground, to the dew,
  3. This air is mixed with cold air mass coming from elsewhere.

In the picture below you can observe the first and second case.

6. Dynamics of the atmosphere.
The factor that determines the movement of the Earth's atmosphere is the solar energy. The sun rays pass through air without heating it, but they warm up the ground and seas. Are those that emit the absorbed heat and which warp up the air by contact with them. When air is heated it expands, which means that the molecules are being separated. It makes it less density, meaning that in the same volume now, there is less molecules, and therefore this volume weights less. Therefore, this warm air rises over the cold air. Another characteristic of hot air is that it admits inside more steam than cold air. In summary, hot air tends to rise and, of it's over the body of water, gets very humid. But in the contrary cold air tends to fall and cannot hold as much moisture as hot air.

When a zone is heated, the warm air rises and its place is taken by air cooler and thicker than the surrounding areas. As the hot air rises, it cools and tends to move to a place formerly occupied by cold air. These air currents are called convection currents and form a closed cycle called convection cell.

An example of convection current occurs in coastal locations. In them, by day there is a wind going from sea up to the mountain, since mountain warms up faster than the sea, the air above warms and rises, and is replaced by cold air coming from sea (sea breeze). However, at night there is a wind that goes from mountain to sea (land breeze), because the mountain cools faster than the sea, so the air above cools down and goes to sea to replace the air that is still warm there, that goes up.

7. Storms and anticyclones. The air weight and, therefore, exerts pressure on the planet's surface. It's called an atmospheric pressure which is the weight per unit area exerted by atmosphere. In cells convection, hot air zones are areas of low atmospheric pressure, because of the hot air being less dense (contains few molecules per unit volume) exert little pressure. Otherwise, cold air zones are areas of high air pressure, since the more dense cold air (containing many molecules per unit volume) exerts more pressure. Areas of low pressure are called cyclones or storms and high pressure areas are called anticyclones.

When two different pressure areas communicate, the air moves from the area where is more pressure (anticyclones) to where there is less pressure (depressions). This is what causes the wind (moving air masses). As an outside air arrives, can produce rainfalls, the storm zones are unstable time zones, and anticyclone zones are stable.

The convection currents are not only set in small areas, but also at the level of the entire planet. On Earth can be differentiated the convective polar cell and equatorial convective cell. This explains the global dynamics of the atmosphere and many characteristic of different climates.


Crossword. (Spanish activity. Activity in English is under construction)
Multiple Choice Test. (Spanish activity. Activity in English is under construction)
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