María y las Ciencias Naturales: THE GASEOUS PART OF THE EARTH

The gases of the atmosphere

The Earth’s atmosphere is a mixture of gases called air. When it is not contaminated, air is odourless, colourless and tasteless.

The atmosphere has gases in different proportions:

Nitrogen (N₂), 78%: this gas has no colour, no smell, and no taste.

Oxygen (O₂), 21%: it is in the air and water. Plants produce O₂ during photosynthesis. It is necessary for combustion and respiration.

Water vapour (H₂O_v) and other gases, 0.97%: water vapour comes from the evaporation of oceans, lakes, and rivers and also from plants transpiration. Ozone (O₃): It is another gas. Electrical storms and the interaction of the sun’s ultraviolet rays with oxygen produce O₃.

Carbon dioxide (CO₂), 0.03%: this gas has no colour or smell and is soluble in water. It is produce by plant and animal respiration and the combustion of certain substances. It is essential for photosynthesis.

The layers of the atmosphere

•IONOSPHERE. This layer is also known as the THERMOSPHERE.

It is the highest and the thickest layer. Its temperature increases to 1,000ºC due to X-rays and gamma rays from the Sun.

It reflects radio and television waves back to the Earth.

Comets appear here.

•MESOSPHERE. About 40 km thick.

It contains clouds of ice and dust.

•STRATOSPHERE. About 30 km thick.

There is an increase in temperature from -70ºC at its lower limit to 0ºC at its higher limit.

The ozone layer is here = ozonosphere. The ozone layer protects us from ultraviolet rays.

•TROPOSPHERE. Very thin (it is 12 km thick), but contains most of the

gases of the atmosphere (80% of the total mass of the atmosphere).

Most meteorological phenomena occur here.

Atmospheric pressure

Air is matter: it has mass and weight

Atmospheric pressure = the weight of the atmosphere on the Earth

We do not notice atmospheric pressure because:

§ we have adapted to it.

§ it is equal in all directions.

Pressure can be measured.

The international unit for pressure is the pascal (Pa).
When we refer to atmospheric pressure we normally use the atmosphere (atm) or the millibar (mb)

1 atm = 1013 hPa = 1013 mb

Atmospheric pressure varies with altitude

The atmosphere is a mixture of compressible gases.

The lower layers support the weight of the upper layers. The gases here are compressed and denser.
Pressure at the top of a mountain is less than at he beach.

Normal pressure is the pressure at sea level (1 atm).
Pressure above this is high pressure.
Pressure below this is low pressure.

Atmospheric pressure varies with temperature

Warm air rises, and the space left behind is filled by other air masses that are nearby. This cause the formation of a low pressure area or depression.

Cold air descends and disperses when it reaches the surface. This causes the formation or a high pressure area or anticyclone.
The air moves from the areas of high pressure to the areas of low pressure, causing air currents.