The Atmosphere

The Atmosphere: Definition

Atmosphere is defined as the gaseous layer that surrounds the planet. It is made up of a mixture of different gases that are retained close to the Earth by the planet's gravity force.

The atmosphere is very important for life on our planet, because it provides gases to living things. It also absorbs ultraviolet radiation, that it is extremely deleterious. The atmosphere, besides, warms the Earth's surface through heat retention and reduces extreme changes of temperature between day and night: it keeps the heat accumulated during the day, so the global temperature doesn't descend dramatically during the night.

Finally, the atmosphere is essential to complete the water cycle. It has not only water in gaseous state (water vapour), but also liquid or even solid water water stored in the clouds.

Chemical Components

The atmosphere is made up of a mixture of gases. The composition of the gaseous layer has changed throughout the Earth's history. The primitive atmosphere was rich in carbon dioxide (CO₂), ammonia (NH₃), methane (CH₄) and water vapour. Oxygen was, however, rare.

The atmospheric water vapour condensed when the Earth's temperature descended, causing intense precipitations and forming the current oceans.

The reduction of the volcanic activity and the photosynthetic activity of living beings that appeared three thousand million years ago changed the gas proportions. Living beings use carbon dioxide to produce organic matter and release oxygen. Due to this, the relative amount of carbon dioxide dropped and the oxygen raised for million years.

The most abundant gas in the current atmosphere is the Nitrogen (N₂). It is 78% of the total mass. This gas is colourless and odourless. Although nitrogen is an essential element for living beings, nearly none of them are able to use the atmospheric nitrogen (only some microorganisms).

The second most abundant gas is the oxygen (O₂). It is 21%of the total mass. As we have just studied, most of the atmospheric oxygen comes from the photosynthetic activity of autotrophic living beings (living beings that can produce their own food). It is transcendental to the respiration of many living beings.

The rest of the gases are only 1% of total mass. Some of them are extremely important gases. Carbon dioxide (CO₂), for instance, is necessary for photosynthesis, but it is also one of the main responsible for the greenhouse effect. 

The water vapour (H₂O) is generated during the water cycle. The amount of water vapour in the atmosphere is called humidity and it is related to the apparent temperature (the higher the amount of water vapour, the higher the transmission of heat by the air).

The ozone (O₃) is rare, but very important because it absorbs the ultraviolet rays, that are dangerous for living beings.

Hydrogen is very rare, because it reacts with the oxygen to form water.

Other gases are inert. Helium (He), Argon (Ar), Neon (Ne) or Krypton (Kr) are noble gases that don't react with any other substance. Due to this, the amount of these inert gases has been nearly the same throughout the whole Earth's history.

Atmosphere: Layers 

The atmosphere can be divided into five consecutive layers:

• Troposphere.
• Stratosphere.
• Mesosphere.
• Thermosphere.
• Exosphere.

Troposphere

The troposphere is the lowest and thinnest layer of the atmosphere. It is 17 kilometres in height on average. It is thicker in the equator and thinner in the poles.

The troposphere contains 75% of the total atmosphere's mass. It is quite thin, but gases are very condensed. Its temperature declines with the altitude, around 6.5°C per kilometre.

It is the layer where the meteorological phenomena occurs, so it is responsible for the Earth's weather.

It is bound above by the tropopause, that separates this layer to the stratosphere.

Stratosphere

The stratosphere is the second sayer of the atmosphere. It is 40 kilometres thick on average (from 12 kilometres to 55 kilometres, more or less). It has very low atmospheric pressure and its temperature is lower at the tropopause, around -60°C, and rises with the height, reaching 0°C in the higher limit.

The atmospheric conditions in this layer are very stable, there are only  some peculiar clouds and there are not relevant atmospheric phenomena. But it contains the ozone layer, that protects the Earth from ultraviolet radiations.

It is bound above by the stratopause, that separates this layer to the mesosphere.

Mesosphere

This layer lies directly against the stratosphere. It is 50 kilometres thick on average, it extends from 50 to 100 kilometres of altitude. It is thinner in summer. During this season, its upper limit descends to 85 kilometres.

It has a very low amount of gases, so its atmospheric pressure is very low. Its average temperature descends with increasing height, from 0°C in the tropopause to -143°C in the upper boundary, called mesopause.

Although the atmospheric phenomena are rare in this layer, there are typical mesospheric structures called noctilucen clouds (or polar mesospheric clouds), that are made of ice crystals. 

The upper part of the mesosphere belongs to a atmospheric layer called ionosphere (a part of the ionosphere is in the mesosphere, another part is in the thermosphere).

The mesosphere is bound above by the mesopause, that separates this layer to the thermosphere.

Noctilucten Clouds

Thermosphere

This layer, that lays just above the mesopause, is very thick: more than 400 kilometres. Its low boundary is between 80 and 100 kilometres high and the upper limit is more than 500 kilometres high.

It has few gases, only vestigial particles. As a result, the atmospheric pressure is extremely low. Its temperature increases with increasing height due to the absorption of solar electromagnetic radiation. The temperature near the upper boundary can reach 2500°C during the day although there is not warm sensation. This fact results from the extremely low amount of matter: there are not particles capable of transmitting heat.

Many artificial satellites or devices such as the International Space Station orbit in the thermosphere.

Exosphere

The exosphere is the atmospheric volume surrounding the Earth, made up of residual particles orbiting the planet by gravitational attraction.

It doesn't have a definite upper boundary. For this reason, the exosphere is frequently considered a part of the outer space.

Atmospheric Conditions

The general tropospheric conditions are not constant, but they change. These changes in the atmospheric conditions are very important, because they are related to weather.

The most relevant atmospheric conditions are:

• Atmospheric pressure.
• Temperature.
• Wind.
• Humidity.
• Clouds.
• Precipitations.

Atmospheric Pressure

The atmospheric pressure is defined as the force of the air on a surface by the weight of the atmospheric air column above that surface. Summing up, it is the weight of the air on a concrete point.

The unit for pressure in the International System (IS) is the Pascal (Pa). Meteorologists,  however, more frequently use other units, mainly bars (bar) or the millesimal unit derived from it, the millibars (mbar). It is measured by an instrument called a barometer.

The atmospheric pressure is variable. It depends on the air density, so it changes when the air temperature changes. It also depends on the amount of air above the surface. For this reason, air pressure at sea level tends to be higher, because the air column is also higher.

Variations in the air pressure are related to changes in weather. Weather maps have lines that connect points with the same atmospheric pressure. In these maps we can find high pressure centres, usually marked with a H, and low pressure centres, usually marked with a L.

The air pressure is related to the movement of air masses or, in other words, it is related to the wind. The air always move from high pressure centres to low pressure centres. As a result, clouds move from high pressure centres to low pressure centres (carried by the wind currents). Thus, low pressure centres are zones where clouds tend to accumulate. So high pressure centres are usually associated to fair weather, whereas low pressure centres are associated to bad weather.

This is how isobaric maps and barometers help us to predict the weather. But there are other essential parameters to complete the prediction, such as the temperature.

There are three types of maps to forecast the weather: isobaric maps, that we have just studied, graphic maps, with symbols that show how the weather is going to be, and real maps, taken by satellites, showing the distribution of clouds above the Earth.

Temperature

The atmospheric temperature is the amount of heat stored in the gases. The International unit for temperature is the Kelvin (K) although in our regular life we usually use other units,  above all degrees Celsius (°C). The instrument used to measure temperature is called  thermometer.

Changes in the air temperature lead to changes in air properties. Hotter air has less density, so hot reduces the atmospheric pressure. Colder air has more density, so cold increases the atmospheric pressure. Hot air, besides, tends to ascend to upper atmospheric layers, because of its low density.

The air temperature is also related to the amount of water vapour of the atmosphere. Colder air can support less water vapour. It the air temperature is extremely cold, the water vapour freezes, so the humidity descends. The driest air of the planet can be found in the Antarctic.

Difference of temperature between different points or zones of the planet are related to the movement of large masses of air, due to changes in the atmospheric pressure. The air in the equator tends to become hotter, then in ascends and moves towards hotter places. The air in the poles tends to become colder, then it descends and move towards hotter places.

All these movements cause atmospheric currents, that are related to the movement of clouds and the local climate in different parts of the planet.

Wind

The wind is the air in movement. As we have just studied, it comes from differences of atmospheric pressure. Wind is responsible for the movement of clouds. The movement of air masses with different temperatures causes air currents. These currents are responsible not only for the transportation of clouds, but also for the transmission of heat from one place to another.

There are two important characteristics of the wind in one particular region: the direction and the speed. The wind direction is measured by wind vanes. The wind speed is measured by the anemometer. The wind speed is related to the strength of the wind. 

Wind can be classified according to its strength. From weaker to stronger, we can define:

• Breeze.
• Gale.
• Storm.
• Hurricane.
• Typhoon.

Humidity

Humidity is defined as the amount of water vapour in the atmosphere. The water vapour is invisible, it can only be seen when it condenses. The most usual system to measure the atmospheric humidity is using a percentage. When this percentage reaches 100% the water vapour condenses, changing from gaseous to liquid state. 

The instrument used to measure the atmospheric humidity is called hygrometer.

The atmospheric humidity is related to the thermal sensation. The higher amount of water vapour in the air, the better heat transmission. Due to this, environments with high humidity increase the thermal sensation. Thus, cold temperatures are perceived as colder and high temperatures are perceived as hotter.

Clouds

Clouds are visible masses of tiny drops of water suspended in the atmosphere. They are formed, in general, by the condensation of atmospheric water vapour.

The condensation takes place when the temperature decreases. This process is specially frequent in high parts of the troposphere, but it can occur at any altitude, even very close to the Earth's surface, forming fog.

In fact, condensation can take place under different conditions of humidity, pressure or temperature. This is the reason why there are different types of clouds.

According to the height, there are low, medium and high level clouds.

Low level clouds.

• Stratus: they are grey, flat and uniform clouds. They show horizontal layering with uniform base. Thick stratus can produce precipitations. When these clouds are very low, they form fog.

• Cumulus: they are low clouds with vertical development. They are cotton-like clouds and usually indicate fair weather. They can also be precursors of other clouds, such as cumulonimbus.

• Stratocumulus: they are hybrid of stratus and cumulus, characterised by dark large masses. They don't usually to produce precipitations, but they are typically visible after rains or before storms.

• Nimbostratus: dense clouds, similar to stratus but thicker. They are made of multiple layers or strata. They produce precipitations, mainly rain and snow.

• Cumulonimbus: thick cotton like clouds, made of multiple layers or strata. They are cumulus with high vertical development. They are related to atmospheric instability, heavy rains and storms.

Middle level clouds.

• Altostratus: flat, grey clouds forming layers. They are similar to stratus, but higher. Due to these characteristics, they are usually translucent. They can produce light precipitations.

• Altocumulus: high white or grey globular clouds, similar to cumulus, but at high altitude. They are related to variable weather.

High level clouds.

• Cirrus: thin and long clouds, similar to white strands. They are always white or light grey. They are made of tiny ice crystals. Although they usually indicate that weather may soon to deteriorate, they can also precede warm fonts. Cirrus are also formed during tropical cyclones.

• Cirrostratus: widespread cirrus or, in other words, high stratus made of ice crystals. They indicate high humidity in high tropospheric layers. They are a sign of precipitations in the following hours, although they can also be related to warm fronts.

• Cirrocumulus: they are made up of liquid water and water crystals mixed. They are high white cotton like clouds. When they form dense groups, they precede rains. When they are isolated, they are related to fair weather.

Precipitations

When the tiny drops of water that made the clouds condense, forming bigger water drops or flakes and little balls of ice, they fall from the clouds due to the gravity force. This process is called precipitation.

The device used to measure the amount of precipitation (mainly of liquid precipitation) is called pluviometer or rain gauge.

There are three types of precipitations: rain, snow and hail.

Rain

Rain is the precipitation formed by liquid water droplets. It occurs when the tiny drops of water that made the clouds condense, forming bigger drops that fall as a result of the gravity force. This condensation can take place due to changes of pressure or temperature.

Rain is the most responsible for the deposition of fresh water. 

Snow

Snow is a precipitation in form of flakes of crystalline water ice. The tiny drops of water that made the clouds condense and freeze slowly. These condensed frozen drops form the crystalline flakes.

The flakes fall from the clouds. Obviously, snow is related to cold weather, because water solidifies under cero degrees Celsius. And it is always formed from clouds with high vertical component.

Hail

Hail is a precipitation in form of ice balls. It is related clouds with high vertical component and also high amount of water.

Hail is formed when the little drops of water condense and freeze abruptly due to low temperatures, but air currents make them ascend instead of falling. These little frozen drops can adsorb more water and freeze again, producing bigger ice balls that fall from the cloud.   

Atmosphere and climate

Different parts of the planet have different climatological characteristics, mainly depending on the latitude, but also on other factors, such as altitude or distance from the sea or air currents.

Latitude

Latitude is defined as the distance of one point or region from the equator.

The latitude is related to the solar radiation intensity. The solar rays reach the Earth in more perpendicular way in equatorial regions. Due to this, radiation is more intense in equatorial zones and less intense in the poles.

So that, the furthest the distance from the equator, the lower the average temperature. 

Altitude

Altitude is defined as the vertical distance from the sea level. The average temperature descends with the altitude. In other words, the higher the altitude, the lower the average temperature.

This is the reason why the higher mountains of the planet have perpetual snow. The line for perpetual snow is not clearly defined, it depends on the latitude or location of the mountain. 

Distance from the sea

The sea moderates changes of temperature, because liquid water heats and cools slowly. Summing up, sea water preserves temperature.

Near the sea, winter tends to be not so cold and summers not so hot. Inland regions, however, tend to have colder winters and hotter summers.

Air and ocean currents

On the one hand, the ocean in equatorial zones heats the air. On the other hand, the oceans in polar zones cools the air. These differences of temperature cause air movements. Hot air tends to ascend, whereas cold air tends to stand close to the earth. 

The difference of the water temperature also causes ocean currents. Sea currents change the air temperature, modifying the main air currents or causing other secondary air movements.