The Hydrosphere

The Hydrosphere: Definition and Distribution

The hydrosphere is defined as the water that covers the Earth's surface. Water in the Earth can be found in three different states: liquid, solid (called ice) and gas (called water vapour).

70% of total Earth's surface is covered by water. Most of this water is salt water, located in the seas and oceans. In fact, 97% of total water is salt water. The other 3% is fresh water that can be found in different locations and states. 

68.8% of total fresh water is solid water: ice and snow. This kind of water can be found, above all, in the poles, but also in high mountains as snow covering the ground or glaciers (large and erosive rivers made of solid water).

30.1% of fresh water is groundwater, water located in pores of deep rocks, underground currents, aquifers, etc.  

0.45% of total fresh water can be found in the atmosphere, as water vapour or tiny drops of water in the clouds. Other 0.45% can be found in living beings: water is the main component of all the known living things in this planet.

Finally only 0.3% of total fresh water is surface fresh water. Only this percentage is really available for regular human consumption. 

87% of surface water is located in lakes and other similar wetlands. 11% is located in swamps. Only 2% can be found in rivers and other surface currents, such as torrents or streams. 

Water: Properties

Water is a substance with some relevant characteristics and properties that make it essential to understand its role in the planet and living things.

Universal solvent

Water is called universal solvent, because it can dissolve many different substances. It can dissolve nearly any kind of salt and many organic substances. It can also support many chemical reactions, or in other words, many chemical reactions can occur in water.

This is one of the reason why water is the most abundant component of living things. All the chemical reactions essential for in living beings take place in water. Water, besides, dissolves many organic or inorganic substances that make up the living beings.

Specific heat capacity

Water has a high specific heat capacity. This means that water preserves heat, so it warms and cools quite slowly.

This characteristic is related to how seas and oceans prevents from abrupt climatic changes of the atmosphere.

High surface tension

Surface tension is the force of the water surface that makes it extend on a surface as much  as possible. Water has a relevant surface tension. As a result, it has a great force of adhesion and also a great cohesion.

This is related to how water flows, its viscosity and its erosive capacity.

Sea Water

Characteristics

97% of total water on Earth is salt water located in seas and oceans. The main characteristic of this water are:

• Components and salinity.
• Temperature.

Components and salinity

Apart from water molecules (H₂O), sea water has other components. The most relevant ones are salts and electrolytes, that are related to the salinity. But sea water has other components, such as gases and organic molecules.

Sea water has different dissolved gases. Oxygen (O₂) is one of the most abundant dissolved gases and it is essential for living beings that need this molecule for respiration. Nitrogen and carbon dioxide are quite abundant too.

The organic molecules dissolved in sea water come from the vital activity of living beings or from several human activities (in this case, we talk about contaminants).

Salinity is defined as the concentration of salts in the water. Salts are inorganic substances. The most abundant one is, by far, sodium chloride (NaCl). Sea water has higher concentration of salts than surface water or groundwater. Due to this, sea water is also called salt water.

The concentration of salts, above all of sodium chloride, is one of the most important characteristics of sea water. Different oceans have different salinity. We can say, in general, that the colder the sea is, the lower amount of salts it has. Due to this, The Artic Sea is characterised by its low salinity. The Dead Sea is  the sea with highest salinity.

The average salinity of sea water is around 35 grams of salt per litre of water. Although the essential components of salt are, as we have just studied, sodium and chlorine, there are other components, such as magnesium, calcium, sulphur or potassium.

Temperature

The temperature of sea water depends on the latitude (distance from the equator) and the depth.

As far as latitude is concerned, sea water temperature decreases with latitude. In other words, it is hotter near equator and it is colder in the poles. This fact is related to some water movements called sea currents.

As far as depth is concerned, the water temperature decreases with depth. At 1000 meters deep, the temperature is lower to -2°C. In fact, it is still liquid because pressure and salinity prevents it from freezing.

Sea water movements

The most relevant sea movements are:

• Waves.
• Ocean Currents.
• Tides.

Waves

Waves are surface movements of water in seas or oceans. They mainly result from wind wind flowing over the seas. Waves can move from thousands kilometres before reaching the coast. 

Waves can have different sizes depending on the strength and direction of the wind. Wave size also depends on the coast features.

Waves are relevant erosive factors. They are related to the formation of cliffs and beaches and how they evolve or change.

Waves are, besides, very important because they oxygenate the water.

Ocean Currents

Ocean currents are continuous directed movements of seawater. They are large masses of water that move like rivers through the oceans.

These currents are mainly produced by the winds and differences in the temperature and salinity of the water. These differences cause variation in the density of the water and this variable density is the main reason for its movement.

The Coriolis effect is another relevant factor. This effect comes from the centrifugal forces caused by the Earth's rotation. The centrifugal forces affect, above all, the fluids, such as atmospheric gases and seawater. It affects the direction and trajectory of the current.

There are also vertical currents. These are vertical movements of seawater. There are two different currents, the surface current that involves the first four hundred meters, and the deep current that involves water deeper than four hundred meters. These two layers of seawater are nearly independent and water does not tend to move from one layer to other. In fact, some water properties, such as salinity and temperature, change abruptly causing a sort of boundary between these two regions. 

Tides

Tides are periodical rises and falls of seas levels. There are one or two high and low tides per day that are caused by gravitational forces exerted by the Moon and the Sun. The Moon's gravitational effect is much stronger than the Sun's one.

The amplitude of the tides depend on two factors: the relative position of the Moon and the Sun and the characteristics of the shore.

Tides are higher when the Moon and the Sun are aligned with the Earth, and are lower when  they are not in a line.

The structure of the shore is also a relevant factor. When the shore directly connects to an large ocean, tides tend to be higher. Tides in the Cantabrian Sea, for instance, are higher than tides in the Mediterranean Sea because the Cantabrian Sea is directly connected to the Atlantic Ocean, that has much more water mass than the Mediterranean Sea and as a result, the effect of the Moon and the Sun is also bigger.

Fresh Water

It is called continental water, and its main characteristic is the low concentration of dissolved salts. There are several types of fresh water:

• Solid water (ice).
• Groundwater.
• Lakes.
• Wetlands.
• Streams, torrents and rivers.

Solid Water

It is the most abundant type of fresh water. It can be found in glaciers, caps, ice sheets or icebergs.

Solid water is exclusive to cold places. The atmospheric temperature depends on the altitude and the latitude and the coldest locations are at high altitude or high latitude. Due to this, ice and snow can merely be found in covering high mountains or near the poles. In fact, mountains above four thousand meters and polar regions have permanent snow or ice. 

Glaciers are characteristic structures made of a permanent mass of ice in continuous movement. The ice in the glaciers moves, flows like the water in a river, but very slowly. Although it is not a fast movement, it has an extreme erosive power. They characteristically wear deep and wide U shaped valleys.

Groundwater

Groundwater is defined as the water that can be found beneath the Earth's surface. It is stored in soil pores, fractures of rocks or in underground rivers or lakes.

Groundwater can sometimes form underground caves and galleries.

When it emerges to the surface, it can form springs, seeps, wells or wetlands. It usually feeds rivers, streams and lakes.

The line that separates the lower parts of the soil filled with water to the upper parts of the soil with no water in the pores is called phreatic level. 

Lakes

Lakes are large masses of water surrounded by land. They can be fed by rivers, glaciers or groundwaters. Rivers and groundwaters can also drain lakes.

They are always located in basins or depressions and they can have variable size and depth, from few square metres to many square kilometres. The largest lake of the world is the Caspian Sea, that is 371,000 Km2. 

Wetlands

Wetlands are areas permanently or seasonally saturated with water. There are many different types of wetlands:

• Swamps: forested wetlands.
• Marshes: wetlands covered by herbaceous plants.
• Bogs: wetlands rich in discomposed plants and other decaying materials.
• Fens: wetlands dominated by grass and sedges.

Rivers and Torrents

Natural water courses of freshwater. Rivers are permanent water courses. Torrents and streams are intermittent or discontinuous water courses with variable courses.

They are responsible for surface runoff, they transport water from the land to the sea. They are also a relevant erosive factor, not as strong as glaciers but much more frequents. They wear V shaped valleys in he upper course and U shaped valleys in the lower course.  

Water Cycle

The water cycle includes all the process that promotes changes in water and its transformations from one type of water to another.

• Evaporation: the Sun warms water, transforming it into water vapour. This process occurs, above all, in seawater, because it is extremely abundant and covers 75% of Earth's surface. But it also takes place in freshwater.
• Evapotranspiration: the water from living beings also evaporates. It is a low amount of water.
• Condensation: the atmospheric water vapour is condensed, transforming the vapour into tiny drops of water that made the clouds.
• Transport: the wind moves the clouds. This is specially relevant in the sea, so clouds formed above the sea due to the evaporation and condensation can move from the sea to the land.
• Precipitation: when the tiny drops of water of the clouds cool and condense, forming bigger drops that fall. This process can occur in many different places, but is very usual in higher zones of the atmosphere and when the clouds shock with the mountains. The water falls forming rain, snow or hail.
• Surface Runoff: this is the movement of the surface water from the land to the sea. Surface runoff forms rivers, streams and torrents.
• Infiltration: the infiltration is the movement of water from the Earth's surface to deeper parts of the ground, forming groundwater.
• Deep Runoff: this is the movement of groundwater to the sea. Due to this, groundwater returns from the land to the oceans.

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.