The Atmosphere: Weather

From the sun shining to the rain pouring, the weather affects our day to day lives. In some cases, even our mood depends on the weather! An overcast day is associated with feeling miserable whilst blue skies and sunshine can leave us feeling positive. This blog post will focus on the science that causes our weather.

Clouds are formed when liquid water from the surface of the Earth is evaporated into the atmosphere. The water then condenses as water droplets onto particles of dust in the troposphere known as cloud condensation nuclei. If temperatures drop to -20°C or less, some of the water droplets freeze into ice crystals. The water droplets are about the size of the droplets found in fog. There are 3 categories for classifying clouds;

High clouds
These clouds form at the top of the troposphere between 6km and 10km, where the air temperature is very cold (see previous blog post). Because of the low temperatures, the clouds are composed of ice crystals. They usually have a wispy appearance as a result of strong winds. 

Medium clouds
Medium clouds form between altitudes of 2km and 6km. They are made of a mixture of ice crystals and water droplets.

Low clouds
Low clouds form below 6km and are formed of water droplets due to higher temperatures near the surface of the Earth. The rising of moist air create towering cumulonimbus clouds which bring heavy rain, thunder and lightning.

Click here for a cloud fact sheet


Precipitation is the name given to any type of water that falls from clouds; it usually comes from nimbostratus and cumulonimbus. The water droplet or ice crystals fall when they grow large enough for their fall velocity (the speed at which they fall) to exceed the opposing updraft caused by the evaporation of water. Rain is precipitation that falls in the form of water droplets. If temperatures are low enough for the water droplets to freeze, the precipitation falls in the form of sleet. Snow is made of ice crystals that form a complex structure within the cloud. Many of these complex structures are what forms a snowflake; its pattern depends on the temperature and humidity of the air.

Wind is the movement of air from an area of high pressure to an area of low pressure; this drives all weather. The pressure change is caused by convection currents; air heated by the land expands, becomes less dense and rises, leaving behind an area of low pressure. When the air eventually cools, it becomes dense and falls (this is known as subsidence), creating an area of high pressure. Areas of high pressure are associated with calm weather as well as clear skies. Areas of lower pressure usually have high winds, warm air as well as precipitation. Still don't understand convection?

Equatorial Low Pressure Trough:
At regions near the equator at 0°-10°North and South, the pressure is low due to the heat energy from the sun causing the air to expand and rise. The ascending air produces clouds which are responsible for heavy rainfall. It is for this reason rain forests such as the Amazon experience 23cm of rainfall annually.

Subtropical High Pressure Cells:
The subtropical region is located between 20°N/S and 35°N/S contains hot, dry air which moves from the tropical region. The heavy rain at the equator removes the moisture from this region, therefore the air is dry. 

Subpolar Low Pressure Cells:
At latitudes of 60° N/S the weather is cool and wet due to the meeting of cool air masses from high latitudes and warm air masses from lower latitudes. The low pressure causes cyclonic storms.

Polar High Pressure Cells:
At polar regions, 90° N/S, the air is extremely cold and dry. 

We are able to forecast the weather by studying these pressure cells. Below is a video explaining how the climate system works.

The Atmosphere: The Layers

The atmosphere is divided into layers with varying air pressure and temperature. There are 5 distinct layers recognised today. As altitude increases, air pressure and density decreases. This is due to the force of gravity becoming weaker on the atmospheric particles as the distance from the centre of the Earth increases. In fact, humans living at high altitudes have experienced consequences due to the low air density. Pregnant women living at high altitudes in Spain were miscarrying due to the low oxygen levels; there was insufficient oxygen available for both the mother and the fetus. However, some humans have evolved for survival through generations. For example, tea pickers in Darjiling, India have a greater lung capacity and their blood has a higher affinity for oxygen which enables them to pick tea and carry heavy loads at high altitudes. Despite this, humans are incapable for exceeding 5 kilometres without pressure suits and oxygen masks. 

Air pressure calculator

Troposphere

This is the lowest layer of the atmosphere and is also the most shallow with a depth of 10 kilometres above sea level. It is the most dense layer of the atmosphere and contains 75% of atmospheric mass, including 99% of the atmospheric water vapour. As the diagram above shows, the temperature decreases significantly from the average surface temperature of 14°C to as low as -60°C. As it is below freezing at the top of the layer, water is converted from its gaseous form and freezes into solid ice. The solid ice cannot remain in the atmosphere and drops down back into the troposphere in the form of vapour. This is known as the cold trap and it is what prevents the Earth from losing its water. The troposphere is where all weather takes place.

Stratosphere

This layer is 20 kilometres deep, twice that of the troposphere, and contains only 24% of atmospheric mass. Unlike the in the troposphere, the temperature of the stratosphere increases from -60°C to -3°C. This is due to the ozone layer which absorbs UV rays and releases energy in the form of heat, it is an exothermic reaction. Ozone (O₃) is a trioxygen gas which was formed from the photodissociation of oxygen gas (O₂) into two free oxygen atoms. These atoms bond to oxygen gas to form ozone.  Above the stratosphere, air pressure is negligible.

Mesosphere

The depth of the mesosphere is 30 kilometres above sea level. Temperature decreases with altitude in this layer; the temperature in the upper mesosphere is -100°C. This is the layer in which meteors burn. The traces of water present in the mesosphere is able to form ice crystals on the meteoric dust and debris, creating noctilucent clouds. These clouds appear illuminated against the dark sky once the sun is below the horizon; because of their high altitudes, they are able to reflect the sunlight after the sun has set. The mesosphere also contains the aurora which is formed from the solar wind being deflected by the Earth's magnetic field. The charged particles from the solar wind cause atmospheric molecules to become ionised and photons (packets of light energy) are emitted. This creates a spectacular light display in the night sky and only occurs at high latitudes due to the Earth's magnetic field lines.

Diagram showing geometry for noctilucent clouds
Credits: NASA Science

Thermosphere
80 kilometres deep, the thermosphere is named after its high temperature of around 1000°C. The high temperature is caused by UV rays from the sun ionising particles to release thermal energy; this region is known as the ionosphere. As the name suggests, the ionosphere contains ionised (electrically charged) particles. These particles can reflect radio waves that are sent from one point on Earth to be received at another point beyond the horizon. This is very useful for communication; the radio waves are used to transmit television signal and of course radio signal.

Exosphere

Although not always categorised as a layer of the atmosphere, the exosphere is a region between the thermosphere and outer space which extends up to 700 kilometres above sea level. It consists of atmospheric molecules which are gravitationally bound to the Earth but have a very low density; the density is too low for them to behave as even a gas! There is no definite temperature in the exosphere as the molecules do not collide and each have different energies. The International Space Station orbits the Earth in the exosphere.

Aurora Australis (the aurora occurring in the Southern Hemisphere) as viewed from the International Space Station.
Credits: NASA

The Atmosphere: Evolution of the Atmosphere

A digital image showing a cross section of the Earth.
Credits: Discovery Channel

You may wonder why we have an atmosphere and where the gases came from. The Earth was formed 4.6 billion years ago through particles of dust being bound together by gravity. The heavy elements sank to the core as they experienced the greatest gravitational pull whilst lighter ones surrounded the core forming the mantle. The lightest elements of them all, such as hydrogen and helium, formed the primitive atmosphere. This was a very thin layer of gas which disappeared with time. One hypothesis states that because the early Earth was very hot, the atmospheric particles had a lot of kinetic energy. This kinetic energy may have been enough for the particles to overcome the gravitational forces bounding them to the Earth. The velocity required for the atmospheric particles to overcome the Earth's gravitational field is known as the escape velocity. Another hypothesis mentions that this primitive atmosphere was stripped by the solar wind which is a stream of charged particles released from the sun with high velocities. The solar wind may have transferred enough energy to the atmospheric particles for them to achieve escape velocity.Today, we are not affected by the solar wind because of the magnetic field generated by the churning of the molten iron core inside the Earth. The magnetic field deflects any charged particles away from Earth. 
Want to know how the Earth was formed?

Illustration of the magnetic field lines surrounding Earth.
Credits: National Geographic

The primitive atmosphere was soon replaced by the secondary atmosphere. This was formed from frequent volcanic activity which was the Earth's mechanism for removal of volatile gases dissolved in the magma (liquid rock beneath the Earth's crust). The gases released by the eruptions consisted mainly of water vapour as well as nitrogen, carbon dioxide, hydrogen, sulphur dioxide, chlorine, hydrogen sulphide, methane, ammonia and carbon monoxide. Although no oxygen was released from the volcanoes, it is thought that there were traces of oxygen in the early secondary atmosphere. The oxygen was a product of the photodissociation (also known as photolysis) of water molecules and carbon dioxide molecules. Photodissociation is a process by which UV rays from the sun break down a molecule into its components:

The OH (hydroxide) and O (oxygen) chemically bond to form H (hydrogen) and O₂ (dioxygen/oxygen gas). However, it was not until 3.5 billion years ago (3.5 x 10⁹ years ago) that this oxygen started to become increasingly abundant. This was when primitive micro-organisms that had adaptations to withstand the extreme environments on early Earth began to photosynthesise. Photosynthesis is the process by which autotrophic organisms produce their own food in order to survive. These early life forms hadn't yet evolved to survive the oxic (oxygen-containing) environment they had started to create. For this reason, they remained below the ground and retreated within sediments which are now known as stromatolites.

Basic photosynthetic reaction

Peter Sawyer's painting illustrates the hostile environments on the primitive Earth. In the foreground lay stromatolites containing the earliest life forms which were able to withstand the environment caused by volcanic eruptions and an anoxic (low oxygen) atmosphere.

The graph above shows how the amount of atmospheric oxygen changed with time. As we already know, the first photosynthesis release oxygen into the atmosphere 3.5 billion years ago, you may wonder why the graph does not show an increase in atmospheric oxygen. The reason there is no initial increase is because this oxygen was absorbed in the ocean and by seabed rock which consisted mainly of iron. This caused the iron to become oxidised to form an iron oxide known as magnetide.

Banded iron formation in seabed rock. The variation between each layer of magnetide sediment is caused by the fluctuations in the oxygen and iron levels in the ocean.
Credits: Stefan Lalonde

It was not until 1 billion years ago that atmospheric oxygen began to increase until it reached the current amount of 20.9%. As the atmospheric oxygen levels increased during this time, organisms began to evolve and begin to respire aerobically (using oxygen). Today, the Earth is dominated by oxygen-tolerant photosynthesisers on which the entire biosphere depends.

The Atmosphere: An Introduction

The atmosphere is the gaseous blanket surrounding the entire planet. The word atmosphere is derived from the Greek word Atmo meaning air. 

Pie chart showing the chemical composition of the atmosphere.
Credits: Patti Isaacs Management and Production

As the pie chart above illustrates, oxygen is the second most abundant gas in our atmosphere. Oxygen is vital for life on planet Earth; all living organisms require this gas for aerobic respiration in order produce the energy required for metabolic reactions. 0.17% of the atmosphere consists of other gases including neon, helium, methane, krypton, hydrogen, nitrous oxide, xenon, ozone, iodine, carbon monoxide and ammonia. Water vapour is also present at low altitudes in the troposphere. The stratosphere, which is the layer above the troposphere, contains a layer of ozone gas. The ozone layer is responsible for filtering high energy ultraviolet (UV) rays which have enough energy to damage the DNA inside cells. This is why exposure to high levels of UV radiation can cause skin cancer. 

Aerobic respiration

The atmosphere is also a shield against meteoroids which are the solid debris any comets or asteroids leave in their path whilst travelling through space. Meteoroids travel at high speeds of up to 70 kilometres per second. Upon entering the Earth's upper atmosphere, the mesosphere, a meteoroid is forced to slow down as the molecules of atmospheric gases provide the drag force friction. The friction has a heating effect and causes the meteoroid to ignite; it is now formally known as a meteor.

This time-lapse photograph shows a meteor shower. A meteor shower occurs when the Earth passes through a belt of debris from a passing comet or asteroid. As a result, many meteors (also known as shooting stars) can be seen in the sky. 
Credits: Kenneth Brandon

Our atmosphere is responsible for the weather we experience on Earth. Different areas of the globe experience different weather patterns which are essentially driven by energy from the sun. Weather is caused by movement of air masses; warm air is carried from the equator to the poles. Areas of mid-high latitudes such as the United Kingdom can experience both cool air from the Arctic and warm air from the equator. Also, it is due to the wind plants were able to colonise land and evolve; wind is responsible for the transportation of seeds and minerals across great distances. Today, wind carries sand and minerals for fertilisation from the Sahara to the Amazon rainforest across the Pacific Ocean.

Air masses moving towards the British Isles. 
Credits: Met Office

What is Earth Science?

Image via NASA

Earth Science is a multidisciplinary field of study; it applies Chemistry, Physics, Biology and even Maths to planet Earth. It involves looking at rocks (as boring as that may sound to some, they're a lot more interesting than they look!), volcanoes, earthquakes, oceans, ecosystems, atmospheric processes, glaciers, soils.. and the list goes on. It is quite fascinating to see how our planets systems work. 
Due to the currently changing climate, there has never been a more important time to learn more about planet Earth. The more we know and learn about our own planet, the easier it will be for us to understand other planets (so yes planetary astronomers, this blog may be useful for you too!). The blog posts will focus on a certain topic, with several posts for each specific topic. For example, if looking at rocks, one post may focus on dating rocks whilst another will be based on plate tectonics.
I hope you find this useful whether you require the information for your homework project or simply have an interest in the world around you.
Enjoy!