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