Where has summer gone?
In the space of only a few short days, the sun has decided to call curtains on 2013. The crystal clear skies that characterised June, July and August have now disappeared, to be replaced by dense, grey and ominous cloud cover. With them they bring a British favourite: Rain
But what exactly is rain and how does it form?
Rain is liquid water that falls to Earth from clouds. Water droplets inside a cloud condense, getting bigger and bigger until they are heavy enough to fall under gravity. We witness this phenomenon directly, as we observe the clouds darkening over time.
Changing from water vapour into liquid water requires a surface to induce the condensation process. This is similar to a trick used by Chemists when making crystals. They make a tiny scratch at the bottom of a beaker, which initiates crystallisation. As a result, the crystal will “crash out” of the liquid.
However, it is not possible to scratch air. Instead, tiny particles provide the platform for water to condense. These particles are called Cloud Condensation Nuclei (CNN), but are also known as Cloud Seeds. They are microscopic particles of clay, salt or pollutant, around 0.2 µm in size – about the same size as the smallest known bacterium.
Amazingly, if the water droplet forms around a coloured CNN (e.g: red clay) the rain will have a colour. Due to the incredibly small size of the cloud seeds, we do not see the colour; it is invisible.
To our eyes, the rain appears just like normal water. When warm moist air in the atmosphere cools, the water droplets condense in the exact same process that occurs on a window in a steamed up room.
But the water that is unleashed as rain is not pure water.
Up high in the atmosphere, the rainwater comes into contact with many molecules. These include ions such as chloride (Cl–), sodium (Na+), Calcium (Ca2+) and Magnesium (Mg2+). These ions get tangled up and dissolved into the water.
And it’s not just ions that get stuck inside the water. Nitrogen based molecules like ammonia (NH3) and its positive cation ammonium (NH4+), are also found dissolved in rain.
What’s more, chemical reactions can occur deep inside the cloud between water and contaminants. Well known examples of this are the pollutants Sulphur Dioxide (SO2) and Nitrogen oxide (NO and NO2). When these molecules react with water, acid is formed, sulphuric and nitric respectively.
Once made, the acid falls to earth as acid rain, a substance which has dire consequences for ecosystems.
The chemical make-up of rain varies based on location and year on year. A multitude of environmental factors are responsible for dissolved ion concentration and the pH of rain. Monitoring this changing constitution is of particular importance to government organisations.
In the UK, the group responsible for measuring our rain is the United Kingdom Eutrophying and Acidifying Pollutants (UKEAP) a network project run by Defra, the Department for Environment and Rural Affairs.
While UKEAP keep an eye on what is inside the rain, the Met Office maintains records about the amount of rain that falls each year.
This is something that is vastly different depending on where you are in the world. From the Atacama desert, to the Indian town of Mawsynram, reportedly the wettest place on Earth, the range of rainfall around the globe is dramatic.
At least with our temperamental weather in the UK, we can count on one thing: the rain will always come to herald an end to the Great British summer.