Leeds Centre for Crystallization

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Crystallization in the Atmosphere

Atmospheric ice nucleation and crystallisation
The nucleation and crystallisation of ice in the Earth’s atmosphere strongly affects cloud properties, but remains very poorly understood.

Many clouds in the Earth's atmosphere are below 0°C, but the particles within these clouds are often made of liquid water. It is well known that ice melts at 0°C, but water supercools well below 0°C. Water droplets in clouds can exist in a metastable or supercoole d state right down to -37°C. However, if the right type of solid particle (ice nucleus) is present ice formation can be catalysed at much higher temperatures.

The properties of clouds are therefore strongly influenced by the presence of ice nuclei. However, the ice nucleating ability of the types of particles which serve as nuclei, such as mineral dust, soot or even bacteria are in general poorly quantified. A recent review article summarises our knowledge of ice nucleation in mixed phase clouds (Murray, Chem. Soc. Rev., 2012).

Cirrus clouds are thin ice clouds which occur in the upper troposphere at roughly the cruising altitude of jet aircraft. Contrails – the trail of ice crystal left behind a jet aeroplane – are man-made cirrus clouds. The temperature in the upper troposphere can fall to -90°C in the tropics. In fact, the tropical tropopause is the coldest environment in the Earth's lower atmosphere and the clouds that form under these extreme conditions have unique, but poorly understood properties. The temperature is so cold that a metastable form (non-hexagonal) of ice can form (Malkin et al., PNAS, 2012).

There are several reasons why we should care about cirrus clouds. Firstly, they affect climate by altering the flow of energy into and out of the Earth's atmosphere. Many clouds in the Earth's atmosphere cool the planet in the day and insulate at night; some types of cirrus can have a warming effect even in the day. The impact of human activities on these clouds such as air travel are at present unknown. A second important reason for studying these ice clouds is that they play a role in controlling water entering the stratosphere. The stratosphere contains the protective ozone layer and water vapour affects the chemistry of this important gas. Cirrus form at the top of the troposphere and when the ice crystals fall (this can be seen happening in many cirrus with the naked eye) they take the water with them while the dehydrated air can enter the stratosphere.

During summer at high latitudes the temperature of the upper mesosphere can fall to below -150°C and is the coldest natural place on Earth. The mesopause is a region around 80-90 km with pressures less than one millionth of an atmosphere. These clouds can be seen with the naked eye only after the sun has set and the lower atmosphere is in shadow. As can be seen in the photo on this page (taken with a simple point and shoot digital camera), they are a very beautiful phenomenon.

Noctilucent clouds were first recorded in 1885, several years after the enormous Krakatau volcanic eruption. There has been a lot of speculation over the years about these clouds being an indicator of climate change because parts of the mesosphere are extremely sensitive to anthropogenic carbon dioxide and methane.

An outstanding questions in mesospheric cloud research is the ice particles nucleate. There are a number of candidate mechanisms including nucleation on meteoric smoke, nucleation induced by ions, clustering on other very polar molecules or even nucleation directly from the vapour phase.