Cloud physics has for a long time been an important segment of atmospheric science. It is common knowledge that clouds are crucial for our understanding of weather and climate. Clouds are also interesting by themselves (not to mention that they are beautiful). Complexity is hidden behind the common picture of these beautiful and interesting objects. The typical school textbook definition that a cloud is 'a set of droplets or particles suspended in the atmosphere' is not adequate. Clouds are complicated phenomena in which dynamics, turbulence, microphysics, thermodynamics and radiative transfer interact on a wide range of scales, from sub-micron to kilometres. Some of these interactions are subtle and others are more straightforward. Large and small-scale motions lead to activation of cloud condensation nuclei, condensational growth and collisions; small changes in composition and concentration of atmospheric aerosol lead to significant differences in radiative properties of the clouds and influence rainfall formation. It is justified to look at a cloud as a composite, nonlinear system which involves many interactions and feedback. This system is actively linked into a web of atmospheric, oceanic and even cosmic interactions.

Due to the complexity of the cloud system, present-day descriptions of clouds suffer from simplifications, inadequate parameterizations, and omissions. Sometimes the most fundamental physics hidden behind these simplifications and parameterizations is not known, and a wide scope of view can sometimes prevent a 'microscopic', deep insight into the detail. Only the expertise offered by scientists focused on particular elementary processes involved in this complicated pattern of interactions allows us to shape elements of the puzzle from which a general picture of clouds can be created. To be useful, every element of the puzzle must be shaped precisely. This often creates problems in communication between the sciences responsible for shaping elements of the puzzle, and those which combine them. Scales, assumptions and the conditions used in order to describe a particular single process of interest must be consistent with the conditions in clouds.

The papers in this focus issue of New Journal of Physics collectively demonstrate (i) the variation in scientific approaches towards investigating cloud processes, (ii) the various stages of shaping elements of the puzzle, and (iii) some attempts to put the pieces together. These papers present just a small subset of loosely arranged elements in an initial stage of puzzle creation.

Addressed by this issue is one of the important problems in our understanding of cloud processes—the interaction between cloud particles and turbulence. There is currently a gap between the cloud physics community and scientists working in wind tunnels, on turbulence theory and particle interactions. This collection is intended to narrow this gap by bringing together work by theoreticians, modelers, laboratory experimentalists and those who measure and observe actual processes in clouds. It forms a collage of contributions showing various approaches to cloud processes including:

• theoretical works with possible applications to clouds (Bistagnino and Boffetta, Gustavsson et al),
• an attempt to construct a phenomenological description of clouds and rain (Lovejoy and Schertzer),
• simplified models designed to parameterize turbulence micro- and macro-effects (Celani et al, Derevyanko et al),
• focused theoretical research aimed at particular cloud processes (Ayala et al, parts I and II, Wang et al),
• laboratory and modeling studies of complex cloud processes (Malinowski et al).

This collage is far from being complete but, hopefully, should give the reader a representative impression of the current state of knowledge in the field. We hope it will be useful to all scientists whose work is inspired by cloud processes.

Focus on Cloud Physics Contents

The development of ice in a cumulus cloud over southwest England
Yahui Huang, Alan M Blyth, Philip R A Brown, Tom W Choularton, Paul Connolly, Alan M Gadian, Hazel Jones, John Latham, Zhiqiang Cui and Ken Carslaw

The equivalent size of cloud condensation nuclei
Antonio Celani, Andrea Mazzino and Marco Tizzi

Laboratory and modeling studies of cloud–clear air interfacial mixing: anisotropy of small-scale turbulence due to evaporative cooling
Szymon P Malinowski, Miroslaw Andrejczuk, Wojciech W Grabowski, Piotr Korczyk, Tomasz A Kowalewski and Piotr K Smolarkiewicz

Evolution of non-uniformly seeded warm clouds in idealized turbulent conditions
Stanislav Derevyanko, Gregory Falkovich and Sergei Turitsyn

Lagrangian statistics in two-dimensional free turbulent convection
A Bistagnino and G Boffetta

Turbulence, raindrops and the l^1/2 number density law
S Lovejoy and D Schertzer

Effects of turbulence on the geometric collision rate of sedimenting droplets. Part 2. Theory and parameterization
Orlando Ayala, Bogdan Rosa and Lian-Ping Wang

Effects of turbulence on the geometric collision rate of sedimenting droplets. Part 1. Results from direct numerical simulation
Orlando Ayala, Bogdan Rosa, Lian-Ping Wang and Wojciech W Grabowski

Collisions of particles advected in random flows
K Gustavsson, B Mehlig and M Wilkinson

Turbulent collision efficiency of heavy particles relevant to cloud droplets
Lian-Ping Wang, Orlando Ayala, Bogdan Rosa and Wojciech W Grabowski