This article originally appeared in IF Magazine #139 (Feb-March 2011).

As a practical effect, rain might seem pretty straightforward, but if it’s not 100 per cent, it could detract from an otherwise convincing production.

Used in everything from feature films to TVCs, “movie rain” has been around since the beginning of filmmaking. Traditionally, SFX companies have used rain systems developed out of firefighting systems – which, while they work, have a number of drawbacks.

“These systems came out of America; they’re great for fires, but have significant problems for film work,” says special effects veteran of more than 25 years, David Trethewey. “Firefighting systems are designed to put out fires. They throw an enormous amount of water into a space to cool it and quench the fire, and they’re not particularly concerned with how that water gets there – with the pattern that comes out of the nozzle – they just want to get water into the space.”

Trethewey says the commonly-used “spinning heads” also have a major drawback because they create a cone-shaped pattern over the set – they have a hole in the middle of shot, right where the actors usually are.

“You can counter that with an overlapping system of two heads, but that just doubles the problems – twice the water, twice the pumps, twice the drainage problems. While they’ve been used for years, I was never happy with spinning heads. The pattern is not perfect, they use heaps of water, you need good drainage which a location might not have and you need big pumping power to run them. All that means is higher cost to the production”

Sydney-based Trethewey decided to create his own version of a dust suppression system that is used in mining and construction, where full pattern coverage and water consumption are important issues.

“What dust suppression systems are trying to achieve is maximum coverage at minimum cost in water and pumping power, and with fewer drainage problems,” he says. “So the system I have developed out of that sector gives a beautiful, even coverage at lower cost – it’s far superior to what is traditionally used. I've spent a long time in R&D [research and development] tweaking things so that you get a good size droplet along with the full coverage.”

Called the Oscillating Rain Head System, each head uses about one-seventh the water of the commonly used “spinning heads” and gives wider coverage per head.

First used on Nash Edgerton’s The Square and regularly on Seven’s Home and Away, the standard set-up gives a 20×15 metre storm density, full-pattern coverage with three heads using 150 litres per minute. The spinning heads give 10×10 metres and have a hole in the middle.

To match Trethewey’s basic system coverage you would need four spinners using about 1400 litres per minute. “The maths speaks for itself,” he says.

Getting the best out of any rain system relies on good preparation. “It’s absolutely vital to do a location recce,” says Trethewey.

The recce determines the best spot for the cherry picker, how drainage is going to work, issues like power lines and hydrant access and where backlighting will go.

“The secret to really great rain on screen, apart from a good rain rig, is lighting and framing,” he says. “Get that right and you’ll get great looking rain.”

When asked how CGI has affected the physical effects sector, Trethewey is sanguine.

“No doubt it has taken away some of our work… but it has created as much as it has taken,” he says. “You can’t make big blockbusters these days without CGI, but CGI mostly looks after the wide shots. And for every wide shot of a snow storm or rain storm, you’ll likely need close-ups with actors, and you can’t do close-ups realistically in CGI. It’s much cheaper to have real rain or snow falling on actors in close-up.”

Productions are starting to realise that actors need a real environment to get the best performance possible, so there’s a swing back to real SFX to give actors that environment.

“People fell in love with CGI when it first came out, but now they’re starting to get the mix right between that and in-camera SFX. Physical SFX will always have a place – it’s a matter of choosing which is best for each shot.”