ActionAid
The advert - created by international development charity ActionAid in conjunction with communications agency Weber Shandwick - is displayed on the iconic Piccadilly Circus screens next to brands like Samsung and McDonald's when a nearby Met Office weather station tells the advert's computer that it's raining.
When the system registers rainfall in the local area, it triggers a "reaction" in the software, prompting the ActionAid advert to be broadcast on a screen belonging to the digital brand Storm.
Once rainfall is detected, the existing ad schedule is disrupted, to reveal a tailored message depending on the level of precipitation - drizzle, light rain or heavy rain.
The rotating adverts are designed to highlight the devastating impact climate change is having on the children of Bangladesh, where intensifying floods, cyclones and storms are killing hundreds of people every year. They depict young children that have been caught up in deadly floods and lost toys that have been washed away.
Research suggests that 50 children are already drowning every day in Bangladesh as the country becomes more waterlogged.
Other Piccadilly Circus advertisers may be disappointed to learn that their adverts will be interrupted the moment it starts raining.
ActionAid
The advertising campaign also aims to encourage the public to get involved with the London People's Climate march on Sunday.
The advert started running on Tuesday and will continue for five more days.