Artistic license. It’s the justification that allows filmmakers to break the rules of logic and science. It’s the "thing" that, when coupled with the audience’s required suspension of disbelief, makes the story or plot point work when it technically shouldn’t. Narrative filmmaking isn’t documentary - stories don’t have to be founded on exact science. And with that, we get the nuclear blast scene in Kevin Macdonald’s How I Live Now (2013).
In the film, the kids are said to live about a six-hour drive from London. As they’re playing and cajoling one afternoon in beautiful English countryside, the wind picks up. The sky darkens. They hear a rumble, and white stuff immediately starts to fall (it’s nuclear fallout ash). The youngest of the family, Piper (Harley Bird), makes a juvenile comment about the falling "snow," as Eddie (George MacKay), the eldest of the family, rushes everyone into the house.
In reality, a nuclear explosion first fills the sky with light - something completely nonexistent in the film. After that, the sound is heard, since sound travels slower than light. The shockwave would be felt, and after a while - possibly hours later - fallout ash would arrive. That delay is particularly true if one was as far away from the blast as the kids in How I Live Now are said to be. Being covered in fallout ash is also likely to cause illness, which the characters in the film do not experience.
But does How I Live Now’s inverted sense of science really matter? No, not especially.
How I Live Now isn’t about the nuclear bomb, or even about the subsequent war. The specifics about the conflict, its combatants, its enemies, and its purpose are left ambiguous for a reason - the narrative focuses on the personal, human effects of conflict, not the thing itself. The way the nuclear explosion played out in the film is poetic to the purpose of the YA love story being told. The blast’s style is congruent with the perspective of the film’s characters. They live the scene, and every scene that follows, in a state of panic, confusion, and fear. They fail to understand the larger picture of what’s going on, and the nuke’s confused presentation is representative of that.
As Justin Chang of Variety notes, “Haunting and grimly poetic, the scene works because Macdonald so scrupulously adheres to his characters’ restricted vantage, allowing the audience to share in the confusion and terror of suddenly being caught up in events beyond their understanding.”
That’s the perspective of the rest of the film, all the way until its final “post-war” moments. This is a story of children living through a horrendous event, and that’s the angle everything in the film takes. Details are fuzzy, motivations are unclear, and science is misunderstood - just as all would be to a child surviving such atrocities.