Reading the Scene in Blood
When blood strikes a surface, it leaves a stain whose shape encodes the angle of impact. A drop falling straight down creates a circle. A drop arriving at an oblique angle creates an ellipse — the more elongated the stain, the shallower the angle. This simple trigonometric relationship, arcsin(width/length), is the foundation of bloodstain pattern analysis (BPA), a discipline used in crime scene reconstruction worldwide.
From Stain to Source
A single stain reveals the angle; multiple stains reveal the origin. By drawing lines through the long axis of several stains and extending them backward, analysts find the area of convergence — the 2D point on the surface directly below the blood source. Combined with the calculated impact angles, trigonometry then reconstructs the 3D point of origin: the exact location in space where the blood was shed.
Velocity and Mechanism
The size of blood spatters correlates with the force that created them. Gravitational dripping produces large drops (>3 mm). Blunt force trauma generates medium-sized spatters (1–3 mm). Gunshot wounds create a fine mist of tiny droplets (<1 mm) along with larger satellite spatters. Analysts use these size distributions to infer the type of weapon or mechanism involved.
The Physics of Blood Drops
Blood behaves as a non-Newtonian fluid with a surface tension of approximately 0.058 N/m. When a drop impacts a surface, it deforms according to the Weber number — the ratio of inertial forces to surface tension. Low Weber numbers produce clean ellipses; high Weber numbers cause spines, satellite droplets, and complex splash patterns that complicate angle calculation.