The simulations use data from the Oak Ridge National Laboratory (which is sponsored by the US Department of Energy) that shows how many people are likely to inhabit an area within a 24-hour period.
Wellerstein previously told Business Insider that his tool is meant to give people "a realistic understanding of what nuclear weapons can and can't do." But it comes with some limitations. The models, for instance, don't take into account weather conditions, which can affect the altitude of a mushroom cloud.
Here's a key to help you understand his tool:
Yellow: Fireball (590-foot radius) — Less less than one millionth of one second after a bomb explodes, it would emit a giant orange fireball filled with hot air and weapon debris. Any buildings, objects, and people caught within this radius would likely burst into flames.
Green: Radiation (0.74-mile radius) — Within at least 15 minutes of a blast, clouds of dust and sand-like radioactive particles — what's referred to as nuclear fallout — would reach the ground. Nuclear fallout can expose people to radiation poisoning, which can damage the body's cells and prove fatal. Wellerstein estimates that between 50% and 90% of people within this radius could die from the acute effects of radiation.
Blue-gray: Air blast (1.04-mile radius) — Air blasts are powerful enough to topple residential buildings. There would likely also be widespread injuries and fatalities within this radius.
Orange: Thermal radiation (1.18-mile radius) — People caught within this radius could experience third-degree burns, severe scarring, or disablement.