Motivation

Many people think of dangerous heat only in terms of temperature. Actually, it is the relationship of temperature and humidity (and other factors that influence them) that is important to understand the impact of heat on the body.

The two main measures that capture this relationship are Heat Index and Wet Bulb Globe Temperature (WBGT). While WBGT is more accurate, Heat Index is more common — you might know it as “feels like“ temperature.

My (intuitive) hypothesis is that reducing heat stress to a single number is misleading: It prevents us from building intuition for the heat index on a given day. I wanted to design a heat index chart that showed:

1. the relationship between temperature and humidity rather than just a single “feels like" number.
2. the forecasted temperate, humidity, and heat index over the course of a day.

I prototyped the chart using Observable Plot (D3) and the AccuWeather API.

Outcome

A two-dimensional data table with the axes temperature and humidity lists the heat index in each cell. Cells are color-coded based on a heat risk scale.
On top of the map, a line traces the heat index throughout the day. Every third hour is marked with a label "9AM", "12PM", and so on.

While it's definitely an unfamiliar chart, the visualization allows to see at a glance at what points during the day temperature and humidity create dangerous conditions for outdoor activity.

Next steps

The layout capabilities of Observable are limited. A custom implementation would allow to improve the user experience considerably, for example by adding visual guides that clarify the direction of the line (ie. the progression of the day).

More details and code:
Go to websiteobservablehq.com/@philippschmitt/visualizing-heat-risk

This is an experiment only. I know very little about meteorology or public health. I also haven't spoken to any experts.