We often hear about the temperature increases caused by climate change. However, what we perceive as heat or cold is also affected by the amount of humidity in the air because evaporation through skin plays a large part of how we regulate our internal thermal equilibrium.
When we look at how this has changed in the past 70 years, we can see how drastic the changes have been. The National Weather Service (NWS) defines heat index with four levels of severity, from caution to extreme danger, as shown in the chart below.
For many cities, the number of hours of heat index in the 'Danger' or 'Extreme Danger' has doubled or tripled since 1950.