H_Formula

 Copilot:

Just curious; how does the US model get a humidity reading if it doesn't use the dew point??

Great question, Louise — this is where the meteorological math gets sneaky-smart!

The U.S. heat index formula uses relative humidity (RH) directly, while the Canadian humidex uses dew point, which is more stable throughout the day. But here’s the twist: dew point and relative humidity are mathematically linked, so you can get from one to the other if you know the temperature.

Here’s the short version:

The dew point is the temperature at which air becomes fully saturated.

The relative humidity is how close the current air temperature is to that dew point, expressed as a percentage.

If you’re handed air temperature (T) and dew point (Td), you can compute RH using this formula:

                                                                         

 Where e(T) is the vapor pressure at temperature T. So if an app only gets dew point and temperature from a weather station, it can derive RH — and then use it in the U.S. model.

In other words, even if the heat index doesn't explicitly use dew point, it can hitch a ride on it by first converting to RH.