Heat Index
Heat Index
[A flowchart is shown beneath its title:] How to Calculate the "Heat Index":
[There are four boxes with arrows between. The first rectangular box is at the top, and an arrow points straight down:] Measure the Temperature
[From there an arrow goes straight down to a decision diamond:] Does it look hot enough?
[Two arrow goes out from the diamond. One goes straight down to another rectangular box. Both the arrow and the box has labels:] No Add a few degrees
[From the bottom box an arrow goes back to the diamond. The other arrow from the diamond goes right to a final box with no arrows.  Again both the arrow and the box has labels:] Yes Done

Heat index, like wind chill, is a way to combine multiple factors, in this case temperature and humidity, to get a single number indicating what the air "feels like." This page gives a table, a formula, and lots more explanation.
Human skin does not directly detect temperature - only the rate of heat gain or loss.  This is why a piece of metal feels cooler than a piece of plastic or wood at the same exact temperature (below body temperature, above it is the other way around) - the metal conducts heat away from the higher body temperature at a higher rate than a good insulator does.
So in warm weather, it's not just the temperature that matters for comfort.  The humidity and wind speed also factor into it.  When humidity is high, sweat evaporation is less effective at cooling us off than in a "dry heat" with low humidity.
Hence, meteorologists use a combination of temperature and humidity to come up with the "heat index" value...and a combination of temperature and wind speed to produce a "wind chill" number.
Neither scale is particularly scientific in terms of measuring how people feel - but both are a more accurate representation of comfort levels than temperature alone.
The joke here is that these numbers seem entirely subjectively chosen - and in a sense, they really are, although they are calculated from an actual formula (a multivariate fit to a mathematical model of the human body - with nine terms!) and not by guesswork as the flow chart implies.
The title text suggests another way it is calculated: In general the effective temperature is calculated based on the conductivity of heat based on humidity. This is a legitimate method of determining how hot something feels because the heat conductance of water is higher than dry air and humans perceive more heat when the humidity is higher. But humans also tend to exaggerate and so Randall implies to add still a bunch more to satisfy the subjective sentiment.
[A flowchart is shown beneath its title:] How to Calculate the "Heat Index":
[There are four boxes with arrows between. The first rectangular box is at the top, and an arrow points straight down:] Measure the Temperature
[From there an arrow goes straight down to a decision diamond:] Does it look hot enough?
[Two arrow goes out from the diamond. One goes straight down to another rectangular box. Both the arrow and the box has labels:] No Add a few degrees
[From the bottom box an arrow goes back to the diamond. The other arrow from the diamond goes right to a final box with no arrows.  Again both the arrow and the box has labels:] Yes Done
