@Stevee1982 is correct. It's the Temperature and Dew Point added together. Sometimes people instead look at relative humidity and say it's going to be 95% or 100%. But saying the relative humidity without the temperature doesn't always tell the whole story on comparatively how one day feels versus another. A 80F temp with 95% humidity is going to feel a ton worse than 30F with 95% humidity. Similarly, an 80F day with 20% humidity and 80F day with 90% humidity are not going to feel the same even though the temp is the same. But what about a temp of 60 and dew point of 60 vs a temp of 80 and dew point of 40? How will those feel compared to each other? It's not a perfect system. One day's T+D of 130 and another's 130 won't necessarily feel the same, but it's a good system for pre-determining pace expectations. For most people, once the T+D goes above 100 is when someone's ideal pacing starts to fall.
I made this chart from my own personal data:
View attachment 448079
The x-axis is pace. The y-axis is T+D. The pink dots were theoretical pacing based on adjustments to a 3:00 hr marathon easy pace. The green dots were theoretical pacing based on adjustments to a 3:05 hr marathon easy pace. The blue dots were real actual data from easy days over a month's span. During that time period, the T+D varied quite a bit. My effort and HR remained relatively constant on all these runs, yet the pace was all over the place ranging from a 8:24-8:49 min/mile. The blue dots matched well with the shape of the theoretical pace adjustments. Which means the research lined up well with my own personal data.
So the long and short of it. According to these adjustments:
View attachment 448081
A T+D of 148 would have a starting point of a 4.5% adjustment from ideal. So let's say on a cool autumn morning where the T+D is less than 100, you could run a HM at 8:50 pace. According to this, on a T+D day of 148 a good place to start for pacing strategy would be a 9:14 min/mile. Essentially, what would feel like 8:50 pace on an ideal day would be more like 9:14 on this non-ideal day.
The adjustments are not perfect and not the same for everyone. But represent a good starting place to see where adjustments need to be made. You can use your own personal data over the last several weeks to see how the different T+D values effect you personally. Relative cloudiness, solar radiation, and heat acclimation are also factors to consider in how one should adjust pace expectations in non-ideal weather conditions.