 We're going to look at a graph of temperature, dew point, and relative humidity in State College, Pennsylvania from 0z on October 6, 2016, which is the time along the left side of the graph through 0z on October 7th, which is the time along the right side of the graph. The top portion here plots temperature, dew point, and relative humidity, and the scale for temperature and dew point is along the left side while the scale for relative humidity is along the right. So what relationships can we see from this graph? For starters, note that fog was reported in State College throughout much of the night, which means that net condensation was occurring. We would expect our relative humidity observation to be 100%, and indeed that was the case. You actually can't see the relative humidity trace through much of the night because it's right along the top of the graph. If relative humidity is 100%, that must mean that the temperature and dew point were equal, and we can see that's the case too. From about 4z through about 16z, the temperature and dew point traces were indistinguishable because they were equal. So through much of the night, the temperature and dew point were equal, relative humidity was 100%, net condensation occurred, and fog was reported. Now look at what happens after 16z. The dew point trace remains relatively flat, which means dew points remain roughly constant because the amount of water vapor in the air didn't change. But while that was happening, relative humidity plunged from near 100% down to less than 60% by 21z. Why did that happen? The air warmed up. Temperature kept climbing through the morning and the afternoon from the low 50s to near 70 degrees. With dew point remaining constant, the warming of the air caused the difference between temperature and dew point to grow, and relative humidity decreased. Note also that after 16z, fog was no longer reported because net evaporation began occurring as the air warmed up and relative humidity declined. After 21z, temperatures started to decline as evenings set in, and note that there was a corresponding increase in relative humidity even though dew points changed very little. The cooling of the air increased the relative humidity as the difference between temperature and dew point became smaller. The bottom line is that the amount of water vapor present changed very little throughout the daytime and yet relative humidity changed quite a bit from 100% to less than 60% because of changes in temperature. So relative humidity does not tell us how much water vapor is in the air, but it does tell us how close the temperature and dew point are to each other, which tells us how close we are to net condensation potentially occurring.