Posted by: lcowie | November 16, 2010

Moisture and Relative Humidity

When studying for the BPI Building Analyst exam, you should spend some time looking over relative humidity and moisture. Moisture is easy to understand because you can picture examples of moisture movement in your head – rain is bulk moisture, windows have condensation, showers produce water vapor. Relative humidity was a bit more difficult for me to wrap my head around because it is more of a conceptual theory; it’s tough to picture.

Relative humidity is a measure of the amount of water in the air. Air temperature affects relative humidity because warmer air can hold more water. When air cools, relative humidity rises if the amount of water in the air stays constant. That last sentence was always hard for me to picture. Fortunately, my BPI training instructor included a graphic in his presentation, which showed various stages of temperature and how that temperature influences the amount of relative humidity. I encourage you to look over this graphic because I remember a few questions on the exam concerning this topic.

Ultimately, just remember that cool air holds less water. You will reach a dew point (saturation) as the temperature cools, and this is when you will see moisture or condensation. You can try to think about the weather outside. On those days when it is really hot and humid, the temperature will cool later in the day and THIS is when it will start raining (think Florida!). The temperature cools to a point where the air is saturated with water vapor and must release it in the form of rain.

You’ll also want to take into account sensible heat and latent heat. I think of sensible heat as the regular heat from the temperature and latent heat as wet and sweaty humidity. Arizona is more comfortable at 100 degrees than Louisiana at 90 degrees because of the differences in sensible heat and latent heat. The air in Louisiana has a lot more moisture in it because humidity is much higher. That moisture holds a lot of energy. So, the total energy in the air in Louisiana is higher than the energy in the air in Arizona.

Pictures help! During my written exam, I used one sheet of scrap paper. On half the sheet, I wrote the numbers of the questions that gave me trouble, as well as the topic of the question. On the other half of the sheet, I drew pictures. I’m one of those visual-learning types, and I need to draw pictures to understand what the question is asking of me. You can draw pictures for most concepts, just as I said about the graphic earlier.

For this concept, I drew two bars (like a bar graph). One bar is Louisiana and the other is Arizona. I divided each bar somewhere inside. The bottom part of the divided bar is sensible heat, and the top part is latent heat. I look primarily at the latent heat to get an understanding of how it will be influenced by the temperature. If there is a larger portion for latent heat, then it will be more humid and uncomfortable. More sensible heat, coupled with less latent heat, is an ok environment. Afterall, a latent heat of 30-50% isn’t that bad. Anything over that is a little excessive.

Leave a comment if you have any other explanations or picture examples referring to relative humidity!



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