water) starts to form on the cool window surface. As the temperature of the air adjacent to this surface is pulled beyond the saturation line, its limit for holding moisture is reached and "dew" (i.e. (See Defining Humidity Ratio and Relative Humidity for a more in-depth explanation.)Īn example of this condensation occurrence would be when hot-humid indoor air comes into contact with a cool window surface during the winter. This concept of the relative humidity (see the red lines on Figure 1) of an air-vapor mix at any given dry-bulb temperature helps one determine the saturation level of the air-vapor mix relative to its current capacity. As the air temperature drops, it has less capacity to hold moisture. As air gets warmer and expands, it has the ability to hold more moisture. Also, air can only hold so much moisture. Proportionately, it is a very small amount of moisture relative to the overall air mass, but it is in there. This is called the humidity ratio.Īir always has some amount of moisture in it. Latent heat is expressed on the psych chart in terms of pounds of moisture per pound of dry air. (See Thermal Dynamics: Visualizing Sensible Versus Latent Heat for a more in-depth explanation.) On the other hand, latent heat is the “wet” heat captured in the air as water undergoes phase change from liquid to vapor via evaporation or boiling. This is the heat that one can "sense" with a conventional thermometer, as the term suggests. Sensible heat is the “dry” heat in the air and relates directly to the dry-bulb temperature.
How much latent (moisture-related) heat is in the air? (See the cyan lines on Figure 1.).How much sensible (dry-bulb) heat is in the air? (See the green lines on Figure 1.).