The maximum amount of water vapor that can be held in air is affected by temperature the higher the temperature, the greater the amount of water vapor air can hold before reaching saturation. It is an indicator of the presence of dew, frost, fog, and precipitation. Humidity is defined as the amount of water vapor (gaseous phase of water) in the air. For references, see the documents in Appendix A or click on the links in Figure 2 ( brown color).Related Wind Chill Calculator | Heat Index Calculator Analysis Relative Humidity Given Dew Point and Temperatureįigure 2 shows how I computed the relative humidity given the air temperature and dew point. Specific humidity is approximately equal to the "mixing ratio", which is defined as the ratio of the mass of water vapor in an air parcel to the mass of dry air for the same parcel. Specific Humidity (symbol SH) Specific humidity is the ratio of water vapor mass ( m v) to the air parcel's total (i.e., including dry) mass ( m a) and is sometimes referred to as the humidity ratio. Absolute humidity in the atmosphere ranges from near zero to roughly 30 grams per cubic meter when the air is saturated at 30 ☌ ( Source). It does not take temperature into consideration. Absolute Humidity (symbol AH) Absolute humidity is the total mass of water vapor present in a given volume of air. Relative humidity depends on temperature and the pressure of the system of interest ( Source). Relative Humidity (symbol RH) Relative humidity is the ratio of the partial pressure of water vapor to the equilibrium vapor pressure of water at the same temperature. It is the temperature to which air must be cooled at constant pressure and water content to reach saturation ( Source). Dew Temperature (symbol tdew) The dew temperature is the temperature at which dew forms and is a measure of atmospheric moisture. Symbolically it is defined as, where m v is the mass of water vapor, and m d is the mass of dry air ( Source). It is often expressed in terms of gram of water vapor per kg of dry air. Definitions mixing fraction (symbol w) AKA Humidity Ratio (HR) Ratio of the mass of water vapor to the mass of dry air. Technically, this term is known as the mixing fraction, but it is very close in value to the specific humidity. Strictly speaking, the requirement is specified in terms of kg of water vapor versus kg of dry air. There is a small error in this statement. During the period of descending temperatures – i.e. Here is an excerpt from GR-487 that calls out the specific humidity requirement.Īt temperatures above 32° F (90☏), the relative humidity may be limited to that corresponding to a specific humidity of 0.024 kg of water per kg of dry air. Here is my Mathcad work if you are interested. I was a bit surprised that I could not find a table that contained this information – I pulled out Mathcad and solved the problem on my own. This was a quick Friday afternoon calculation. Since my test gear can control relative humidity, I need to derive a relationship between relative humidity and the specific humidity, which I show in Figure 1. For a given specific humidity, the relative humidity will vary with temperature. 24 grams of water vapor per per kilogram of air). In a test specification based on GR-487, a humidity test is called out where we need to have a fixed specific humidity (i.e. I often have to interpret odd test requirements. Figure 1: Relative Humidity Vs Temperature ForĪ Fix Specific Humidity (24 grams of Water
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