There are several methods of examining tropospheric moisture. They are all important to determining the quantity of moisture in the troposphere and/or determining how close the troposphere is to saturation. Three indicators of tropospheric moisture are the relative humidity, dewpoint and precipitable water.
Relative humidity tells the forecaster how close the troposphere is to saturation. On the model progs, RH is given as the average 850 to 500 mb RH. If this value is high, it indicates the troposphere is near saturation through a deep layer of the troposphere. Surface RH (commonly reported with the local weather) tells how close the troposphere is to saturation but with a catch (it only tells the forecaster how close the air is to saturation at the earth's surface and does not give any indication to how deep in the troposphere the saturated or unsaturated conditions exist). The average 850 to 500 millibar has a problem of its own: It is an average. A region (or slice) between 850 and 500 millibars can be saturated and the relative humidity shows as low because there is a deeper slice of dry air between 850 and 500 millibars. There is one guarantee however, if the 850 to 500 mb RH is high it does indicate there is not any significant dry air from 850 to 500 millibars. High RH's on the model progs primarily result from "deep" lifting associated with synoptic scale low-pressure systems and other large scale synoptic uplift mechanisms such as strong low level convergence.
Another moisture indicator is that of dewpoint. Dewpoint is plotted on the models on the surface and 1000 millibar progs. If the dewpoint is close to the temperature and the 850 to 500 mb RH is high, then this indicates the troposphere truly is saturated through all the low and middle levels. Even slight lifting will result in clouds and precipitation. The exact amount of moisture in the troposphere (for a slice in the troposphere) can be derived from the dewpoint value. This is accomplished through what is known as the saturation mixing ratio. For each dewpoint value, there exists a specific quantity of water vapor in the troposphere. As dewpoint increases, the quantity of water vapor in the air increases exponentially. When looking at the models, the forecaster can gain insight into the moisture content at specific levels of the troposphere and gain insight into average RH values, but there is another tool that combines all three ways of measuring moisture into one chart: It is called the Skew-T Log-P diagram.
From this diagram, a relative humidity and dewpoint for EVERY level in the troposphere can be determined as well as a bulk precipitable water value (PW). The precipitable water is defined as the amount of moisture (in inches or centimeters) that would accumulate at the earth's surface if all water vapor in the troposphere above a specific area was condensed and brought down to the surface. A Skew-T diagram will show regions in the troposphere, which are near or far from saturation. It gives a complete picture of the moisture content of the troposphere. In the next Haby Hint we will apply these three moisture variables (RH, dewpoint, and PW) to forecasting thunderstorms, precipitation amounts, clouds, high and low temperature, moisture advection and more.
The next Haby Hint is at: http://www.theweatherprediction.com/habyhints/114/