Simple Skew-T Techniques -- A Primer February 2001

Below are a few rules for deriving information from a skew-T diagram.

1. Potential temperature. From any pressure level, follow the dry adiabat down to 1000 hPa. The temperature at 1000 hPa is the potential temperature.
2. Relative humidity. For a pressure level, look at the sounding for temperature and dewpoint temperature. The temperature corresponds to a certain water vapor mixing ratio that would be at saturation for that temperature, w_s, while the dewpoint temperature corresponds to the water vapor mixing ratio that actually exists. Find w and w_s by looking at the water vapor mixing ratio that correspond to the temperatures at that pressure. The relative humidity is given by R.H. = 100 x w/w_s.
3. If you are given two of the three quantities dewpoint temperature, temperature, or relative humidity, you can find the other two using the process in 2.
4. Water vapor partial pressure. If you want to know the water vapor partial pressure for a given temperature, you can find it by following the temperature up to 622 hPa. When p = 622, e(T) = w (at 622 hPa).
5. Lifting condensation Level (LCL). Follow the dry adiabat up from the temperature and the water vapor mixing ratio up from the dewpoint temperature. Where the two intersect is the LCL.
6. Equivalent potential temperature, Q _e . Once you are at the LCL, follow the moist (saturated) adiabat all the way up to where the moist adiabats become parallel to the dry adiabats. Because Q _e is the potential temperature equivalent to the case when all the moisture has become liquid or ice and fallen out, Q _e is the same as the dry adiabat with which it coincides.
7. Wet bulb temperature, T_w. Under saturated conditions, T = T_d = T_w. Below the LCL, these three are all different. To find T_w for any pressure level below the LCL, follow the moist adiabat to that level from the LCL. To find Q _w, the wet bulb potential temperature, follow the moist adiabat to p=1000 hPa.
8. Air ascending above the LCL. Moist air that ascends above the LCL does so along the moist adiabat. The total amount of water remains constant, if it does not rain, but the amount of water vapor changes, since the temperature is decreasing and, by the Clausius Clapeyron Equation, the e_s depends only on T. Thus, w_s = w decreases as the liquid water content (LWC) increases.
9. Rain and then descent of the air parcel. Suppose that rain occurs for a saturated air parcel. The total water content is no longer constant, but the amount of water vapor is, because the water vapor mixing ratio depends only on T and p. If a certain percentage of the liquid or solid water falls out, the total remaining water is the sum of the water vapor and the remaining solid or liquid water. As the air parcel descends, it will do so along the moist adiabat UNTIL it reaches the new LCL, which occurs at a pressure level for which w =new total water, i.e., all the water is again in the form of vapor. But, because the amount of total water is less, the LCL will occur at a lower w than before, and the LCL will be higher. As the air parcel continues to descend, and all the water is in the vapor form, the temperature changes according to the dry adiabat and the water vapor mixing ratio remains constant. Q _new > Q _intial.