The two graphs in figure 6-2-23 may be used valuate the maximum probable icing in a cloud after you have determined that icing conditions

are favorable. These graphs were constructed from the formula:

where LWC = liquid water content, in grams per cubic meter;

W₀ = saturation mixing ratio at cloud base, in grams per kilogram;

W₁ = saturation mixing ratio at an evaluated level in the cloud;

P₁ = atmospheric pressure at the evaluated level within the cloud; and

T₁ = temperature (kelvin) at the evaluated flight level.

The graphs, which compensate for adjusted lapse rates, were constructed for values of LWC tabulated against cloud heights. The cumulus graph has been constructed for clouds having bases near 950 millibars; and the stratus graph, for clouds with bases near 850 millibars. Devia-tions for these types of clouds with bases at different levels are very slight and may be ignored. The LWC values indicated on the graph are the critical values necessary to produce the icing intensities indicated by the areas they separate. The location of the LWC lines is based on studies of moisture distribution in clouds at different levels and the typical lapse rates within those types of clouds.

When you have determined that icing exists by the Minus 8D method, locate the base of the cloud. Read the temperature at the cloud base. From the cloud base temperature, go straight up the graph to determine the maximum probable icing in the cloud for the height above the cloud base. For instance, with a stratiform cloud base temperature of 10C, you would evaluate no icing up to 3,000 feet or so above the cloud base, moderate icing from 3,000 feet to 5,000 feet, and severe icing from 5,000 feet upwards. Adding these values to the actual height of the cloud base will give you the heights of the probable maximum icing intensity. Figure 6-2-24 shows the maximum probable intensity of icing analysis for the example we used in the Minus 8D method. In this example, lets assume a nimbostratus cloud base at 900 millibars (3,300 feet) and a cloud base temperature of 0C. Using the stratus graph, we would evaluate light icing to 2,700 feet above the cloud base, with moderate icing above 2,700 feet. Since the cloud base is at 3,300 feet, the graph would tell us that the maximum probable icing in this case is light icing from 3,300 feet to 6,000 feet (3,300 + 2,700), with moderate icing from 6,000 feet upward. But the Minus 8D method shows icing only from 6,400 feet to 10,800 feet. Therefore, our analysis would combine the two techniques, resulting in moderate icing from 6,400 feet to 10,800 feet. Use the criteria presented in lesson 1 to assign a type to this icing.

Figure 6-2-24.-Example combining Minus 8D analysis, maximum probable icing analysis, and icing type analysis.

If any doubt exists as to the probable intensity of icing occurring, you may use the formula for determining LWC and actually evaluate a level or two to find LWC. An LWC value less than 0.68 indicates light icing; between 0.68 and 1.33, moderate icing; and greater than 1.33, severe icing. Remember to determine the mixing ratio values from the frost point temperaturenot the dew point temperature. Actual LWC calculations would yield values for actual icing conditions not the maximum probable icing.