Icing computations on the skew T

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ICING COMPUTATIONS ON THE SKEW T

You learned in the previous lesson how to evaluate icing type, and you were given some sub-jective rules for icing intensity. We must use more than "guesstimation" when flying safety is at stake. In this section we will discuss two icing analysis techniques for use on the Skew T. The Minus 8D technique is a method which gives a simple Yes or No evaluation of conditions favor-able for icing. The second technique will yield a qualitative analysis of icing intensity. Although there are a lot of calculations that must be done in this technique, it is far better to use this technique than estimation when attempting to analyze and forecast hazardous conditions such as icing. One of my most memorable days on the Flight Forecast Counter occurred one winter day, when analysis of data, including several Skew T’s, indicated severe clear and mixed icing conditions north of NAS Norfolk, extending well north of Washington, D.C. I used the "Flight Not Recommended" stamp on three dash-one briefs to end the arguments by pilots who just "had to" fly to the D.C. area. Although two of the three pilots became very irate, I felt great about my decision after hearing of a tragic accident involving a major passenger jet that crashed into the Potomac River because of icing. They could have ended their flights the same way. If I had used estimation instead of qualitative techniques, I would not have been as sure about my forecast and may have let those determined pilots fly that day.

Minus 8D Icing Analysis

Physical observations of clouds often fail to give any indication of the composition of the clouds—whether they are water, water and ice mixed, or entirely made up of ice crystals. In any convective cloud, it is reasonable to assume that they are made of a mixture of ice and water at temperatures below freezing. In the stratus cloud, however, it is not safe to assume this. Clouds com-posed entirely of ice present little icing hazard. Clouds below freezing composed of mixed water and ice present a great icing hazard. Therefore, it is helpful if you know the cloud composition before you brief a flight through the cloud.

Figure 6-2-22.-Example of minus 8D icing analysis.

In a mixed ice and supercooled water cloud, evaluation of the dew point and the frost point would show that the cloud may be near or at saturation with respect to water but that the cloud would be supersaturated with respect to ice. We can assume that a cloud that is saturated with respect to ice would be subsaturated with respect to water and that it would consist entirely of ice crystals. Now, we could evaluate the humidity at all levels based on the dew point and frost point to see where icing will occur; or, we can use the Minus 8D Icing Analysis to show at a glance those areas that are supersaturated with respect to ice (are composed of supercooled water) and are therefore, icing areas. The procedure I find easiest follows:

1. Task the Skew T plotter to enter the reported dew point depression (as the chart is being plotted) just to the left of the dew point temperature plot for all levels where the air temperature is below freezing. An alternative is to calculate the difference between the plotted dew point temperature and the air temperature and enter this value (always a positive number) just to the left of the plotted dew point temperature. In this case, the value is called D even though it is the dew point depression, It should be found to the nearest tenth.

2. Multiply D by –8 and plot this value (°C) the appropriate temperature on the same pressure level. For instance, if your dew point depression, or D, is 1.1, you would multiply this by –8 to find a product of –8.8°C. This would be plotted on the appropriate isotherm. The color used for the plot is established locally. I have seen green used most often.

3. Connect all of your plotted –8D values

4. Conditions are favorable for aircraft icing where the line falls on the right side of the temperature curve. Evaluate flight levels for the base of each icing layer and the thickness of each layer, in thousands of feet, using the plotted pressure altitude curve. Pass this information to the forecaster.

See figure 6-2-22 for an example of a –8D analysis of an icing layer. This technique does not indicate the intensity or type of icing in a layer, only that icing conditions are favorable or unfavorable.

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