December 1999 9-4 Table 9-1 Thunderstorm Potential Lifted Index (LI) Severe Potential K Index (KI) Thunderstorm Probability 0 to -2 Weak < 15 near 0% 15 - 19 20% -2 to -6 Moderate 20 - 25 21% - 40% 26 - 30 41% - 60% 31 - 35 61% - 80% < -6 Strong 36 - 40 81% - 90% > 40 near 100%
PRECIPITABLE WATER PANEL The precipitable water panel (Figure 9-3) is the upper right panel of the composite moisture stability chart. This panel is an analysis of the quantity of water vapor in the atmosphere from the surface to the 500 mb (hPa) level (18,000 feet MSL). The quantity of water vapor is shown as precipitable water, which is the amount of liquid water that would result if all the water vapor were condensed.
Two constant factors affect precipitable water reports. Warm air is capable of holding higher quantities of water vapor than cold air. Therefore, warm air masses generally have more precipitable water than cold air masses. For example, precipitable water values are higher during summer months than during winter months. Also, high elevation stations have smaller vertical columns of air between surface and 500 mb (hPa) than low elevation stations. Therefore, higher elevation stations tend to have lower precipitable water than lower stations.
December 1999 9-5
PLOTTED DATA Precipitable water values are plotted above each station symbol to the nearest hundredth of an inch. The percent relative to normal for the month is plotted below the station symbol. Blackened circles indicate stations with precipitable water values of 1.00 inch or more. An “M” plotted above the station symbol indicates missing data.
ANALYSIS Isopleths (lines of equal values) of precipitable water are drawn and labeled for every 0.25 inches with heavier isopleths drawn at 0.50-inch intervals.
USING THE CHART This panel is used to determine the quantity of water vapor in the air between the surface and 500 mb (hPa) (18,000 feet MSL). Higher moisture content indicates “more fuel” for convective conditions. In Figure 9-3, Glasgow, MT, has a plot of “.22/100.” This indicates that 22 hundreds of an inch of precipitable water is present, which is the average for the month. At Oklahoma City, OK, the “.72/196” indicates that 72 hundredths of an inch of precipitable water is present, which is 196 percent of normal (about double) for any day during this month. At Aberdeen, SD, the percent of normal value is not plotted due to insufficient climatological data.
FREEZING LEVEL PANEL The freezing level panel (Figure 9-4) is the lower left panel of the composite moisture stability chart.
This panel is an analysis of observed freezing levels. The freezing level is the height above MSL at which the temperature is zero degrees Celsius.
Freezing levels are affected by air mass temperatures. Colder air masses have lower freezing levels, and warmer air masses have higher freezing levels. Freezing levels change with the movement of contrasting cold and warm air masses. For example, freezing levels tend to lower behind cold fronts and rise ahead of warm fronts.
Generally, a station has one freezing level. Relative to the freezing level, the lower levels have abovefreezing temperatures, and the upper levels have below-freezing temperatures. During very cold periods, all temperatures over a station may be below freezing and there would be no freezing level.
During colder periods of the year, and with certain weather systems such as fronts, stations may have more than one freezing level. There would be several layers of air with alternating above-freezing and below-freezing temperatures. A report from such a station would contain multiple freezing levels. Table 9-2 illustrates a vertical temperature profile drawn relative to zero degrees Celsius which contains multiple freezing levels. In this table there are two layers with above-freezing temperatures and two layers with below-freezing temperatures. Above-freezing layers extend from the surface to 3,000 feet MSL and from 6,000 to 9,000 feet MSL. Below-freezing layers extend from 3,000 to 6,000 feet MSL and above 9,000 feet MSL.
December 1999 9-6 Table 9-2 Vertical Temperature Profile with Freezing Levels 90 60 30 < 0° 0° > 0° Plotted Data Height 9,000 Feet MSL 6,000 Feet MSL 3,000 Feet MSL
December 1999 9-7
PLOTTED DATA Observed freezing levels are plotted on the chart in hundreds of feet MSL. Multiple freezing level events have plots for each freezing level. BF is plotted on the chart to indicate below-freezing temperatures at the surface. "M" indicates missing data. Note in Table 9-2 the freezing level plots associated with the illustrated vertical temperature profile. Table 9-3 provides examples of several station plots for various types of freezing level conditions.
ANALYSIS Freezing levels are analyzed with contours (lines of constant height) and are drawn as solid lines. The lines are drawn with intervals of 4,000 feet beginning with 4,000 feet. Multiple freezing levels are analyzed for the lowest freezing level. Contours are labeled in hundreds of feet MSL. The surface freezing level is drawn and labeled as the 32-degree Fahrenheit (0° C)isotherm. The surface freezing level line encloses stations with BF data plots.
USING THE PANEL The freezing level chart is used to assess freezing level heights and their values relative to flight profiles.
In Figure 9-4, Salt Lake City, UT, is an example where all temperatures above the station were below freezing (below 0° C or 32° F.) Lake Charles, LA, depicts a single freezing level at 11,500 feet MSL.
North Platte, NE, is an example of multiple freezing levels. The temperatures were below freezing at the surface but warmed to above freezing between 4,400 and 6,100 feet MSL. Above 6,100 feet MSL the temperatures were again below freezing.
In areas with single freezing levels, flights above the freezing level will be in below-freezing temperatures, and flights below the freezing level will be in above-freezing temperatures. In areas with multiple freezing levels, there are multiple layers of above- and below-freezing temperatures. According to Figure 9-4, a flight en route from Seattle, WA, to Portland, OR, at 7,000 feet would be flying above the freezing level and in below-freezing temperatures. A flight en route at 7,000 feet from Atlanta, GA, to Washington, DC, would be flying below the freezing level and in above-freezing temperatures.
Special care must be exercised to properly identify the altitudes of layers with above and below freezing temperatures when there is a potential for icing conditions.
December 1999 9-8 Table 9-3 Plotting Freezing Levels Plotted Interpretation BF Entire observation is below freezing (0 degrees Celsius).
28 Freezing level is at 2,800 feet MSL; temperatures below freezing above 2,800 feet MSL. All significant levels are missing.
120 Freezing level at 12,000 feet; temperatures above 12,000 feet are below freezing. Some mandatory levels are missing.
110 51 BF Temperatures are below freezing from the surface to 5,100 feet; above freezing from 5,100 to 11,000 feet MSL;
and below freezing above 11,000 feet MSL.
90 34 3 Lowest freezing level is at 300 feet; below freezing from 300 feet to 3,400 feet; above freezing from 3,400 to 9,000 feet; and below freezing above 9,000 feet.
M Data is missing.
December 1999 9-9
AVERAGE RELATIVE HUMIDITY PANEL The average relative humidity panel (Figure 9-5) is the lower right panel of the composite moisture stability chart. This panel is an analysis of the average relative humidity for the layer surface to 500 mb (hPa).
Relative humidity is the ratio of the quantity of water vapor in a sample of air compared to the air's capacity to hold water vapor expressed in percent. The air's capacity to hold water vapor depends primarily on its temperature and, to a lesser extent, its pressure. Warm air can hold more water vapor than cold air. Air at lower pressure can hold more water vapor than air at higher pressure.
Average relative humidities of the layer are changed primarily by vertical motion of air. Upward motion increases relative humidities, and downward motion decreases relative humidities.
Relative humidity is an indicator of the degree to which air is saturated. Air is saturated when it contains all of the water vapor it can hold. High relative humidities (moist air) identify air which is at or close to saturation. Air with high relative humidites often contain clouds and may produce precipitation. Low relative humidities (dry air) identify air that is not close to saturation. Low relative humidity air tends to be free of clouds.
PLOTTED DATA The average relative humidity is plotted above each station symbol. Blackened circles indicate stations with humidities of 50 percent and higher. An “M” indicates the value is missing.
ANALYSIS Isopleths of relative humidity, called isohumes, are drawn and labeled every 10 percent, with more heavily shaded isohumes drawn for values of 10, 50, and 90 percent.
USING THE PANEL This panel is used to determine the average relative humidity of air from the surface to 500 mb (hPa).
Areas with high average relative humidities have a higher probability of thick clouds and possibly precipitation. Areas with low average relative humidities have a lower probability of thick clouds, although shallow cloud layers may be present. Weather-producing systems, such as lows and fronts, which are moving into areas with high average relative humidities have a high probability of developing clouds and precipitation. Significant values of average relative humidities which support the possibility of developing clouds and precipitation are 50% and higher with unstable air, and 70% and higher for stable air. Weather-producing systems affecting areas with low average relative humidities, 30% and less, may produce only a few clouds, if any. According to Figure 9-5, much of Arkansas has very moist air with average relative humidities greater than 90%, while western Arizona has dry air with average relative humidities less than 30%.
High values of relative humidity do not necessarily mean high values of water vapor content (precipitable water). For example in Figure 9-3, Oakland, CA, had less water vapor content than Miami, FL (.64 and
1.43 respectively). However, in Figure 9-5, the average relative humidities are the same for both stations.
If rain were falling at both stations, the result would likely be lighter precipitation totals for Oakland.
December 1999 9-10
USING THE CHART This chart is used to identify the distribution of moisture, stability, and freezing level properties of the atmosphere. These properties and their association with weather systems provide important insights into existing and forecast weather conditions as well as possible aviation weather hazards.
Generally these properties tend to move with the associated weather systems, such as lows, highs, and fronts. Contrasting property values within weather systems are redistributed relative to the systems by advecting winds. Also, changes in property values relative to the systems can occur as a result of development and dissipation processes. In some instances property values will remain stationary relative to geographical features, such as mountains and coastal regions.
December 1999 150 150 PERCENT BEE WATEL 166 80 100 125 150 'PERCENT 12 ANALYSIS 12Z WED 07 JAN 1998 70 20 20 100 N * ALL SIG. LEVELS MISSING • SOME MANDATORY LEVELS MISSING. SOYE HAND. AND SIE. LEVELS MISSING PRECIPITABLE WATER 60 60 50 50 1200Z MED OF AN 1228 AVG SFC-TO-500MB RANALYSIS Figure 9-1. Composite Moisture Stability Chart.
Ì 20 Î55‹ LIFTED INDEX g5W K INDEX 32F FREELZOIG LEVEL 85W ANALYSIS 12Z WED 07 JAN 1998 do pro WED 07 1200Z WED 07 LIFTED INDEX ANALYSIS N100 D181 32F 1200Z ED OZ JAN 1992F FREEZING LEVEL N100 0181 9-11
December 1999
US DEPT OF COMMI LIFTED INDEX 85W K INDEX Non LIFTED INDEX ANALYSIS 12Z WED 07 JAN 1998WED 19 ! Figure 9-2. Stability Panel.
13 12007 WED 07'JÀN 1098 N100 0181 9-12
December 1999 14 288 150 PRECIPITABLE WATER 166 PERCENT OF NORMAL 150 ¿•76' 68 1.36 100 125 Figure 9-3. Precipitable Water Panel.
• 80 1.00 ANALYSIS 12Z WED 07 JAN 1998 167 075
- 18 75 • 27 70- 050 106 . 23 8ž • SOME HAND OR SEE LEVELS MESSING 025
• 09 40 . 21 64 * ALL SIG. LEVELS MISSING JÀN 1998 PRECIPITABLE WATER 00 1ž8 • 29 64 1200Z WED 07 050
TERCE SHINGTON 9-13
December 1999 154 142 151 85W Ỉ15\42Ó 4⅞\11 .22 FReEZING LEvEL 108 115 FREEZING LEVEL ANALYSIS 12Z WED 07 JAN 1998 Figure 9-4. Freezing Level Panel.
JZE 00 18 32F 40 2Z JAN 1998 1200Z WED N100 0181 9-14
December 1999 AVERAG RELATIVE HUMIDITY 85W (SFC TO 500MB) 61 12Z WED 07 JAM 1998 Figure 9-5. Average Relative Humidity Panel.
70 70 50 ž6 1200Z MED OF JAN 1298 AVG SFC-TO-500MB RANALYSIS 88 9-15