December 1999 8-4 route. Evaluate temperatures by identifying isotherm values and patterns. Evaluate areas with moist air and cloud potential by identifying station circles shaded black.
Consider the potential for hazardous flight conditions. Evaluate the potential for icing. Freezing temperatures and visible liquid forms of moisture produce icing. Evaluate the potential for turbulence.
In addition to convective conditions and strong surface winds, turbulence is also associated with wind shear and mountain waves. Wind shear occurs with strong curved flow and speed shear. Strong lows and troughs and strong isotach gradients are indicators of strong shear. Vertical wind shear can be identified by comparing winds on vertically adjacent charts. Mountain waves are caused by strong perpendicular flow across mountain crests. Use winds on the pressure charts near mountain crest level to evaluate mountain wave potential.
Pressure patterns cause and characterize much of the weather. As a general rule, lows and troughs are associated with clouds and precipitation, while highs and ridges are associated with good weather.
However, this rule is more complicated when pressure patterns change with height. Compare pressure pattern features on the various pressure charts with other weather charts, such as the weather depiction and radar summary charts. Note the association of pressure patterns on each chart with the weather.
Pressure systems, winds, temperature, and moisture change with time. For example, pressure systems move, change size, and change intensity. Forecast products predict these changes. Compare observed conditions with forecast conditions and be aware of these changes.
December 1999 8-5 Code Explanation WIND: Plotted wind direction and speed by symbol. Direction is to the nearest 10 degrees and speed is to the nearest 5 knots. (See Figure 5-3 for the explanation of the symbols.) If the direction or speed is missing, the wind symbol is omitted and an "M" is plotted in the Hc space. If speed is less than 3 knots, the wind is light and variable, the wind symbol is omitted, and an "LV" is plotted in the Hc space.
HGT: Plotted height of the constant pressure surface in meters above mean sea level. (See Table 8-1 for decoding.) If data is missing, nothing is plotted in this position.
TT: Plotted temperature to the nearest whole degree Celsius. A below-zero temperature is prefaced with a minus sign. Position is left blank if data is missing. A bracketed computergenerated temperature is plotted on the 850 mb/hPa chart in mountainous regions when stations have elevations above the 850 mb/hPa pressure level. If two temperatures are plotted, one above the other, the top temperature is used in the analysis.
T-D: Plotted temperature-dew point spread to the nearest whole degree Celsius. An "X" is plotted when the air is extremely dry. The position is left blank when the information is missing.
Hc: Plot of constant pressure surface height change which occurred during the previous 12 hours in tens of meters. For example, a +04 means the height of the surface rose 40 meters and a -12 means the height fell by 120 meters. Hc data is superseded by "LV" or "M" when pertinent.
CIRCLE: Identifies station position. Shaded black when T-D spread is 5 degrees or less (moist).
Unshaded when spread is more than 5 degrees.
Figure 8-1. Radiosonde Data Station Plot.
December 1999 8-6 Table 8-1 Features of the Constant Pressure Charts - U.S.
Pressure (millibars/hectoPascals) Pressure Altitude in feet (flight level) Pressure Altitude in meters Temperature/ Dew Point Spread Isotachs Contour Interval (meters) Prefix to Plotted Value Suffix to Plotted Value Plotted Height Decode Examples of Station Station Height Height Plot Plotting 850 700 500 300 250 200 5,000 10,000 18,000 30,000 34,000 39,000 1,500 3,000 5,500 9,000 10,500 12,000 yes yes yes yes** yes** yes** no no no yes yes yes 30 30 60 120 120 120 1 2 or 3* _____ _____ 1 1 ___ ___ 0 0 0 0 530 180 582 948 063 164 1,530 3,180 5,820 9,480 10,630 11,640 NOTE:
1. The pressure altitudes are rounded to the nearest 1,000 for feet and to the nearest 500 for meters.
2. All heights are above mean sea level (MSL).
3. * Prefix a “2” or “3,” whichever brings the height closer to 3,000 meters.
4. ** Omitted when the air is too cold (temperature less than -41 degrees).
December 1999 8-7 Table 8-2 Examples of Radiosonde Plotted Data 22 479 09 129 -19 558 -46 919 -55 037 -60 191 X X 850 mb 700 mb 500 mb 300 mb 250 mb 200 mb WIND light and 010/20 KTS 210/60 KTS 270/25 KTS 240/30 KTS missing variable TT 22° C 9° C -19° C -46° C -55° C -60° C T-D 4° C 17° C >29° C not plotted not plotted not plotted DEW POINT 18° C -8° C dry dry dry HGT 1,479 m 3,129 m 5,580 m 9,190 m 10,370 m 11,910 m Hc not plotted - 30 m + 30 m + 100 m - 10 m not plotted 4 LV 17 -03 + 03 + 10 -01 M
December 1999
WASHTHETON
- 05 Scal HEIGHTS/TEMPERATURE VALID OOZ FHU 08 JAN S Figure 8-2. 850 Millibar/HectoPascal Analysis.
230455/28044 10 470 4100458 95-459 19-382-82639 1604190 05079-014022 07,484 56493 6 131 085160
ANALYSIS +15 850MB D030 8-8
6661 eque -1Z0780 40000 HEIGHTS/TEMPERATURE VALID 00Z THU 08 JAN 1998 Figure 8-3. 700 Millibar/HectoPascal Analysis.
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ANALYSIS 700MB DO26
December 1999 TOO ETA48 ш33:540 HEIGHTS/TEMPERATURE VALID 00Z THU 08 JAN 1998 Figure 8-4. 500 Millibar/HectoPascal Analysis.
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ANALYSIS 500MB D020 8-10
December 1999 Lap 09 Z7hu ANALYSIS HEIGHTS/ISOTACHS VALID 00Z THU 08 JAN 1998 Figure 8-5. 300 Millibar/HectoPascal Analysis.
65 86ã 50- 52 Здк 50,085 -4ö* 300мB D049 8-11
December 1999 45 70790 1916013 - 60/9 022 48. 026 250. ANALYSIS HEIGHTS/ISOTACHS VALID 00Z THU 08 JAN 1998 Figure 8-6. 250 Millibar/HectoPascal Analysis.
55- 984 996 699817650003 008 20 à 056 ,206 550MB D053 ..
068 8-12
December 1999 9-1 Section 9
COMPOSITE MOISTURE STABILITY CHART The composite moisture stability chart (Figure 9-1) is a chart composed of four panels which depict stability, precipitable water, freezing level, and average relative humidity conditions. This computergenerated chart contains information obtained from upper-air observation data and is available twice daily with valid times of 00Z and 12Z.
The availability of upper-air data for analysis (on all panels) is indicated by the shape of the station symbols. Use the legend on the precipitable water panel (Figure 9-3) for the explanation of symbols common to all four panels. Mandatory levels referred to in the legend are the routinely used levels of surface; 1,000; 925; 850; 700; and 500 mb (hPa). Significant levels are nonroutine levels at which significant changes occur in the vertical profile of atmospheric properties during each observation.
STABILITY PANEL The stability panel (Figure 9-2) is the upper left panel of the composite moisture stability chart. This panel contains two indexes that characterize the moisture and stability of the air. These indexes are the K index (KI) and the lifted index (LI).
K INDEX (KI) The K index (KI) provides moisture and stability information. KI values range from high positive values to low negative values. A high positive KI implies moist and unstable air. A low or negative KI implies dry and stable air. KIs are considered high when values are at and above +20 and low when values are less than +20.
The KI is calculated by the summation of three terms:
KI = (850 mb/hPa temp - 500 mb/hPa temp) + (850 Mb/hPa dew point)
- (700 mb/hPa temp/dew point spread) The first term (850 mb/hPa temp - 500 mb/hPa temp) describes the vertical temperature profile. The term compares the temperature at about 5,000 feet mean sea level (MSL) with the temperature at about 18,000 feet MSL. The larger the temperature difference, the less stable the air, and the higher the KI.
The smaller the temperature difference, the more stable the air, and the lower the KI.
The second term, 850 mb (hPa) dew point, is a measure of the quantity of low-level moisture. The higher the dew point, the higher the moisture, and the higher the KI. The lower the dew point, the lower the moisture, and the lower the KI.
The third term, 700 mb (hPa) temp/dew point spread, is a measure of the level of saturation at 700 mb (hPa). The smaller the spread, the higher the level of saturation, and the higher the KI. The greater the spread, the lower the level of saturation (drier air), and the lower the KI.
The KI can change significantly over a short time period of time due to temperature and moisture changes.
December 1999 9-2
LIFTED INDEX (LI) The lifted index (LI) is a common measure of atmospheric stability. The LI is obtained by hypothetically displacing a surface parcel upward to 500 mb (hPa) (about 18,000 feet MSL) and evaluating its stability at that level. A surface parcel is a small sample of air with representative surface temperature and moisture conditions. As the parcel is "lifted" it cools due to expansion. The temperature the parcel would have at 500 mb (hPa) is then subtracted from the actual (observed) 500 mb (hPa) temperature.
This difference is the LI. LI values can be positive, negative, or zero. The LI does not identify the parcel's stability behavior at any of the intermediate altitudes between the surface and 500 mb (hPa).
A positive LI means a lifted surface parcel of air is stable. With a positive LI, the parcel would be colder and more dense than the surrounding air at 500 mb (hPa). A more dense parcel would resist upward motion. The stable surface parcel is like a rock at the bottom of a pool which, being more dense than the water, would resist being displaced upward. The more positive the LI, the more stable the air. Large positive values (+8 or greater) would indicate very stable air.
A negative LI means a lifted surface parcel of air is unstable. With a negative LI, the parcel would be warmer and less dense than the surrounding air at 500 mb (hPa). A parcel which is less dense than the surrounding air would continue to rise and possibly gain increasing upward speed until stabilizing at some higher altitude. The unstable surface parcel is like a cork at the bottom of a pool which, being less dense than the water, would accelerate upward to the surface of the pool. Large negative values (-6 or less) would indicate very unstable air.
A zero LI means a lifted surface parcel of air is neutrally stable. With a zero LI, the lifted parcel would have the same temperature and density as the air at 500 mb (hPa) and have a tendency to neither rise or sink. A neutrally stable parcel offers no resistance to vertical motion and, without further influence, would remain at the displaced level.
Temperature and moisture changes in the atmosphere change lifted index values. LIs decrease (become less stable) by increasing the surface temperatures, increasing surface dew points (moisture), and/or decreasing 500 mb (hPa) temperatures. Cold lows and troughs aloft with warm humid surface conditions tend to be associated with unstable air. LIs increase (become more stable) by decreasing surface temperatures, decreasing surface dew points, and/or increasing 500 mb (hPa) temperatures. Warm highs and ridges aloft with cool, dry surface conditions tend to be associated with stable air. Note that the LI can change considerably just by daytime heating and nighttime cooling of surface air. Daytime heating will decrease the LI, and nighttime cooling will increase the LI.
PLOTTED DATA Figure 9-2 shows the two stability indexes that are computed for each upper-air station. The LI is plotted above the station symbol, and the KI is plotted below the symbol. Station circles are blackened for LI values of zero or less. An "M" indicates the value is missing
STABILITY ANALYSIS The analysis is based on the LI only and highlights weakly stable and unstable areas. Solid lines are drawn for values of +4 and less at intervals of 4 (+4, 0, -4, -8, etc.).
December 1999 9-3
USING THE PANEL The KI and LI can be used in combination to assess moisture and stability properties of air masses. Air masses can be classified as moist and stable, moist and unstable, dry and stable, and dry and unstable.
When used in combination, the KI, although containing stability information, is used primarily to classify moisture information, and the LI primarily to classify stability information. See Figure 9-2. Aberdeen, SD, has air characterized as dry and stable. The KI is 3 (dry) and the LI is 19 (stable). Melbourne, FL, is an example of dry and unstable air. The KI is 8 (dry) and the LI is -1 (unstable). Moist and unstable air is depicted at Key West, FL. The KI is 29 (moist) and the LI is -3 (unstable). The last example, Albany, NY, indicates moist and stable air. The KI is 31 (moist) and the LI is 15 (stable).
Moisture and stability properties of air masses characterize the weather. High KIs are associated with the potential for clouds and precipitation. Weather associated with high LIs and stable air are stratiform clouds and steady precipitation. Weather associated with low and negative LIs are unstable air, convective clouds, and showery precipitation.
The KI and LI can also be used to evaluate thunderstorm information. The KI is an indicator of the probability of thunderstorms (Table 9-1). Higher KIs imply higher probabilities. Lower KIs imply lower probabilities. The low and negative LIs are indicators of intensities of thunderstorms, if they occur.
Higher negative LIs imply greater instability and stronger updrafts in thunderstorms. High positive LIs suggest little, if any, chance of thunderstorms.
Air masses classified with negative LIs do not always contain thunderstorms. This can occur for several reasons. Thunderstorm development is inhibited when a layer of stable air is located between the surface and 500 mb (hPa). This stable layer is referred to as a "cap." Inadequate amounts of moisture may also limit thunderstorm development in the presence of negative LIs. It is also possible to have a positive LI and still have thunderstorms develop. This can happen when a layer of air aloft located above stable surface air, such as above a front, is unstable and is sufficiently lifted, or if temperature and moisture conditions change rapidly and stabilities decrease.
Seasons affect the use of the KI regarding thunderstorm information. During the warmer seasons of spring, summer, and fall, a high KI generally indicates conditions are favorable for thunderstorms (Table 9-1). During winter with cold temperatures, fairly high values do not necessarily mean conditions are favorable for thunderstorms. Cold 850 mb (hPa) temperatures mean low dew points (low moisture.) The temperature profile term can generate high KI values, but low dew points may mean inadequate moisture to support thunderstorm development.