SkewT Plots ----------- A Skew T plot is a standard plot used by meteorologists to analyze data from a balloon sounding. This is a plot of temperature with height as denoted by pressure. Standard pressure levels and approximate heights: Pressure Approximate Height Approximate Temp Sea level 0 m 0 ft 15 C 59 F 1000 mb 100 m 300 ft 15 C 59 F 850 mb 1500 m 5000 ft 5 C 41 F 700 mb 3000 m 10000 ft -5 C 23 F 500 mb 5000 m 18000 ft -20 C -4 F 300 mb 9000 m 30000 ft -45 C -49 F 200 mb 12000 m 40000 ft -55 C -67 F 100 mb 16000 m 53000 ft -56 C -69 F The pressure lines are plotted horizontally in blue and are also on an inverse log scale. The concept of Skew T means that the temperature is not plotted vertically but angles off to the right at a 45 degree angle. The temperature lines of the Skew T are in blue. The green lines are called dry adiabats. The light blue dashed lines are saturation adiabats. The yellow dashed lines are lines of constant mixing ratio. The sounding is plotted as two white lines. The right line is the temperature profile. The left line is the dewpoint profile. The winds are plotted as wind barbs (See Weather Symbols Legend) with height on the right edge of the plot. Specific sounding related parameters are plotted in the upper right. These values are: TP: Tropopause level as pressure in mb MW: Maximum wind level as pressure in mb LCL: Surface lifted condensation level in mb PW: Precipitable water in inches TH: 1000-500 mb thickness LI: Lifted index SI: Showalter index TT: Total totals index KI: K index SW: Sweat index Sounding diagrams provide a important means for determining the stability of the atmosphere above a specific location. By using the concept of an air parcel, lifting it or lowering it and comparing the resulting parcel conditions to the conditions of the surrounding environment as defined by the balloon sounding. Skew T are in blue. The green lines are called dry adiabats. Parcels of unsaturated air tend to follow the dry adiabat lines (green lines) as they ascend or descend. The saturation adiabats (light blue) lines show how parcels saturated with water vapor will ascend or descend. Descending parcels will tend to unsaturate immediately. The mixing ratio lines (yellow) relate to the amount of water vapor in a parcel in grams of water vapor per kilograms of dry air. Parcels of air attempt to maintain a constant mixing ratio as they ascend or descend. Generally, a parcel will rise, following the dry adiabat until it saturates. This occurs when dry adiabat crosses the initial mixing ratio line. This is considered the LCL. If lifting continues, the parcel cools following the saturation adiabat. If the parcel descends, it will always follow the dry adiabat as it will immediately unsaturate if saturated. By then comparing the parcel temperature to the environment, you can determine whether it is stable (parcel cooler) or unstable (parcel warmer). An unstable parcel will accelerate upwards and is the primary means for thunderstorm development. A stable parcel will decelerate and eventually descend. This is the typical atmospheric condition and it the primary condition in high pressure areas. The descending air desaturates the atmosphere and leads to clearing skies and calm conditions. ATMOSPHERIC STABILITY INDICIES To measure the instability of the atmosphere, several stability indices have been developed. They involve differing means for assessing the stability and result in the following criteria: Lifted Index (LI) LI > 2 No significant activity 0 < LI < 2 Showers probable, isolated thunderstorms possible -2 < LI < 0 Thunderstorms probable -4 < LI < -2 Severe thunderstorms possible LI < -4 Severe thunderstorms probable, tornadoes possible Showalter Index (SI) SI > 3 No significant activity 1 < SI < 3 Showers probable, isolated thunderstorms possible -2 < SI < 1 Thunderstorms probable -6 < SI < -2 Severe thunderstorms possible SI < -6 Tornadoes possible Total Totals Index (TT) 44-45 Isolated moderate thunderstorms 46-47 Scattered moderate / few heavy thunderstorms 48-49 Scattered moderate / few heavy / isolated severe thunderstorms 50-51 Scattered heavy / few severe thunderstorms and isolated tornadoes 52-55 Scattered to numerous heavy / few to scattered severe thunderstorm / few tornadoes > 55 Numerous heavy / scattered severe thunderstorms and scattered tornadoes K Index (KI) K < 15 0% Air mass thunderstorm probability 15-20 <20% Air mass thunderstorm probability 21-25 20-40% Air mass thunderstorm probability 26-30 40-60% Air mass thunderstorm probability 31-35 60-80% Air mass thunderstorm probability 36-40 80-90% Air mass thunderstorm probability K > 40 >90% Air mass thunderstorm probability SWEAT ( Severe Weather thrEAT ) Index (SW) SW < 300 No activity expected 300 < SW < 400 Isolated moderate to heavy thunderstorms 400 < SW < 500 Severe thunderstorms and tornadoes probable SW > 500 Severe thunderstorms and tornadoes likely .