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Potential for Convection

Learn about the Lifted Index which compares theoretical lifted air temperature to upper air temperature, indicating thunderstorm potential. Explore calculations, diagrams, and practical applications for forecasting weather phenomena.

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Potential for Convection

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  1. Potential for Convection Lifted Index

  2. Lifted Index • Compares theoretical lifted temperature of surface air to actual temperature of upper air • If (lifted temperature) > (upper-air temperature), parcel unstably rises

  3. Lifted Index • LI = (upper-air temp)−(lifted air temp) • Meanings > 0: stable air; no thunderstorms 0 to −2: possible thunderstorms with lifting mechanism −2 to −6: thunderstorms likely, possibly severe < −6: severe thunderstorms likely

  4. Determine a L.I.! Educational! Easy! • “Lift” parcel, cool at dry lapse rate until saturated • Continue to “lift,” but at saturated lapse rate (slower T drop) • Compare to actual air temp at 500 mbar Fun!

  5. Making a Stüve Plot Potential for convective storms

  6. Thermodynamic Plot • Solid slanted lines (dry adiabats) show “lapse rate:” temperature drop with elevation gain • If you know surface T,p you know it for the rising parcel

  7. Thermodynamic Plot • Rising moisture-saturated air condenses • Releases heat • Temperature drop is inhibited • Slanted dashed curves: saturated adiabats

  8. Thermodynamic Plot • Mixing ratio: (mass of water vapor)/(mass of air) (g/kg) • Dotted lines: saturation mixing ratios • Dew point T, p at that mixing ratio

  9. Lifting a Surface Parcel • Air rises, expands, and cools along a dry adiabat… • Until it reaches its dew point • Then rises and cools along a saturated adiabat

  10. Finding the Dew Point • Lift along mixing ratio until it meets the dry adiabat • That is when the moisture begins to condense

  11. Lifting a Surface Parcel • Lift along the dry adiabat and the saturation mixing ratio until they meet • Then lift along the saturated adiabat

  12. Lifted Index • Lift surface air along dry adiabat until saturation • Then lift along saturated adiabat to 500 mb • Lifted index = (air temp at 500 mb) – (lifted parcel temp at 500 mb) • Best chance of severe thunderstorms when L.I. < –6 °C

  13. Task • Plot the temperatures and dew points • Lift the surface parcel to 500 mb pressure • Determine the lifted index

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