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Convective Available Potential Energy (CAPE) and Lifted Index (LI) are used to assess the likelihood of thunderstorms developing.
LI gives a measure of stability in the atmosphere. A positive value indicates the atmosphere is stable, a negative value that it is unstable. The lower the LI value the more unstable the atmosphere is and the greater is the potential for thunderstorms. The LI is calculated by subtracting the temperature of a lifted 500mB parcel of air from the 500mB environmental temperature. The 500mB level represents the middle of the atmosphere with approximately half of the air mass below this level and is at about 5500m.
CAPE also gives a measure of instability in the atmosphere. It is the amount of Potential energy (stored energy) held by air molecules in section of the atmosphere (vertical column of air). Higher CAPE values indicate more instability and a greater chance of thunderstorm development. CAPE is shown on charts in Joules per kilogram (J/kg).
CAPE and LI charts for the next 16 days are available on TWO and are updated every 6 hours. These are generated using data from the Global Forecast System (GFS) computer model.
The CAPE and LI chart below is used as an example. The contour lines represent LI with negative values in the south indicating an unstable atmosphere. CAPE is illustrated by the shading with the yellows and oranges indicating higher values and greater instability. This chart suggests a good chance of thunderstorms in the south.
CAPE and LI are measures used to forecast the likelihood of thunderstorms. There isn't a hard and fast rule to apply but basically look for:
1) The LI contours to be negative
2) CAPE shading to be orange with values heading above 1000 J/kg
The latest charts can be viewed on the dedicated CAPE and Lifted Index page. They are also available along with thousands of other plots on the Chart viewer.
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