The Lifted Index is a measure of atmosphere's stability (or instability) and Meteorologists use it to determine the thunderstorm potential. It doesn't accurately predict the intensity of every single storm, but it is a useful tool to estimate the atmosphere's potential to produce severe thunderstorms.
"Parcels" (or bubbles) of air start to rise on their own if they are warmer than the surrounding air. This process is called convection. Consider an air parcel as it begins to rise through the atmosphere after being heated by the sun and the warming ground. The Lifted Index is defined as a rising parcel's temperature when it reaches the 500 millibars level (at about 5,500m or 18,000 feet asl), subtracted from the actual temperature of the environmental air at 500 millibars. If the Lifted Iindex is a large negative number, then the parcel will be much warmer than its surroundings, and will continue to rise. Thunderstorms are fueled by strong rising air, thus the Lifted Index is a good measurement of the atmosphere's potential to produce severe thunderstorms.
|The Lifted Index (LI)|
For example, if the rising parcel has an temperature of -5°C when it reaches 500 millibars, but the actual temperature at 500 millibars is -11°C, then the lifted index is -6 Kelvin (or K) indicating the potential for strong thunderstorms. However, there are no specific threshold values that correlate lifted index to thunderstorm severity. In general a negative Lifted Index indicates an unstable atmosphere, so the larger the negative number, the more unstable the atmosphere is, the stronger a thunderstorm could be. Lifted index values rarely go below -7.
Now, find out about the convection and thunderstorm probablility across the British Isles yourself. Have a look at WeatherOnline's new Lifted Index maps. The table above will help you to 'read' the map and to estimate the thunderstorm risk.
Important ! The Lifted Index is not a measured quantity, it is only a parameter that is theoretically derived. If the Lifted Index is favorable for severe storms but other conditions are not met, then no storms may form at all.