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The Jefferson Stability Index (JI), or the Jefferson Index is a meteorological index that measures the atmospheric instability. It can be used for an indication of cumulonimbus development.

Definition
The Jefferson Index is calculated where $$I$$ is calculated at 0000 GMT from the formula
 * $I = 1.6\theta_{w900}-T_{500}-11$

where $$\theta_{w900}$$ is the wet bulb potential temperature in °C at 900mb; $$T_{500}$$ is the 500mb temperature in °C. $$11$$ is the correlation factor. When the $$I$$ is greater than 30, then thunderstorm development is not expected.

Use
This scale is used to determine the atmospheric instability as mentioned above. The lower the value is, the higher the instibility is, thus there will likely be cumulonimbi development should all of the other ingredients be in place. This can be useful to know what the thunderstorm activities are going to be for that day or in the future with help from several forecast models.