If you’re wondering what the Windy’s CAPE index forecast layer is good for, a meteorology and medicine nerd Luke gives you an answer in a thread on Twitter.

Convective Available Potential Energy is a measure of the energy that can be created if there is enough heat in a cloud to give convection.

The values are given in Joules per Kilogram of air as a general rule.

The greater the energy that is released during convection, the greater will be the charge separation and the more lightning is liable to occur.

CAPE index of lightning risk

• < 1000 -Slight
• 1000 - 2500 - Moderate
• 2500 - 3500 - Very
• > 3500 - Extremely

CAPE is usually used by people at sea. Sailing without checking CAPE can be dangerous. It's a great way of predicting how much lightning an area will get and Windy have a fantastic CAPE setting so check it out.

(Thanks to MailASail for some info)

Please keep in mind that CAPE index, as a measure of instability, is only one of the factors to consider when you forecast the thunderstorms. @Gkikas-LGPZ sums it up nicely.

However, instablility on it's own is not enough to ensure thunderstorms. There needs to be a lifting factor due to orography or frontal activity. Check for other hints such as high humidity and a forecast of showers in the area.

Example: the following images show forecast for same time (25 July 2019 / 00 UTC). Although high CAPE values in the Balearic islands (West Mediterranean Sea), no thunderstorms expected.

CAPE definition

In meteorology, convective available potential energy (CAPE), sometimes, simply, available potential energy (APE), represents potential energy of atmosphere and can be indicator of forming convective clouds and storms.

CAPE is effectively the positive buoyancy of an air parcel (mass of air) and is an indicator of atmospheric instability, which makes it very valuable in predicting severe weather.

Values from 1000 to 2000 can indicate forming of moderate thunderstorms, and over 2000 severe ones.

CAPE for a given region is most often calculated from a thermodynamic or sounding diagram (known as a Skew-T log-P diagram) using air temperature and dew point data usually measured by a weather balloon.

Sounding diagram is also available on Windy, just right click the place and choose Sounding forecast from the contextual menu.

If you prefer Skew-T style sounding diagram, there’s an amazing plugin by John Kealy (project on Github).