When you download a weather forecast from Predictwind or a Grib, you can also download the CAPE values that are predicted. These are very important to have and understand if you are in tropical or other areas that generate thunderstorms.

CAPE or Convective Available Potential Energy, is a measure of how much energy is stored in the atmosphere and in the sea, which is a big heat sink and therefore energy source.

CAPE is like a fuel tank, it specifies how much fuel is available for any forming Thunderstorm. Now its important to note, a high CAPE doesn’t mean you’ll get thunderstorms, anymore than a low one means you won’t.

What it means is that IF you get a thunderstorm how much power it will have. Think of it as the fuel tank and a forming Thunderstorm as the match… if you have both high CAPE and a forming thunderstorm…. seek shelter quickly.

So what is high?

CAPE is measured in Joules/Kg and a figure below 1000J/Kg will see you having more likely rain storms than thunder and lightening. Although that is not guaranteed, you may get a short lived thunderstorm at those levels but they are unlikely to be too powerful.

Above 1000 you should exercise caution above 2000J/KG you should be seeking shelter quickly, above 3000J/Kg people are likely to die if they are not well prepared.