COLORADO SPRINGS, Colo (KOAA) — Weather forecasting is challenging, making it rare to find a single number that can tell you most of what you need to know about any type of weather setup. Convective Available Potential Energy, or CAPE, is an exception.
CAPE is a single number that -with a few caveats- tells our forecast team how likely lightning is above your house. It measures how unstable the atmosphere is by calculating the difference in density between warm, wet air near the ground and cold, dry air further up.
Colorado has some of the best ingredients in the country for thunderstorms. Thanks to the Rocky Mountains, there is plenty of cold, dry air in the upper atmosphere. When warm, wet air close to the ground sits below this cold mountain air, it rises upward like a ball floating in water because it is less dense.
A bigger CAPE number means the warm air is warmer and the cold air is colder, creating a bigger contrast. Higher CAPE numbers tell us directly how unstable the atmosphere is in a given place and how strong a thunderstorm can get. There is no other single number that gives us so much useful information about storm potential. Notably - we can even see this on special vertical graphs, so we can visually determine where in the atmosphere instability is strongest.
We can divide CAPE units into different categories to understand storm potential -
- Between zero and 500 J/kg: Very weak showers with limited lightning.
- Between 500 and 1,500 J/kg: Typical thunderstorms with lightning and small to perhaps quarter-sized hail.
- Between 1,500 and 2,500 J/kg: Widespread severe storms are possible. Most storms that form in this environment will produce significant hail in Colorado.
- Above 3,000: Major severe weather events with very large hail, strong wind, and nearly continuous lightning.
CAPE tells us how much air wants to rise. Big CAPE numbers indicate the air wants to rise violently and aggressively, showing how fast the air will move in a thunderstorm. Powerful updrafts fling water above the height at which airplanes fly, where the air is very cold. This creates lots of ice crystals that crash into each other and spark lightning. Those violent updrafts also hold up ice in the clouds, which produces hail.
While CAPE is not the only ingredient needed for severe weather, it does tell you how strong storms can be if they form. CAPE is literally the "thunderstorm fuel" our team references on-air. Like any fuel, you still need a spark. For a thunderstorm, that's anything that lifts the air enough to get to a place where CAPE is present. A front can do that. So can regular old solar heating on a warm sunny day. An upper level low can also cause air to rise. So it's possible to have CAPE and not get a thunderstorm. But what CAPE does tell you is how strong a storm's updraft can be at a given time if you can get a storm to form.
Colorado's unique environment is critical for turning a strong updraft into a monster storm. This requires a second ingredient: wind shear, which is a change in wind speed and direction with height. While wind shear is hard to get in most of the country (routinely), it happens quite a lot in Colorado. The mountains naturally tend to create an upslope flow on summer afternoons near the ground, while the air in the upper part of the atmosphere flows in from the west. This regular change in wind direction helps us use available storm energy more effectively.
The upshot is: when we forecast, we are not just looking at simulated radar; we are looking for where the pockets of energy are strongest, because that is where the storms form.
This story was reported on- air by a journalist and has been converted to this platform with the assistance of AI. Our editorial team verifies all reporting on all platforms for fairness and accuracy.
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