CAPE: A Key Parameter for Storm Forecasting
When storm clouds start to form, especially in the summer, meteorologists often focus on one key parameter: CAPE (Fig. 1). The abbreviation CAPE (Convective Available Potential Energy) refers to the amount of energy in the atmosphere that can be converted into upward air motion, making it an essential indicator in thunderstorm development. Higher values mean that a given location is likely to experience severe storms, heavy rain, and even hail. Monitoring CAPE (see map) can therefore be a key step in predicting when and where severe weather might strike.
In summer, certain parts of the surface heat up more quickly than their surroundings (for example, an asphalt area compared to a forested surface). A strongly heated surface also warms the adjacent air mass. The hot air above such a surface has a lower density than the relatively cooler surrounding air, so according to Archimedes’ principle, it can rise (similar to the principle of a hot-air balloon). As the rising warm air cools, condensation occurs. However, latent heat is released during condensation, so under the right conditions, the rising air parcel remains warmer than the surrounding air up to the tropopause. This condensation process leads to cloud formation. The magnitude of CAPE depends on the temperature difference between the rising air parcel and the surrounding air. The greater this difference, the higher the CAPE values, and the stronger the updrafts, which increases the risk of large hail in particular.
In addition to CAPE, other parameters are also crucial for developing thunderstorms. Besides sufficient atmospheric moisture, an initiation mechanism is required — for example, a frontal boundary or a convergence line. At the leading edge of frontal boundaries, especially cold fronts, air parcels are forced to rise, which significantly contributes to the formation of thunderstorm clouds. In the absence of a frontal boundary, thunderstorms may form mainly over mountainous areas. For the development of severe thunderstorms in particular, wind shear is also an important factor (see map). Wind shear refers to the difference in wind speed and direction between the surface and higher altitudes (e.g., at 6 km).
CAPE values are expressed in J/kg. In typical thunderstorm situations, CAPE values reach several hundred J/kg, but in extremely unstable environments they can rise to several thousand J/kg (see map). However, as mentioned earlier, when forecasting thunderstorms, it is important to consider not only CAPE but also other parameters such as atmospheric moisture, the presence of frontal boundaries, and wind shear. It is also valuable to look at outputs from multiple models. On Ventusky, CAPE forecasts are available from models like GFS, ECMWF, ICON, and others (Fig. 2).
Higher CAPE values mean a greater risk of large hail formation. With high CAPE, the updraft speeds within thunderstorm clouds are stronger. Faster updrafts allow hailstones to remain suspended in the cloud longer, enabling them to grow larger. Due to climate change, the amount of water vapour—and thus energy—in the atmosphere is increasing. In the future climate, CAPE values are expected to be generally higher, increasing the frequency of severe hail events, especially those involving large hailstones.
