What is the Keraunic Level?

The keraunic level, also known as thunderstorm frequency, is a meteorological and physical parameter. It describes the average number of days per year on which thunderstorms are recorded at a specific location or in a specific region.

The keraunic level is subject to significant regional variations — ranging from less than 1 in the Arctic and Antarctic to as high as 180 near the equator. If local keraunic level indices are plotted on a map, areas of constant levels — lines of equal frequency, known as isokerauns — can be identified.

Depending on this and on additional criteria from the risk analysis for buildings (see point 2), the use of surge protection devices may be recommended or even mandatory.

The term is derived from the Greek word keraunós, meaning “lightning” or “thunderbolt.”

Figure 1: Global distribution of lightning frequency in terms of annual strikes per square kilometer (Reference: NASA/GHRC/NSSTC Lightning Team)

1. Defenition and Meterological Measurement

In meteorology and standardization (particularly according to VDE and IEC), the thunderstorm threshold (Tk) is strictly defined: A day is considered a thunderstorm day as soon as at least one distinct clap of thunder is heard at the relevant measuring station.

The following criteria are taken into account:

  • Acoustic perception:
    Historically, detection has been based on hearing thunder, since sound waves in the atmosphere provide reliable confirmation of a lightning discharge (whether ground-to-cloud or cloud-to-cloud).

  • Independence of intensity:
    It does not matter whether a brief heat thunderstorm with a single flash of lightning and clap of thunder passes through on a given day, or whether a severe weather system lasting several hours does—the day counts as a thunderstorm day.

  • Long-term averages:
    Since the weather varies greatly from year to year, the Keraunic level is calculated as a statistical average over a long observation period (usually 10 to 30 years).

2. Electrical Engineering Significance and Risk Analysis

In electrical engineering—particularly in the field of lightning and surge protection—information on the number of thunderstorm days alone is not sufficient. Designers and engineers need to know the probability that, for example, a building, an overhead power line, or a cellular tower will be struck directly by lightning.

The Keraunic level serves as the basis for calculating the ground lightning density (Ng). The ground lightning density indicates how many lightning strikes per year result in impacts per square kilometer (n/km²*a).

In Central Europe, an empirical approximation formula is often used to estimate the ground lightning density (Ng) from the keraunic level (Tk):

Ng ≈ 0,1 * Tk

In regions with more severe thunderstorms (e.g., the tropics or mountainous areas), the factor may shift, for example, to:

Ng ≈ 0,04 * Tk hoch 1,25

Based on this calculated lightning density, a risk analysis for buildings is conducted in accordance with the DIN EN 62305-2 (VDE 0185-305-2) standard. This analysis determines whether a lightning protection system (external and internal lightning protection, surge protection) is absolutely necessary and which protection class (I up to IV) must be selected.

3. Geographic Distribution

The Keraunic level is extremely dependent on geographic and climatic factors:

  • Central Europe (Germany, Austria, Switzerland):
    In Germany, the long-term average keraunic level typically ranges between 15 and 35. The distribution shows a clear gradient: While the coastal regions in the north (e.g., Schleswig-Holstein) experience thunderstorms less frequently (Tk ≈ 15), the low mountain ranges and, in particular, the Alpine foothills in the south (e.g., Upper Bavaria, Lake Constance region) exhibit significantly higher values (Tk ≈ 30 to 35). In the Swiss Alps or parts of Austria (Styria), even higher values are reached locally.

  • Global Hotspots:
    Worldwide, the highest levels of lightning activity are recorded in tropical and subtropical regions. In parts of Central Africa (the Congo Basin), South America (Venezuela, Catatumbo), and Southeast Asia (Indonesia), the rate ranges from 150 to over 200 thunderstorm days per year.

  • Global minima:
    In the polar regions and over the cold ocean currents, the keraunic level tends toward zero because the thermal convection (upward movement of warm, moist air masses) necessary for thunderstorms is absent there.

4. Modern Developments: From the Keraunic Level to Lightning Detection

Although the Keraunic level remains established as a reference value in many technical standards and reference manuals, in modern practice it is increasingly being supplemented or replaced by precise data from digital lightning detection systems (such as ALDIS/BLIDS).

Modern antenna systems detect the electromagnetic waves emitted during a lightning discharge. This allows the actual ground lightning density (Ng) to be measured today with kilometer-level accuracy and in real time, rather than having to estimate it using the Keraunic level and empirical formulas. Nevertheless, the Keraunic level remains important worldwide as an easily understandable, historical, and climatological reference value.