Summer thunderstorms are often characterized by their intensity and the large quantities of water they produce over a short period of time on a territory. These events may generate floods and significant claims for insurers.

Figures 1 and 2 show typical thunderstorm cells observed in the summer, generally coinciding with hot and humid weather. Such storm cells are characterized by their relatively limited dimensions, and are roughly cylindrical. They comprise three identifiable zones illustrated on figure 3 (Chow et Als. 1988), which consist of an inflow region near the ground, where warm moist air is drawn into the cell (Zone A), an uplift region in the middle, where moisture condenses as the air rises, producing precipitation (Zone B), and an outflow region in the upper atmosphere where outflow of cooler, dryer air occurs (Zone C). Such drier air is circulated back down to the ground, where it contributes to the convective cell air circulation through the bottom of the cloud base.

Figure 1 

Figure 2 

Figure 3

Thicknesses in the order of 8 to 16 km are typical of storm cell cloud covers. Through the analysis of continuity for water vapour and related energy balance (see reference 1), it is possible to calculate that a thunderstorm cell of 5 km in diameter with a cloud cover beginning at an altitude of 1.5 km and terminating at 10 km, with conditions of ground air temperature of 30 oC and atmospheric pressure of 101.3 kPa, can produce a precipitation intensity of 9.8 cm/hour over the ground surface area of the 5 km diameter cell. This represents a volume of 535 m3/second of rainwater flowing onto that territory area.

The condensation of moisture allows the release of its latent heat which, for the volume of water involved in the above calculation, translates into a total energy of 1,335,000 MW. Comparatively, the powers of hydroelectric generating stations Manic 5 and LG2 in Quebec are 2,660 and 5,096 MW respectively. It is therefore obvious that even for a relatively limited sized storm cell, the quantity of the energy released through rain is enormous. One may appreciate the corresponding devastating effects, storm water collection networks often times not being able to manage over a short period of time the quantities of water involved.

It is also typical of flood generating thunderstorms that they occur between weather data stations on a given territory. Under such conditions, the data collected at such stations may not reveal the occurrence of these thunderstorms. Investigations conducted by the insurers following a loss must then resort to collecting data available through the manifestations of these thunderstorms in the field to characterize the event.