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A storm of heavy rain accompanied by lightning, thunder, wind, and sometimes hail. Thunderstorms occur when moist air near the ground becomes heated, especially in the summer, and rises, forming cumulonimbus clouds that produce precipitation. Electrical charges accumulate at the bases of the clouds until lightning is discharged. Air in the path of the lightning expands as a result of being heated, causing thunder. Thunderstorms can also be caused by temperature changes triggered by volcanic eruptions and forest fires, and they occur with much greater frequency at the equatorial regions than in polar regions.