All thunderstorms start with moisture and rising warm air. This typically occurs on a warm summer’s day when the longer hours of sunshine have heated the ground. The warm, moist air immediately above the ground is less dense than the cooler, dryer air above, causing it to rise.
As it lifts, the water vapour it contains begins to cool and condense into water droplets. This causes a cloud to form and also releases heat, giving the air a boost as it continues its ascent, creating a powerful upward current.
Within about 30 minutes or so, a towering thundercloud (cumulonimbus) builds up, reaching heights of up to 10km. Beyond this altitude, the temperature no longer decreases with height, meaning that the rising mass of air is no longer buoyed upward. Instead, it spreads out, producing the anvil shape that’s typical of storm clouds.
As more water vapour condenses out inside the cloud, the water droplets merge and grow, while ice particles also form and combine in the freezing, upper reaches of the cloud. Once the water droplets and ice particles are heavy enough, they begin to fall as rain or hail. In their wake, they generate a current of cold air rushing downward, spreading out and causing strong winds at ground level, as well as a drop in temperature.
Meanwhile, collisions between ice crystals and water droplets inside the cloud knock electrons off the water droplets and lighter ice crystals and transfer them to the bigger ice particles.
As the heavier, negatively charged particles sink, and positively charged particles rise, the top and bottom of the cloud accumulate opposite charges. This creates a voltage (‘potential difference’), which, if high enough, can discharge in what’s known as ‘intra-cloud’ lightning (we see this as sheet lightning).
The negatively charged cloud base also repels electrons on the ground, creating a positive charge there. This potential difference can discharge in a blaze of ‘cloud-to-ground’ lightning. The rapid heating and expansion of the surrounding air causes the characteristic rumble of thunder that accompanies the flash of light.