Cumulonimbus: the thunderstorm cloud

A cumulonimbus can stretch from 1 km above the ground to the tropopause, more than 10 km up, in a single tower.

From cumulus to storm

Every cumulonimbus starts as an ordinary cumulus. If the air is unstable, meaning a rising parcel stays warmer than its surroundings, the parcel keeps rising. Condensation releases heat, which keeps the cloud buoyant, and the tower climbs higher and higher. The WMO describes the result as a heavy and dense cloud of considerable vertical extent, in the form of a mountain or huge towers. Its base is low, often very dark, and its top reaches into the high level.

Why the top goes flat

At some point the rising air meets a lid. In the troposphere the temperature falls with height, but above the tropopause, in the stratosphere, it rises again. NOAA explains that this warmer air on top stops convection, and that the bottom of the stratosphere is readily seen by the anvil-shaped tops of cumulonimbus clouds. The rising air spreads sideways under the lid and the wind stretches it into the anvil, which can run downwind for a long way.

How high the lid sits depends on latitude. NOAA gives the height of the troposphere as 18 to 20 km over the equator, about 9 km at 50° north and south, and just under 6 km over the poles. Temperatures fall from about 17 °C on average at the surface to about −51 °C at the tropopause. So a storm in the tropics can tower far higher than one near the poles, and the ice in the upper part of the cloud is extremely cold.

Thunder, lightning and hail

Inside the tower, strong updrafts carry droplets far above the 0 °C level, where some freeze. Collisions between small soft hail (graupel) and ice crystals separate electric charge (NOAA NSSL), and when the charge grows large enough, lightning jumps within the cloud or to the ground. Thunder is the sound of air heated suddenly by the lightning channel. Hail starts when updrafts carry raindrops up into extremely cold parts of the cloud, where they freeze (NOAA NSSL); the ice keeps growing by collecting supercooled droplets until it is too heavy for the updraft to hold. Heavy showers, gusty winds and sometimes tornadoes also come from cumulonimbus.

Lightning reaches farther than the rain. NOAA warns that it can strike more than 10 miles (about 16 km) from where it is raining, so if you can hear thunder you are already within striking distance. Its advice is to go indoors at the first thunder and stay there until 30 minutes after the last clap. For people who cannot hear thunder, NOAA added a companion rule in 2016: see a flash, dash inside. Forecasters follow these storms on radar, and NOAA's dual-polarisation radars can tell rain, hail and snow apart inside the cloud.