Thunderstorms
Thunderstorms are convective storms with lightning and thunder, powered by rising moist unstable air and capable of heavy rain, hail, damaging wind, and tornadoes.
What thunderstorms are
A thunderstorm is a storm with lightning and thunder, usually produced by a tall cumulonimbus cloud. It begins when warm, moist air rises, cools, and condenses into cloud droplets. If the rising air remains buoyant, the cloud can grow deep enough for rain, ice particles, lightning, gusty wind, and sometimes severe weather.
The three ingredients
Thunderstorms need moisture, instability, and lift. Moisture supplies water vapor for clouds and rain. Instability lets air keep rising after it is lifted. Lift can come from surface heating, fronts, sea breezes, mountains, outflow boundaries, or low pressure systems that push air upward.
Life cycle of a storm
A typical thunderstorm has developing, mature, and dissipating stages. In the developing stage, an updraft builds a towering cloud. In the mature stage, rain and hail begin falling and a downdraft spreads cool air near the ground. In the dissipating stage, the downdraft cuts off the warm inflow that fed the storm.
Lightning and thunder
Inside a growing storm, collisions among ice particles help separate electric charge. When the charge difference becomes large enough, lightning discharges between cloud regions, between clouds, or between cloud and ground. Thunder is the sound wave from air rapidly heated and expanded by lightning.
Single cells, lines, and supercells
Some thunderstorms are short-lived single cells. Others organize into multicell clusters, squall lines, or large mesoscale convective systems that can last for hours. Supercells are long-lived storms with rotating updrafts; they can produce very large hail, damaging wind, flash flooding, and tornadoes.
Hazards
Thunderstorms can produce dangerous cloud-to-ground lightning, heavy rain and flash flooding, hail, straight-line wind, microbursts, derechos, and tornadoes. A storm does not need to be officially severe to be dangerous, especially for people outdoors, on water, on roads, or near trees and power lines.
Watches, warnings, and safety
A severe thunderstorm watch means conditions are favorable for severe storms in and near the watch area. A severe thunderstorm warning means severe weather has been reported or indicated by radar and people should act. The safest response is to move indoors, away from windows, and avoid plumbing, wired electronics, trees, water, and open fields during lightning.
Why they matter
Thunderstorms matter because they are both ordinary and high-impact. They bring needed rain and help move heat through the atmosphere, but they can also cause flash floods, power outages, crop losses, aviation delays, wildfires from lightning, and life-threatening severe weather.