Thunderstorms need warm and moist air to develop. The warm air at the surface creates an unstable environment for the air to rise rapidly.The moisture in this warm air cools and condenses into a cloud, so the more moisture and warm air at the surface, the higher the cumulus cloud can build vertically into a towering cumulus cloud. Check out our Cloud Types blog post for more information on clouds.
If a lifting mechanism is present – a passing front, air moving horizontally over mountains, or even afternoon heat from a hot and sunny day – this warm and moist air rises quickly during the Developing Stage. Rain usually does not yet develop as the storm is still developing during this roughly 10 minute process. While the moisture in the cloud may start to form rain droplets, the updrafts are stronger than the weight of the raindrops and keep the drops suspended in the cloud, where they collide with other drops to become larger and heavier.
When these drops become heavier than the strength of the updraft, the storm has entered the Mature Stage. The rain falling down and pulling cold air with it is called the downdraft. Downdrafts can drastically reduce the surface temperature, even on a hot summer afternoon. As the downdraft hits the ground, it spreads horizontally and can produce a strong gust front with severe-strength straight-line winds exceeding 50 mph. As long as the updraft continues to feed warm air and moisture into the storm, the storm will continue strengthening with lightning, possibly hail, and even tornado development in the strongest storms.
However, in most cases, the downdraft will cut off the supply of warm air to the storm after 20 minutes or so. This marks the Dissipating Stage, with reduced rainfall and less frequent lightning, but strong winds remain a danger. The whole process ranges on average from 30 to 60 minutes. However, in severe thunderstorms, the cycle is longer because the structure is a bit different. When the storm is vertically slanted, the downdrafts will not cut off the inflow of warm air, which can allow the storm to last for hours.
Image Credit: NWS Preparedness Guide