THUNDERSTORM BASICS

Life cycle of a thunderstorm cloud

A thunderstorm is caused by a large vertical cloud mass (cumulonimbus cloud, often called a "Cb" or "thunderhead") that has a flat, dark base from which heavy precipitation is falling.When not obscured by haze or low clouds, the top of a Cb is seen as bright and tall, reaching up to an altitude of 10-16km. The top may be boiling with "crunchy" detail but more often has a fibrous, frozen appearance.

A thunderstorm is a three-dimensional structure but it should be thought of as a constantly evolving process rather than as an object. Each storm cell or cluster is a self-contained system with an organized pattern of updrafts (rising air) and downdrafts (sinking air) that affects the wind flow around it. This system's control over and interaction with the prevailing winds of that day show themselves in changing cloud features which you can see and interpret.

On a typical summer storm day, the sky will have only small cumulus around until early afternoon. A few may become briefly larger, but their tops soon evaporate or their compact structures dwindle. At some point, the number of clouds decreases and fewer, larger ones remain. Several may sprout tall towers or a group may form and grow larger. Then, as if the lid has come off the pot, a "boiling-over" stage begins and a bigger, steadily growing cloud mass emerges. This is the first breakout, in which organized, sustained updrafts take over from weaker, random ones. In 10-20 minutes, the developing Cb will form a short anvil cloud and become a young thundershower.

This first cell probably has a single updraft at its core but there will already be signs of the next stage to come.As the first large cloud matures, new cloud towers form beside it, usually on an axis extending to the S or SW (the upwind side,in the direction the clouds are moving from).This is called the flanking line. As heavy rain descends to the ground,the updraft region in the initial cell is shifted back (upwind) along the flanking line such that the next developing cumulus tower grows to become the dominant one. Towers along this baseline lead into and merge with the storm centre of action, replacing it as they mature. Individual clouds blossom and die but the storm system carries on. It has become a separate, evolving process that survives by regeneration.

The cloud material is carried along by that day's prevailing winds but the local circulation around the storm maintains an orderly balance between the warm inflow air and the cool downdraft outflow air. The inflow to a storm is that air which enters the cloud system to become updrafts. Outflow is the air which comes from downdrafts inside or near a storm. Most regular storms have several cells which maintain them for a half to one hour before cold downdrafts smother the warm updraft and cause the system to collapse entirely.

Anatomy of a storm

Every Cb has a spreading anvil top, a core region, and an inflow-outflow region. Each of these basic parts of the cloud has something to tell us about the storm's strength, age, motion, and degree of organization.

The anvil is a flat, often fibrous cloud sheet which forms at the top of the Cb and spreads ahead of the storm. It consists mostly of frozen particles that have been blown forward by the stronger winds high in the atmosphere. The anvil arrives first, thickening from a white veil to a smooth grey sheet.

Other, lower clouds may arrive along with rumbles of thunder and lighter rain. With stronger storms, rain will become heavier and may be mixed with hail as the clouds darken near the storm's centre. Other, lower clouds may arrive along with rumbles of thunder and lighter rain.

4 W/E An anvil cloud overspreads the sky ahead of the distant,dark,rain core. Note the dark cloud band lower left,indicating inflow and updrafts are on the south side.

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Meteorological Service of Canada - Environment Canada - Government of Canada

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