Why Do Thunderstorms Form?
The thunderstorm is a fascinating and occasionally frightening display of the power of nature, able to generate torrential rain, damaging wind, and flux-capacitor powering lightning. Every year, there are an estimated 16 million thunderstorms worldwide, with about 2,000 occurring somewhere in the world at any given time.
For those of us who live in temperate climates, thunderstorms are nearly synonymous with summer weather. An afternoon seems incomplete without the rolling of thunder somewhere in the distance. But where do they come from, and why? Over human history, cultures have usually invented deities to explain the thunderstorm's origins.
Today, meteorologists have much more rational explanations for these fascinating and powerful natural phenomena.
The Ingredients of a Thunderstorm
There are three basic ingredients necessary for a thunderstorm to form:
- moisture in the atmosphere as water vapor;
- unstable air - a tendency of warm surface air to keep rising when lifted upward;
- a "nudge" - something that causes uplift of a packet of air, such as a low-level cold front or mountain range.
Atmospheric instability is perhaps the most critical of these ingredients. When air masses are stable warm air will cool as it rises and sink back down toward the surface. When an air mass is unstable, an uplifted parcel of warm moist air will remain warmer than the surrounding dry air and continue to rise.
As anyone who has flown in an airplane knows, air temperature decreases with altitude. As the warm air parcel rises through these colder layers of air, it also begins to cool. This causes the water vapor to condense into water droplets, forming a cloud. The condensation process releases heat - known as latent heat of vaporization - into the air parcel, warming it and causing it to continue rising. Eventually the rising air and water droplets will cool to the freezing point, again causing a release of latent heat that allows the air parcel and crystallized water droplets to continue to rise. Even though the air parcel has reached the freezing point, it will keep rising as long as it is warmer than the surrounding air.
The towering cloud produced in this process is known as a cumulonimbus cloud. The rising motion of the air parcel is known as an updraft. This updraft creates a low-pressure area beneath the developing storm cloud, feeding it with additional moisture and warm air. With these ingredients in place, the young storm is ready to advance to the next stage in its life cycle.
The Thunderstorm Life Cycle
There are three stages in the life cycle of a thunderstorm cell: the developing stage, the mature stage, and the dissipating stage. In the developing stage, a warm moist air parcel rises high into the atmosphere and forms a towering cumulonimbus cloud. At this stage, there is no precipitation, but the storm may begin to produce thunder and lightning. The storm will not produce rain until it reaches the mature stage.
The rising air of the growing storm cloud will eventually reach an altitude known as the tropopause - the boundary between the troposphere and the stratosphere. Above this boundary, air temperature no longer decreases with height. Since the surrounding air is no longer getting colder with altitude, the cooling, rising air parcel will eventually reach a height where it is the same temperature as the surrounding air. At this point, it stops rising and begins to spread laterally as it is fed by the updraft of warm moist air from below. This lateral spreading of the top cloud layer is what produces the characteristic anvil shape of a storm cloud. The storm has now reached the mature stage.
The ice crystals circulating at the top of this anvil are kept aloft by the rising updraft from below, growing bigger as they accumulate more and more water. Eventually the ice crystals grow too big and heavy to be held up by the updraft, and they fall out of the cloud. When the falling ice crystals reach the warmer temperatures at lower altitudes, they melt on the way down and fall to the ground as rain. If the updraft is strong enough, however, the ice crystals can grow so big that they are still frozen when they hit the ground - this is known as hail.
The falling rain also cools some of the air in the cloud, causing it to push downward. This is known as a downdraft, and is the cause of the strong winds at ground-level that accompany a thunderstorm.
While the storm is in the mature stage, the updraft at the front of the storm will continue to feed the downdraft at the back of the storm. Eventually, however, the cool wind produced by the downdraft will spread out at ground-level so much that it cuts off the supply of moisture and warm air to the updraft. With no more moisture and heat fueling it, the storm will enter the dissipating stage and begin to die out. For a single storm cell, the entire process from start to finish usually takes about thirty minutes.
What Causes Lightning?
In much the same way that wool socks accumulate an electrical charge when shuffled across a carpet, the ice crystals and water droplets in growing cumulonimbus cloud will accumulate static electricity as they bump into each other and circulate through the air. Some of the ice crystals lose electrons, accumulating a positive charge, while others gain electrons, accumulating a negative charge. The exact cause this difference in electrical charge is still a matter of debate by atmospheric scientists, but the result is unarguably dramatic.
A bolt of lighting happens when these positive and negative charges build up so much that they create an electrostatic discharge - a sudden flow of electrons from the area of negative charge to the positively-charged area. A lightning bolt is, essentially, a scaled-up version of the spark discharged between a wool-clad carpet walker's finger and a metal doorknob.
The scale-up is pretty massive, however. A typical lightning bolt can stretch for more than 1.2 miles (about two kilometers) and heat the air to temperatures of 30,000 degrees Fahrenheit (54,000 degrees Celsius) - five times hotter than the surface of the Sun. The loud boom of thunder accompanying a lightning strike is actually a shock wave produced by this channel of superheated air as it tries to explosively expand, slamming into the cooler air surrounding the bolt.
The peak power of a single lightning strike is about one trillion watts, or one terawatt - enough to power a 100-watt light bulb for three months. Though about 75% of lightning strokes are cloud-to-cloud lightning that never reach the ground, the cloud-to-ground strokes that do reach the ground can be extremely dangerous. Lightning strikes kill an average of 60 people a year and injure 300 others, and can severely damage electronic equipment or set off forest fires.
Types of Thunderstorms
There are four main types of thunderstorms, varying in method of formation and severity.
Single Cell storms are isolated thunderstorms forming from a single parcel of hot, moist air. Also known as "pulse" storms, these are short-lived storms that are mild in severity. Although high winds or small hail can occasionally be produced by a single cell storm, this is extremely rare.
Multi-cell Clusters occur when weather conditions in an area are able to produce multiple storm cells, as the name implies. These can last much longer than a single cell storm, as a multi-cell cluster will continue to propagate new storm cells as older ones drift downwind and dissipate. When wind speeds are low and the rate of cell formation is high, these storms can produce dangerous flash flooding.
Squall lines are a line of thunderstorms, often produced at the leading edge of a cold front. These lines of storms can extend for hundreds of miles, with new storms continually propagating along the edge of the squall line. These storms can produce damaging winds and hail, and occasionally spawn tornadoes. When a squall line travels more than 240 miles, it is known as a derecho.
Supercell storms are the most dangerous type of storm, capable of producing 100 mile-per-hour winds, golf ball-sized hail, and strong tornadoes. Supercells are giant, well-organized single-cell storms produced when wind conditions at higher altitudes vary from surface winds, causing the storm system to twist like a corkscrew.
Whether a single storm or a supercell, the thunderstorm is a fascinating example of the power of nature. The simple ingredients of temperature and water, under just the right conditions, combine to produce these destructive, yet beautiful natural events.
Sources and Further Information
- Severe Weather: NOAA Watch
Thunderstorms, Tornadoes, and Lightning - Nature’s most violent storms.
- NOAA: Questions and Answers about Thunderstorms
A thunderstorm is a rain shower during which you hear thunder. Since thunder comes from lightning, all thunderstorms have lightning.
- Updrafts/Downdrafts - University of Illinois
All thunderstorms require instability (potential) and lift. The lift is the mechanism that releases the instability. Lift is produced by such things as fronts and low pressure troughs, or by air rising upslope.
- Lightning and Atmospheric Electricity Research at GHCC
A Lightning Primer: Characteristics of a Storm
- Thunderstorms Observation Techniques
Most people are able to recognise thunderstorms from their typical characteristics of lightning and thunder. Thunderstorms are not 'objects' that move.
- Storm Prediction Center
Severe weather information from the Storm Prediction Center.