Why Do Thunderclouds (Cumulonimbus) Grow So Tall?
― How the Updraft "Engine" Works
On a summer afternoon, a small white cloud floating in the blue sky can turn into a tower of cloud so huge you have to crane your neck to see the top, in less than an hour. This "thundercloud" is called a cumulonimbus in weather terms, and it is said to sometimes top 10 km in height. What fuels a cloud as it climbs so high?
On a hot midsummer day, have you ever seen a pure white cloud billowing up in the distance? Look away for a while, then look back, and it can seem much bigger than before, stretching upward.
Unlike other clouds, such as a thin layer spread across the whole sky, a thundercloud is a single mass that heaves upward and upward, almost like a living thing. What forces are at work inside it?
In fact, the growth of a thundercloud is said to depend on one more mechanism, beyond the rise of warm air alone.
The sun heats the ground, and the ground warms the air. Pockets of air that have become lighter than their surroundings rise as updrafts.
When water vapor in the rising air cools and turns into droplets, heat is released, warming the surrounding air further and speeding up the rise, it is thought.
Let's go through this "rising leads to more rising" mechanism step by step.
Why Does Warm Air Rise?
When air is warmed, it expands, and its weight per unit volume (its density) drops. A pocket of air lighter than the air around it rises, much as a bubble floats up through water. When strong summer sun heats the ground, the air above it is warmed one batch after another, and strong updrafts form.
Where Clouds Form: The Height at Which Rising Air Cools and Vapor Becomes Droplets
Rising air meets lower pressure higher up, so it expands and cools bit by bit. When the air cools to the temperature at which the water vapor in it starts to turn into droplets (the dew point), the vapor becomes tiny visible droplets, and a cloud begins to form. It is known that the bigger the gap between the ground temperature and the dew point, the higher this level is.
Every time water vapor in the cloud turns into droplets, heat is produced that pushes its growth along.
The "Engine": Heat Released When Water Vapor Turns Into Droplets
When water vapor turns into droplets (condenses), a heat called "latent heat of condensation" is released into the surroundings. This heat keeps the air inside the cloud warmer and lighter than the air around it. Air that stays warm keeps trying to rise, so rising and condensation repeat again and again, and the cloud stretches higher and higher.
So there is a self-accelerating mechanism: the more the air rises, the more heat is produced, and that heat pushes the air up even more. This is thought to be why a thundercloud can grow into a huge tower in just a few tens of minutes to an hour.
The top of a mature cumulonimbus sometimes spreads out into a flat shape called an "anvil cloud". This is said to happen because above a certain height, the surrounding air is a stable layer where the temperature hardly drops any further. The cloud's rising air cannot get past that layer and is thought to spread sideways instead.
Things You Can Check Yourself
- On a sunny summer afternoon, find a cloud billowing upward
- From a safe place, take a photo of the same direction every 10–15 minutes (a smartphone is fine)
- Line the photos up and compare how the cloud's height and shape changed
- Also see whether clouds in the morning differ in how fast they grow from clouds in the afternoon, once the temperature has risen
When a cumulonimbus is developing quickly, it can bring short, heavy rain, thunder and sudden gusts of wind. Watch from indoors, or from somewhere you can get indoors right away.
Summary
A thundercloud (cumulonimbus) forms when air warmed at the ground keeps rising as an updraft. When water vapor in the rising air turns into droplets, heat called latent heat of condensation is released. Through this self-accelerating mechanism, in which the heat pushes the air up even more, the cloud is thought to grow to a huge height in a short time. When it reaches a stable layer high up, it cannot climb any further, and its top spreads sideways.
The sheer power of a thundercloud is not the wind simply carrying it along. The cloud is being pushed up from the inside by heat it makes itself.
Heavy rain that this kind of cloud drops over mountains can make a river suddenly swell in a sunny place far away. How this happens, and how to protect yourself, is explained in this article.
This strong updraft grows not only raindrops but also ice grains. Why hail can fall even in midsummer is explained in detail in Why Do Ice Pellets (Hail) Fall From the Sky in Midsummer?
Also, this cloud is not made only of air flowing upward. Below the cloud, cooled air is falling. That is why a cold wind blows before the rain arrives, as explained in Why Does a Cold Wind Suddenly Blow Just Before an Evening Shower?
For Those Who Want to Know More ― Terms, Numbers and Links to TextbooksFrom middle-school science to topics still being researched, each item is labeled with its level
- Middle schoolCovered in middle-school science
- High schoolCovered in high-school "Basic Earth Science"
- High school+Covered in high-school "Earth Science", or treated as advanced material or a sidebar in textbooks
- UniversityUniversity-level specialist content (atmospheric dynamics) not taught in high school
- ResearchTopics researchers are still investigating, not yet taught as settled fact even at university
Middle schoolTerms: Words Around Thunderclouds
- Updraft: a flow of air that has become lighter than its surroundings and rises.
- Dew point: the temperature at which water vapor in the air starts to turn into droplets.
- Latent heat of condensation: the heat released into the surroundings when water vapor turns into droplets.
High schoolChecking With Formulas: The Height Where a Cloud Forms, and the Time to Grow
We calculate, as rough guides, the height where a cloud begins to form from the gap between temperature and dew point, and the time a cloud takes to grow from the speed of the updraft.
| In symbols (cloud base height) | h ＝ 122 ×（T － Td） |
| In symbols (growth time) | t ＝ H ÷ w |
| In words | Time taken = cloud height ÷ updraft speed |
| Where the formula comes from | Rising air cools by about 10℃ per 1000 m, while the dew point drops by only about 2℃. The gap between them shrinks by 8℃ per 1000 m |
| Height needed to shrink the gap by 1℃ | 1000 ÷ 8 ＝ 125 (m) |
| Hence the coefficient | In practice, this 125 m is used as the value 122. The time formula is "speed × time = distance" solved for time |
Height where cloud forms (m) ＝ 122 ×（temperature − dew point）
| Temperature, dew point | Both are values at the ground (℃) |
| 122 | An empirical coefficient used as a rule of thumb in weather science |
| Gap between temperature and dew point (℃) | 30 − 18 ＝ 12 |
| Height where cloud forms (m) | 122 × 12 ＝ 1464 |
| Result | About 1464 m above the ground (about 1.5 km) |
* This formula is an empirical approximation used as a rule of thumb in weather science.
| Time taken (seconds) | 9000 ÷ 15 ＝ 600 |
| Converting seconds to minutes | 600 ÷ 60 ＝ 10 |
| Result | About 10 minutes |
This is only a rough calculation, but it means a huge cloud 9000 m tall could be pushed up by the updraft in just about 10 minutes. Real thundercloud growth also depends on things like changes in the updraft's strength, so it is said not to proceed at a constant speed matching this time.
High school+Positive Feedback That Keeps Producing "Buoyancy"
When a rising pocket of air stays lighter than the air around it, experts say it "has positive buoyancy." Because latent heat is released, this buoyancy is kept up, and that is thought to be why a strong updraft like that of a cumulonimbus lasts a long time. How easily the air rises is expressed by an index that weather science calls "Convective Available Potential Energy (CAPE)."
UniversityThe Complex Movement of Air Inside a Thundercloud
In atmospheric dynamics, it is known that inside a cumulonimbus there are not only strong updrafts but also downdrafts, carried by falling raindrops, and together they make a tangled flow of air. The relationship between these rising and sinking flows is thought to affect the cloud's lifetime and how local heavy rain falls. The quantity that shows how easily air rises is called convective available potential energy, and it is read as an area on an emagram, a chart of the state of the atmosphere.
📖 Derivation of the formulas and beyond: Convective available potential energy (CAPE) ／ National Oceanic and Atmospheric Administration "JetStream" weather learning resources
ResearchWhat Is Still Unclear
- Technology to predict more accurately, and earlier, when and where a cumulonimbus will develop rapidly is still being researched in meteorology (improving forecasts of so-called "guerrilla downpours").
- Climatology is also studying how the strength and frequency of cumulonimbus development will change as global temperatures rise.
- Work is also under way on numerical simulations, using high-resolution weather radar and supercomputers, to forecast cloud development a short time ahead.
A thundercloud towering in the summer sky holds rich themes where atmospheric dynamics and climatology meet, and research continues on them today.
Links to Textbooks (by Level)
| Level | Subject / unit | Where in this article |
|---|---|---|
| Middle school | Science: how clouds form | Basic terms: updraft, dew point, latent heat of condensation |
| High school | Basic Earth Science: atmosphere and clouds | Calculating cloud-forming height and growth time |
| High school+ | Earth Science: atmospheric stability (advanced) | Positive buoyancy and the idea of CAPE |
| University | Atmospheric dynamics | Structure of updrafts and downdrafts inside a cumulonimbus |
| Research | Meteorology and climatology (under research) | Forecasting rapid development, trends under climate change |
- Explanations of how cumulonimbus clouds develop, in meteorology textbooks and materials.
- Explanations of the approximate formula for cloud-forming height, in earth science textbooks.
- Research reviews on convective available potential energy (CAPE) and cumulonimbus development, in atmospheric dynamics.
- Research reviews on forecasting techniques for local heavy rain (so-called "guerrilla downpours"), in meteorology.
- Research reviews on trends in climate change and convective activity, in climatology.
* The formula for cloud-forming height and the growth time are rough approximate calculations. Actual values are said to vary greatly with the state of the atmosphere on the day.
* This article is a science explainer for general readers. When a cumulonimbus is developing rapidly, there is a risk of lightning strikes, sudden gusts of wind and sudden heavy rain. If the sky changes quickly, move early to a safe place such as inside a sturdy building.