Can We Make a Cloud in the Laboratory?
Meteorology Beyond the Syllabus: Making a Cloud — Why Does Air Suddenly Become Visible?
Look up at almost any British sky and there is a good chance that you will see clouds.
We become so accustomed to them that it is easy to forget just how extraordinary they are.
A cloud can contain an enormous quantity of water, yet remain suspended in the atmosphere. It can appear seemingly from nowhere, grow rapidly, disappear again, or develop into something capable of producing torrential rain, hail or snow.
But perhaps the most interesting question is much simpler:
Why can we see a cloud at all?
There is water vapour in the atmosphere around us virtually all the time. If water vapour makes clouds, why isn't the air around us permanently white and misty?
The answer takes us into some fascinating meteorology involving temperature, pressure, humidity, condensation and the behaviour of rising air.
Better still, we can demonstrate much of it in the laboratory.
Water Vapour Is Invisible
The first misconception worth tackling is one that I regularly encounter when teaching science:
water vapour is not the white material that we see above a boiling kettle.
Water vapour is water in its gaseous state, and it is invisible.
The visible white mist above a kettle consists primarily of tiny liquid water droplets that have formed after the invisible water vapour has cooled and condensed.
That distinction is enormously important when we start thinking about clouds.
A cloud is not simply a mass of water vapour.
It consists of vast numbers of microscopic liquid water droplets, ice crystals, or a mixture of the two.
So our real question becomes:
What makes invisible water vapour suddenly condense into visible droplets?
Let's Make a Cloud
One of the things I particularly enjoy about teaching science in a laboratory is being able to turn an apparently enormous natural phenomenon into something that can be investigated on a bench.
We obviously cannot fit a thunderstorm into the laboratory.
But we can reproduce one of the fundamental physical processes responsible for cloud formation.
A classic demonstration uses a strong transparent container containing moist air. A small amount of water provides a source of water vapour. A suitable method is then used to increase and subsequently reduce the pressure.
There are several versions of the experiment, including commercially produced cloud chambers and demonstrations using robust pressure-rated transparent vessels.
Important safety point: pressure demonstrations should only be performed with apparatus designed to withstand the pressure differences involved. Ordinary glass jars or improvised containers should not be pressurised.
The dramatic moment occurs when the pressure is suddenly reduced.
A faint white cloud can appear inside the container.
Increase the pressure again and it may disappear.
Reduce it again and the cloud returns.
That immediately raises a much more interesting scientific question.
Why?
It Isn't Simply the Pressure
It is tempting to say:
"Reducing the pressure makes a cloud."
But that skips the most interesting physics.
When a parcel of air expands rapidly, it has to do work on its surroundings. If there is insufficient time for much heat to enter from outside, the expansion is approximately adiabatic.
As the air expands, its temperature falls.
This is called adiabatic cooling.
The chain of events is therefore approximately:
Pressure falls -> air expands -> air cools -> relative humidity rises -> saturation is reached -> condensation occurs -> cloud droplets form.
That sequence is one of the keys to understanding real weather.
Relative Humidity — What Does 70% Actually Mean?
Weather forecasts frequently give a figure for relative humidity.
Perhaps:
Relative humidity: 70%
It is easy to interpret that as meaning that 70% of the air consists of water.
It doesn't.
Relative humidity compares the amount of water vapour actually present with the amount required for saturation at that temperature.
In simplified form:
Relative humidity = (actual water vapour / water vapour required for saturation) x 100%
The crucial point is that the amount of water vapour needed for saturation depends strongly on temperature.
Warm air can reach equilibrium with a larger concentration of water vapour than cold air.
Consequently, we can take some moist air, leave the amount of water vapour in it almost unchanged, cool it down and cause its relative humidity to rise.
Eventually:
Relative humidity = 100%
The air has reached saturation.
Cool it further and some of the water vapour can condense.
That is the beginning of our cloud.
The Dew Point
This introduces another weather term that students may have encountered without fully appreciating its importance:
dew point.
The dew point is the temperature to which air must be cooled, at roughly constant pressure and water-vapour content, for it to become saturated.
Imagine that the air temperature is 18°C but the dew point is 12°C.
The air is not saturated.
If that air cools towards 12°C, its relative humidity increases.
At approximately 12°C it reaches saturation.
Further cooling can produce condensation.
This explains much more than clouds.
It helps explain:
dew on grass;
condensation on windows;
mist above water;
fog;
water appearing on the outside of a cold drink;
condensation on bathroom mirrors.
The water appearing on the outside of a cold glass did not leak through the glass.
Water vapour already present in the surrounding air was cooled below its dew point and condensed onto the cold surface.
But Water Needs Somewhere to Condense
There is another part of the story.
Cloud droplets generally do not form completely spontaneously in perfectly clean air.
The atmosphere contains enormous numbers of tiny particles.
These can include:
dust;
sea salt;
smoke particles;
pollen;
biological particles;
sulphates and other aerosols.
Some of these act as cloud condensation nuclei.
Water molecules can collect around these microscopic particles and eventually produce tiny droplets.
This gives us another variable to investigate in our laboratory cloud.
Depending on the apparatus and demonstration method, introducing a very small concentration of suitable aerosol particles can make cloud formation much easier to see.
It produces an excellent comparison.
Moist air + cooling
compared with:
Moist air + cooling + condensation nuclei
The difference can be striking.
It also demonstrates an important principle of experimental science: a phenomenon may depend upon several conditions being satisfied simultaneously.
From a Laboratory Container to a Real Cloud
Now we can scale the experiment up.
Imagine a parcel of warm, moist air near the Earth's surface.
Something causes it to rise.
Perhaps the Sun has warmed the ground.
Perhaps air is being forced over a hill.
Perhaps two air masses are meeting along a weather front.
As the parcel rises, atmospheric pressure decreases.
The rising air expands.
Expansion causes cooling.
Eventually the temperature reaches the dew point.
Water begins condensing onto suitable nuclei.
Millions upon millions of microscopic droplets form.
A cloud becomes visible.
The same fundamental physics that we produced inside a transparent container is occurring kilometres above our heads.
Why Clouds Often Have Flat Bottoms
Once students understand the dew point, another familiar observation becomes much easier to explain.
Look at a group of fair-weather cumulus clouds.
They often have surprisingly flat bases.
Why should clouds forming independently have bases at roughly the same height?
Near the surface, different parcels of rising air may have broadly similar temperature and humidity.
As those parcels rise, they cool.
At approximately the altitude where their temperature reaches the dew point, condensation begins.
That produces the visible cloud base.
The flat underside of a cumulus cloud is therefore not simply an interesting shape.
It is evidence of atmospheric physics that we can actually see.
An Experiment Within the Experiment
This demonstration becomes much more useful educationally if students do more than simply watch a cloud appear.
We can turn it into an investigation.
Ask:
What conditions make the best cloud?
We could compare different starting conditions.
For example:
Investigation 1 — Humidity
Compare relatively dry air with air that has been allowed to become more humid.
Does the cloud form equally easily?
Investigation 2 — Temperature
Does changing the starting temperature affect the result?
Investigation 3 — Condensation nuclei
Compare relatively clean moist air with air containing a controlled, safe source of microscopic condensation nuclei.
Which produces the most visible cloud?
Investigation 4 — Pressure change
Using suitable pressure-rated equipment, investigate whether the magnitude or rate of pressure reduction affects the visibility of the cloud.
Now we have moved from a demonstration to genuine scientific investigation.
Measure It Rather Than Simply Watch It
This is where modern sensors can make the experiment particularly interesting.
Rather than merely saying:
"The cloud appeared when we reduced the pressure,"
we can try to measure what happened.
With suitable sensors and data-logging equipment we could record:
temperature;
pressure;
relative humidity;
time.
Plotting these against time allows students to look for the point at which the visible cloud forms.
A particularly interesting graph would show pressure and temperature changing together.
We should observe that rapid expansion is accompanied by a temperature decrease.
That gives us direct experimental evidence for the physical process we are trying to explain.
For an A-level student, this is considerably more valuable than simply memorising the phrase "rising air cools."
We can actually make air expand, measure its temperature and watch the cloud appear.
Can We Measure the Dew Point?
There is another simple experiment that complements the cloud demonstration beautifully.
Take a shiny metal container containing water and gradually cool it by adding ice while monitoring the temperature.
Watch the outside carefully.
Eventually a faint film of condensation begins to appear.
Record the temperature.
That provides an experimental estimate of the dew point of the surrounding air.
Allow the container to warm again and note the temperature at which the condensation disappears.
Repeating the experiment and taking several measurements gives a better estimate.
We have now turned a term from a weather forecast into something measurable in the laboratory.
Why Fog Is Really a Cloud at Ground Level
Once we understand cloud formation, fog becomes much less mysterious.
Fog is essentially a cloud forming at or very close to the Earth's surface.
Instead of air necessarily rising thousands of metres before cooling sufficiently, the air near the ground reaches saturation.
One familiar mechanism occurs on clear nights.
The ground loses thermal radiation and cools.
Air close to the ground is then cooled.
If its temperature falls to the dew point, water can condense into tiny suspended droplets.
Visibility falls.
Fog forms.
This is why cool, clear and relatively calm nights can sometimes produce mist or fog the following morning.
Mountains Can Make Clouds
The same idea explains why clouds frequently form around hills and mountains.
Moist air moving towards high ground can be forced upwards.
As it rises:
pressure decreases -> air expands -> temperature falls.
If the air cools to its dew point, cloud forms.
Continue the process and precipitation may follow.
This is orographic uplift, and it helps explain why mountainous regions can have very different rainfall patterns on opposite sides of the same range.
Suddenly our small laboratory cloud is helping us understand entire landscapes.
Clouds Are Evidence of Moving Air
One of the most useful changes in thinking is to stop regarding clouds simply as objects.
A cloud is often better regarded as evidence of a process.
It can tell us something about:
where air is rising;
where air is cooling;
where saturation has been reached;
atmospheric stability;
moisture distribution;
fronts and convection.
This is why learning only a catalogue of cloud names misses much of the interesting science.
Yes, recognising cumulus, cumulonimbus, cirrus and stratus is useful.
But it is far more powerful to look at a cloud and ask:
What must the atmosphere be doing to produce that?
A Connection With Sailing
Meteorology becomes particularly interesting when it affects something you actually do.
As a sailor, I am constantly interested in what the sky is telling me about the atmosphere.
Clouds are not merely scenery above the boat.
Developing cumulus can indicate convection. A growing cloud can reveal active rising air. Changes in cloud structure may accompany changes in wind, approaching rain or the arrival of different air.
On inland water, where the wind can already be strongly influenced by trees, banks and surrounding terrain, watching the sky adds another source of information.
The laboratory demonstration therefore connects very naturally with a much larger skill:
learning to read the atmosphere rather than merely receiving a weather forecast.
A Connection With Flying
The same physics matters enormously in aviation.
A rising parcel of moist air can eventually reach its condensation level and form cloud.
Pilots and meteorologists therefore care about quantities such as:
air temperature;
dew point;
cloud base;
atmospheric stability;
humidity;
pressure.
The difference between air temperature and dew point can provide useful information about how close the atmosphere is to saturation.
A simple school laboratory experiment has therefore taken us into real operational meteorology.
Why Doesn't the Cloud Immediately Fall?
This raises another excellent student question.
If a cloud consists of liquid water droplets, why don't they immediately fall?
The answer is largely one of scale.
Cloud droplets are extremely small. Their terminal velocities can consequently be very low, while atmospheric turbulence and upward-moving air can help keep them suspended.
But droplets can collide and combine, and ice processes can also cause precipitation particles to grow.
Eventually some become sufficiently large that gravity wins.
Then we get rain.
So there is another fascinating progression:
water vapour -> condensation -> cloud droplets -> droplet/ice growth -> precipitation.
A visible cloud is only one stage in a much larger atmospheric process.
Why This Goes Beyond the Syllabus
Students studying GCSE and A-level science encounter many of the ingredients separately.
They learn about:
changes of state;
gas pressure;
energy transfer;
latent heat;
particles;
temperature;
convection;
specific heat capacity.
But meteorology provides an opportunity to combine those ideas into a real physical system.
That is one reason I enjoy taking science beyond the formal syllabus.
The objective isn't simply to give students more facts to remember.
It is to show them that the topics they study are connected.
Physics does not stop at the edge of the physics textbook.
Chemistry does not stop when the chemistry lesson ends.
Biology, physics, chemistry, geology, geography and mathematics all meet when we try to understand the real world.
Meteorology is an excellent example.
Questions I Would Ask Students
After producing our laboratory cloud, I would resist the temptation simply to explain everything immediately.
Instead, I might ask:
Why did the cloud appear when the pressure fell?
Was it the pressure itself that caused the condensation?
What happened to the temperature?
Where did the water in the cloud come from?
Why was that water invisible before?
Why might condensation nuclei be necessary?
Would the experiment work as well with very dry air?
Why do clouds form when air rises over a mountain?
Why can a cold glass become wet on the outside?
Why does fog often form overnight?
These questions require students to connect observations with mechanisms.
That is a much deeper form of learning than memorising a definition.
A Cloud Is a Physics Experiment Happening Above Us
Perhaps the most impressive thing about this experiment is how ordinary the phenomenon initially seems.
We see clouds almost every day.
Yet explaining why one exists requires us to think about pressure, temperature, energy, phase changes, humidity, microscopic particles and atmospheric motion.
A cloud is therefore not simply something floating in the sky.
It is visible evidence that the atmosphere is changing.
Somewhere, air has cooled sufficiently for invisible water vapour to become microscopic droplets or ice crystals.
And once students have produced that process themselves in the laboratory, they may never look at a cloudy sky in quite the same way again.
Good science education should do more than teach us the names of things.
It should make us look at an everyday phenomenon and suddenly realise that there is an experiment taking place in front of us.
Sometimes that experiment is happening on the laboratory bench.
And sometimes it is several kilometres above our heads.
Try This Question
Next time you see a cloud forming, don't begin by asking:
"What type of cloud is that?"
Instead ask:
"What is the air doing that has caused that cloud to exist?"
That question takes us from learning meteorology to actually thinking like a meteorologist.
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