Atmospheric Convection Model - Explanatory Narration
Gottfried Mayer, 06/10/25
Today we want to study a laboratory model of atmosphere and learn where the wind comes from and why there is weather.
Does anyone know what wind is? (Moving air)
Can we see the air? (No)
How do we know it is there (We can feel it when it moves relative to our body.)
Imagine you are in the mountains and have an accident and a rescue helicopter tries to land and the pilot wants to know if it is windy and which way the wind blows. How can you show the pilot the wind conditions at the place where you are?
(Make smoke, start a fire)
To study wind we could go to the mountains and observe smoke or we can build a little model of the mountain and the air above the mountain.
In science we often try to study complex systems by building simple models that share some important properties that we are interested in with the real system.
Here our model takes a small slice of a mountain (represented by sand) and air (represented by water)
How can water be a model of air? It is a liquid not a gas and it is much heaver than air. (Scientists found out that both water and air move in similar ways, governed by the same mathematical equations, both are fluids)
How can we make the movement of the water visible? (we can squirt color into the water).
[We squirt food color from the top corners at an angle towards he center]
The movement of the water comes to an end, so there is no wind.
What is a situation in the mountains when we can expect no wind? (early in the morning)
Now we want to model he effect of sunlight on the atmosphere. Sunlight has both visible light as well as heat radiation. We call heat radiation Infra Red or IR. Here we have a heat lamp that will serve as a model of the sun.
[Let everybody feel the heat of the heat lamp radiation].
Now we want to see what happens when the sun warms the mountain.
[Point the heat lamp towards the sand "mountain"]
What do you think will happen? Nothing happens? Well that teaches us that in science we have to learn to be very patient.
[We observe a plume of water that slowly rises]
We know (e.g. from hot air balloons) that warm air or water rises.
The space that is left behind by the raising air will be filled by air that flows in from neighboring areas. If we stand on the mountain we will feel that air as wind.
What happens if to the warm air as it rises up through layers of colder air? It cools down and if it was very humid, he water condensed ad clouds are formed. That is why paraglider pilots as well as eagles look for clouds because they know that underneath there is a thermal that will lift them high up into the sky.
We can also see that at the warm air stops rising it will flow either to the right or to the left. Small differences in the airflow at the bottom can determine if a certain volume of air will move to the left or to the right. This difference can be amplified in the real atmosphere and might determine weather patters two weeks later. That is the reason why people talk about the "Butterfly Effect" of systems that are as unpredictable as the weather: A butterfly could change the airflow rising from the ground and thereby influence the weather at a far away place some time later.
The model that we just studies corresponds to a slice of atmosphere above a single mountain, maybe 1km wide and 1km high.
Now we want to change the perspective and look at a model of a layer of atmosphere that is maybe a square of 20km * 20km.
If we look at the earth from a spaceship or from an airplane that flies very high, we can recognize that the atmosphere actually is a very thin and fragile layer around our globe.
Now we want to demonstrate what the effect that we just studied at a single mountain would look like at a larger area. We need to remember that the visible light from the sun can easily pass through our atmosphere and warm the earth. But the IR heat radiation that is sent back from the warm Earth into outer space can be easily trapped in the atmosphere, especially if it contains greenhouse gases (CO2, methane, water, etc).
In our model that means we can simulate the effect of sunshine by shining IR light at the layer of water from below. The water contains very fine aluminum particles and is called "rheoscopic fluid". That is why we can see the flow patters in the fluid.
As we shine the IR on the bottom of the fluid layer and observe from above we observe that first individual spots appear where a "thermal" of warm fluid was formed and reached the top layer.
Very quickly these individual spots coalesce and form complex "cloud patterns".
It is important to remember that this is a simple model of how the sun moves the air and forms cloud-like patterns. But of course the real atmosphere is much more complex and bigger than the patch represented by our model. Also the earth is rotating and thereby creating the Coriolis effect that will influence the air movement. But most importantly we have the complicated transition between evaporation and condensation in the real atmosphere which is the reason why our understanding of the real atmosphere and how it couples with land and ocean is still poorly understood. As a result computer simulation of weather and climate still produce results that reflect the butterfly effect and change widely between models and simulations.
Once we understand the science of clouds better we can hope to obtain better results from climate simulations.
Clouds are also very important in influencing the climate because they can both create shadows on the Earth and thereby reduce global warming but they can also trap the IR heat radiation and thereby increase global warming. Details about the nature of the clouds (altitude, brightness, size of water or ice particles, etc) will dramatically influence their role for global warming. Once we understand that better we can also start thinking about how we can create conditions that will favor the formation of he type of clouds in areas that are most important for climate change.