1.1. Cauliflower clouds

Not all clouds produce thunderstorms, in fact, fairly few do so. You can spot clouds that indicate a thunderstorm may develop, is developing or has developed, just by looking at their general appearance. They look like cauliflower.

Seriously.

Cauliflower.

Let us take a look at how and why thunderstorms form.


                    

Background image created by Mrsiraphol – Freepik.com

1.1.1. Convection

But before we go into detail, we need to take a few steps back and look at the big picture. To understand thunderstorms, we first must dip our toes into underlying physical phenomena. These are phenomena you encounter in your every day life, often probably without even realizing it. You are certainly familiar with most of them, and in this introduction we will see how they influence the weather and thunderstorms. So before we talk about thunderstorms, we will talk about the environment they form and live in – our atmosphere.

The following two sections (1.1.1 and 1.1.2) will be a bit more technical: some physical processes and properties, but we will not delve into equations. Knowing these is very helpful for understanding of how clouds and thunderstorms work. You can skip these and go straight to clouds and thunderstorms in section 1.1.3.


                    

1.1.1.1. The atmosphere – where do thunderstorms live?

THE ATMOSPHERE

The thin shell of gas around our planet that we call our atmosphere is divided into several layers. While the pressure falls with increasing altitude, the temperature is more dynamic, alternately decreasing and increasing with altitude. The layers are:

  • Troposphere [surface – ~12 km]
  • Stratosphere [~12 – 50 km]
  • Mesosphere [50 – 80 km]
  • Thermosphere [80 – 700 km]
  • Exosphere [700 – 10 000 km]

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Virtually all the weather happens in the troposphere and thunderstorm clouds are (mostly) confined to the troposphere. Therefore, all phenomena discussed here take place in the troposphere unless specifically stated otherwise.

Fun facts:

  • The edge of space – the Karman line – is at 100 km height, within the thermosphere
  • The highest ever balloon ride and parachute jump was made by Alan Eustace in 2014: he rose up to 41.419 m, and made a safe parachute jump.
  • Meteors appear in the thermosphere and mesosphere, usually between 130 and 70 km. Some have been detected as high as almost 170 km, and some penetrate do less than 20 km above the ground, well within the stratosphere.

Now that we have seen the structure of our atmosphere, let us take a look at the part of atmosphere thunderstorms (and we) live in.

1.1.1.2. Buoyancy

We will first define an air parcel. We are going to be using this term a lot. An air parcel is an imaginary volume of air. It can be at any altitude, any temperature, any pressure. Keep this concept in mind.

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The fundamental physical phenomenon behind thunderstorms is convection. This is a process in which less dense fluid (be it a liquid or a gas) rises within a more dense fluid.

Convection is driven by buoyancy. Buoyancy is an upward force experienced by an object immersed in a fluid. The force is equal to the weight of the fluid displaced by the object. If the object is denser than the fluid, it sinks – the object is negatively buoyant. If it is lighter it rises up and floats on the surface – it is positively buoyant.

Let us see some examples.

1.1.1.3. Phase changes of water

Phase changes in water

Water is present in the atmosphere in solid (ice), liquid and gas form. To understand weather and thunderstorms, we need to take a look at how water behaves.

Water changes states from vapour to liquid to solid and vice versa. The change from one state to another is called a phase change. To change water from solid (ice) to liquid (…water) to vapour (steam), you need to add energy (heat).

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Fun fact(s): you need the same amount of heat to melt a kilogram of water ice at 0 °C as you need to heat the same kilogram of melted water from 0 °C to 80 °C. It takes a lot of heat to melt ice! It takes even more heat to evaporate the same amount of water: it takes over 6 times as much energy to evaporate 1 kg of boiling water than to melt 1 kg of water ice!

1.1.1.4. Convection

Convection

As we have seen, warmer air has a lower density than colder air. This makes it buoyant, making it rise. This is what makes hot air balloons rise. If we take a parcel of warm air and put it into cold air, it will rise. If the warm rising air does not cool and / or the surrounding air does not warm, the parcel of warm air will rise indefinitely.

This is obviously not the case. The rising warm air cools and eventually comes to the same temperature as the surrounding air. It then looses buoyancy and ceases to rise. The way in which the rising air cools is a very important factor in meteorology.

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As the warm air rises it cools adiabatically. This means the air cools only due to pressure drop and expansion, the exchange of heat with the surrounding air is negligible. A rising parcel of air cools by about 10 °C (9.8 °C more precisely) for every kilometer it rises. This value, 9.8 °C/km, is called the dry adiabatic lapse rate. This a very rapid cooling rate, causing dry air to come to the temperature of surrounding air quickly.


                    

1.1.1.5. Moist convection

Moist convection

Warm *moist* air, however, behaves somewhat differently. This difference is crucial for the formation of thunderstorms. As the parcel of warm, moist air rises it initially cools by the dry adiabatic lapse rate, i.e. it cools by about 10 °C (9.8 °C) for every km it rises.

However, as it cools, the moisture the air contains begins to condense. As you may recall, the water going from vapour to liquid (to water droplets) releases heat, warming the air. Therefore moist warm air cools less quickly as dry warm air. As it rises and cools, more and more moisture condenses into water droplets, releasing latent heat and slightly warming the air. Additionally, as the moist air rises, water droplets begin to freeze, releasing more latent heat and slightly warming the air further.

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The rate at which rising moist air in which moisture has already began condensing, cools with height is the moist adiabatic lapse rate: it cools on average by less than 6 °C (typically 5.5-6 °C) for every kilometer it rises. So rising moist air cools only about 2/3 as fast as dry air.

Interesting fact: the moist adiabatic lapse rate is not constant – it depends on the temperature and pressure. At 1000 mbar and 20 °C it is 4.3 °C/km, at 0 °C and 600 mbar pressure it is 5.4 °C and -20 °C and 400 mbar pressure it is 7.3 °C/km.

1.1.2.1. Stable and unstable atmosphere

Stable and unstable atmosphere, instability

Atmospheric stability is the resistance of the atmosphere to vertical motion of air. A stable atmosphere inhibits vertical motion. An unstable atmosphere encourages vertical motion. The stability depends on how the air temperature changes with altitude (the temperature lapse rate).

  • Very stable: temperature increases with altitude, a temperature inversion. Air at ground level is negatively buoyant and does not rise. If air is forced to rise, it will sink back towards the ground again.
  • Stable: temperature lapse rate is less than the dry adiabatic lapse rate (i.e. it falls less than by 9.8 °C for every kilometer you go up), but temperature does decrease with altitude. Air at ground level is negatively buoyant and does not rise. If air is forced to rise, it will sink back towards the ground again.
  • Neutral: temperature is the same as the dry adiabatic lapse rate. I.e. if air is forced to rise, it will cool at the same rate as the temperature around it drops. It will be neutrally buoyant.
  • Unstable: temperature lapse rate is greater than the adiabatic lapse rate (9.8 °C/km). Air at ground level is buoyant and will rise.
  • Very unstable: temperature lapse rate is much greater than the adiabatic lapse rate (9.8 °C/km). Air at ground level is buoyant and will rise rapidly.

1.1.2.2. Initiation of convection – how thunderstorms start (daytime heating, fronts, etc)

Initiation of convection

On a warm day, convection begins when the air close to the surface warms enough to become buoyant and start rising. This process is called free convection. The temperature at which free convection begins is called the convective temperature. When the convective temperature is reached on a clear, hot and humid day, the atmosphere becomes widespread explosive development of thunderstorms begins. The height at which moisture in the rising air condenses is called the convective condensation level (CCL).

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Air parcels at the surface buoyantly lift and require no other mechanisms to start lifting. There are other ways of getting air parcels at the surface to begin lifting, even if the air near the surface is stable (it is below convective temperature). There needs to be another other lifting mechanisms that pushes the air to a height where it becomes buoyant and begins to rise. The height at which moisture in the rising air condenses is now called the lifted condensation level (LCL)