Author: SW101

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....

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...

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...

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. The fundamental physical phenomenon behind thunderstorms is convection. This...

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...

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...

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,...

1.1.2. Atmospheric instability

1.1.2. Atmospheric instability

As air near the ground heats, it becomes more and more unstable and becomes more and more likely to rise convectivelly. We now take a look at when the atmosphere is stable, less stable and unstable, what that actually means and how convection initiates.                     

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...

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...