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Buoyancy and phase changes in water

Convection is driven by buoyancy. So 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, or any pressure. Keep this concept in mind.

The fundamental physical phenomenon behind thunderstorms is convection. This is when a less dense fluid (liquid or a gas) rises within a more dense fluid.

BUOYANCY

 

Buoyancy is an upward force experienced by an object immersed in a fluid. The force is equal to the weight of the fluid the object displaces. If the object is denser than the fluid, it sinks – it is negatively buoyant. If it is lighter, it rises up and floats on the surface – it is positively buoyant.
Let us see some examples.

A simple example of positive buoyancy is to place a piece of wood on water. Wood is less dense than water. Therefore it floats. Buoyancy counteracts the weight, and the net force on the piece of wood is zero.

If you submerge a block of wood below the surface and release it, it will buoyantly rise to the surface. Buoyancy is larger than the piece’s weight. Therefore, there is an upward net force that makes the wood rise.

Hot air balloons are another example of buoyancy, much closer to the driving force of thunderstorms. Hot air balloons are filled with – hot air. The hotter the air (or any gas), the more it expands and the lower its density. As the hot air (Tb) in the balloon is less dense than the cooler (Ts) surrounding air, it is positively buoyant and forced to rise – taking the balloon and passengers with it*.

weather-baloon-buoyancy

A hot air balloon uses gas burners to heat the air. In nature, daytime warming of the air near the ground causes the warm air to rise buoyantly.

* – minor nitpick: while any air parcel with a temperature higher than the surrounding air buoyantly rises, in the example with the balloon, the warm air needs to be sufficiently warmer than the surrounding air for the buoyancy to overcome the weight of the balloon.

Now, back to that air parcel. If the air parcel is warmer than the surrounding air, it will – the same as the air in the hot air balloon – buoyantly rise.

It will keep rising as long as it remains buoyant in the surrounding air.

Recap: So far, we have discovered that a parcel of warm air rises when surrounded by colder air. This is because warm air is less dense than cold air and rises due to buoyancy. This process is called convection.

Now, we will look at how water in the air behaves and how it influences convection.

PHASE CHANGES IN WATER

 

In solid (ice), liquid, and gas forms, water is present in the atmosphere. To understand weather and thunderstorms, we must look at how water behaves.

Water changes states from vapor 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 vapor (steam), you need to add energy (heat).

water-phase-changes

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

You need to add heat to melt ice and evaporate water. In the other direction, heat is released as water condenses from vapor to liquid and freezes from liquid to ice.

So if water droplets condense from vapor, heat is released, warming the air. Heat is also released if water droplets freeze into ice, warming the air. On the other hand, if ice (snowflakes or hail, for example) melts, heat is absorbed from the air, cooling it. If water droplets in the air evaporate, heat is absorbed, cooling the air – this process is called evaporation cooling.

There is also a shortcut: water can transition directly from solid to vapor – this process is called sublimation. Energy-wise, it is the same as other processes: it takes energy to evaporate ice, and energy is released when vapor goes directly into ice.

Recap: Phase changes of water can release or absorb heat. Heat needs to be added to get ice to melt into liquid water or to get liquid water to vaporize (i.e., heat is absorbed by the water). When water vapor condenses into liquid water, heat is released. When liquid water freezes into ice, heat is also released.