There is thunder and lightning, loud noise wakes you up in the middle of the night. You hear heavy rain and look out of the window. Suddenly you see a bolt coming out of the blue and striking next to a tree. Your friend and you enjoy the color and light play. The next strike is tremendous and you hear your friend saying that the lightning came from the group and went upwards. This is silly! How can a lightning bold strike from the ground upwards when every little child knows the bold comes from the cloud and moves towards ground?
Unfortunately the show came to an end and you cannot proof him wrong – or was he right?
Much light. Such wow!
Let’s best honest. Thunderstorms can be a very pleasant occurrence but frightening the same time. What exactly is lightening and how does it work?
In order to understand the process of lightning we need to have closer understanding of evaporation and condensation.
Evaporation is the process of a liquid turning into gaseous state. This occurs for instance when you boil water, it will turn from liquid into vapor. This process needs energy as the molecules in the liquid need more kinetic energy to break from their liquid-phase molecular forces. The same process happens to molecules on a surface. Since these molecules are the one closest to sunlight exposure, they receive enough energy to break their bonds and evaporate, turning into gas.
The opposite processes is called condensation and occurs when a gaseous phase is cooled. The steam turns into liquid state. Evaporation and condensation are essential for the water cycle which plays an significant role in today's article.
Fig.1 Transition of water into its different states Source
The process of lightning requires electrification of clouds. Lightning usually occurs in special type of clouds, so called Cumulonimbus clouds. This is a process which scientifically is not as save as houses but is commonly accepted among scientist. In the following I will try to explain how clouds can become electrified.
As already mentioned water vapor rises. The higher the vapor gets the colder temperatures it is exposed to. This is why humid air is required to develop Cumulonimbus clouds, which can rise up to 13 km high. At a certain point the water vapor may turn into ice crystals or put together with other water molecules to droplets - rain. These ice crystals are so light that they move upwards by the updrafts while downdrafts pull larger particles as water drops or soft hail down. The collision between ice crystals and hail leave the crystal with a positive charge and the droplets with a negative charge (the crystals transfer electrons to the falling, larger particles). This causes the cloud to have a negative charge at the bottom and a positive charge at the top.
This is how the electrification of clouds work. Because of the separation of charges an electric field occurs between earth and cloud and even within the cloud itself.
Fig.2 Ice crystals and soft hail colliding, causing charges within the cloud Source
Fig.3 Separation of charges in a cloud Source
The next step that occurs is that a stepped leader (-) propagates from the clouds bottom towards ground. This leader is not visible and ionizes the air, making it easier for the electric charges to flow as it creates a conductive path. The closer the stepper leader approaches towards ground, the stronger the electric field at ground level becomes. In fact there is not only one single stepped leader but man y leaders. We don't see a straight lightning bolt but several ones. This is because the air is not ionized equally. Since same charges repel and opposite attract, the ground gets positively charged. Objects on the ground form upstream leaders (+) and these moves towards the stepped leader. When both connect a discharge happens and the current can be up to 100 000 Ampere! When you are in the middle on an lightning get as low as possible and tuck your body. This might help you since the upstream leader does not continue to grow and you keep the distance between upstream and stepped leader lower
Fig.4 Stepped leader and upward stream coming together Source
There is a simple explanation for the sound of thunder. During the discharge the air gets heat up to 30 000 degree Celsius and creates a plasma. Did you know the bolt temperature is hotter than the surface of the sun? This process happens in less that one hundred millionth of a second. This extremely fast expansion of gas caused by the heat results in an shockwave which we notice in form of thunder. Did you know that you can estimate the distance of the lightning simply by counting the seconds between flash and thunder? Dividing this number by three give you the distance in kilometers! Try it out next time and lets hope you can count at least to three.
To get back to our question: Can a lightning strike go from ground to cloud? Yes, it can! When there is a tall object that is bound to earth such as skyscrapers, the initial strike will happen the other way around. A stepped leader will propagate from the ground towards clouds. There are also cloud to cloud strikes.
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