As the title suggests, I'm curious to know, approximately how many photons are emitted in a single lightning strike?
[Physics] How many photons are emitted by a lightning strike
estimationlightningphotonsthermal-radiation
Related Solutions
The wikipedia article is quite good on this subject.
For any discharge in the air the molecules of the air must be ionized. This ionization happens during thunderstorms because of the high static electric fields carried by the clouds which generate "streamers", i.e. paths for the electrons to flow downwards. Corresponding streamers are formed by conductors and high points on the ground with positive charge again generated by the high fields of the storm cloud, the positive ions flow upward and the path for a discharge is set.
A lightning flash terminates [& discharge occurs] on a tree while an un-attached streamer is visible on the earth surface projection to the left.
When the electric field of the storm passes over the ground, high points that are also grounded have higher fields then the ground and can form streamers. Lightning rods work, by generating upward streamers more efficiently since they are highly conducting and the field at the top is very high due to the geometry.
[Upward streamers]
When a stepped leader approaches the ground, the presence of opposite charges on the ground enhances the strength of the electric field. The electric field is strongest on grounded objects whose tops are closest to the base of the thundercloud, such as trees and tall buildings. If the electric field is strong enough, a positively charged ionic channel, called a positive or upward streamer, can develop from these points. This was first theorized by Heinz Kasemir.
As negatively charged leaders approach, increasing the localized electric field strength, grounded objects already experiencing corona discharge exceed a threshold and form upward streamers. Once any downward leader connects to any upward leader available, a process referred to as "attachment", a circuit is formed and discharge may occur. Photographs have been taken on which unattached streamers are clearly visible. The unattached downward leaders are also visible in branched lightning, none of which are connected to the earth, although it may appear they are.
Return stroke
Once a channel of ionized air is established between the cloud and ground this becomes a path of least resistance and allows for a much greater current to propagate from the Earth back up the leader into the cloud. This is the return stroke and it is the most luminous and noticeable part of the lightning discharge.
Discharge
When the electric field becomes strong enough, an electrical discharge (the bolt of lightning) occurs within clouds or between clouds and the ground. During the strike, successive portions of air become a conductive discharge channel as the electrons and positive ions of air molecules are pulled away from each other and forced to flow in opposite directions.
The electrical discharge (averaging 30 kA for negative or 300 kA for positive lightning, and travelling at around 1×108 m/s) rapidly superheats the discharge channel, causing the air to expand rapidly and produce a shock wave heard as thunder. The rolling and gradually dissipating rumble of thunder is caused by the time delay of sound coming from different portions of a long stroke
It is wise not to be swimming during a thunderstorm because the water being flat the most discontinuous conductive object will be the head. Also if during a thunderstorm in the open air one's hair becomes electrified it is wise to fall on the ground and keep away from high objects like trees.
A Faraday cage is made to form an alternate path for electricity to travel from source to ground without travelling through the object inside the cage.
So if the Cage is complete then there is no problem in touching the inner surface. That is why people wear cloths out of metallic mesh which also acts as a Faraday cage (check for EMF shield clothing). But remember the cage should be complete, which means the circuit of the cage should be complete.
Best Answer
From How Big Is A Lightning Bolt? we see that a lightining bolt is “an inch wide and five miles long”, and at “50,000 °F”. So in useful units, approximately 3 cm diameter, 8 kilometer long, 28000 K hot.
If we consider that the heat is mostly due to black body radiation (for a perfect black body with an emissivity of $\epsilon = 1$), then the power will be given by the Stef-Boltzmann law:
$$P = A \epsilon \sigma T^4$$
The area, $A$ of the lightning bolt (a cylinder, of course) is given by
$$A= 2 \pi\times(3 \text{ cm})\times 8 \text{ km} \sim 1500 \text{ m}^2$$
And so,
$$5.2 \times 10^{13}\; \text { Watts of power.}$$
Lets say, it lasts 10 miliseconds, so its around $\sim 5 \times10^{11}$ J.
Now to calculate it the amount of photons properly, you would have to consider the spectrum of the black body radiation, and convert the energy density to number of photons using Planks law. I will just use the rule of thumb that "1 Watt of monochromatic visible light is approx $10^{18}$ photons per second".
And so, it would be around:
$$\sim 10^{29}\ \text{ photons.}$$