Lightning is essentially an electrical discharge through gases. During the course of formation and segregation of clouds, some clouds lose electrons and acquire positive charge and others collect those electrons and become negatively charged.
Due to the electrostatic repulsion of similar charges, the static electricity is condensed more on the surface zones of the clouds. During rain, the clouds move very swiftly causing frequent close encounters. When clouds holding positive and negative static electrical charges approach one another, they constitute to form capacitors regionally.
A capacitor discharge its accumulated charge once its potential and the charge accumulated reach their limits. The discharge is sudden and occurs in a short span of time. Essentially, the situation is like a built-in high voltage CR (Capacitor-Resistor) circuit in which charge flows(discharges) through the clouds(resistor) once the capacitor is maximally charged.
During the course of the discharge that takes place in ( say t) seconds, there is a current I, flowing across the clouds which have a high potential difference, V between them.
Under these circumstances, the electrical energy is converted into heat, which is quantitatively equal to iVt. Even if the time t is short, the voltage V and the current i are in scales of thousands and hence the heat liberated amounts to temperatures of thousands of degrees Fahrenheit.
The temperatures are greater than even those on the surface of the sun (10,000 degrees Fahrenheit). The heat liberated devastates molecules and particles of the discharged portions of the clouds into a plasma state, which emits bright polychromatic(white) radiation (lightning light) and sends intense pressure variations in the atmosphere that results in the exploding loud noise(thunders).
Since the discharge through charged capacitors(clouds) is sudden and the current and voltage are very large, the heat liberated reaches suddenly to een 50,000 degrees Fahrenheit during lightning.
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