Thunder and lightning

Thunder is the sound caused by lightning. Depending on the distance and nature of the lightning, thunder can range from a sharp, loud crack to a long, low rumble (brontide). The sudden increase in pressure and temperature from lightning produces rapid expansion of the air surrounding and within a bolt of lightning. In turn, this expansion of air creates a sonic shock wave, similar to a sonic boom, which produces the sound of thunder, often referred to as a clap, crack, peal of thunder, or boom.

The cause of thunder has been the subject of centuries of speculation and scientific inquiry. The first recorded theory is attributed to the Greek philosopher Aristotle in the fourth century BC, and an early speculation was that it was caused by the collision of clouds. Subsequently, numerous other theories were proposed. By the mid-19th century, the accepted theory was that lightning produced a vacuum.

In the 20th century a consensus evolved that thunder must begin with a shock wave in the air due to the sudden thermal expansion of the plasma in the lightning channel. The temperature inside the lightning channel, measured by spectral analysis, varies during its 50 μs existence, rising sharply from an initial temperature of about 20,000 K to about 30,000 K, then dropping away gradually to about 10,000 K. The average is about 20,400 K (20,100 °C; 36,300 °F). This heating causes a rapid outward expansion, impacting the surrounding cooler air at a speed faster than sound would otherwise travel. The resultant outward-moving pulse is a shock wave, similar in principle to the shock wave formed by an explosion, or at the front of a supersonic aircraft.

Experimental studies of simulated lightning have produced results largely consistent with this model, though there is continued debate about the precise physical mechanisms of the process. Other causes have also been proposed, relying on electrodynamic effects of the massive current acting on the plasma in the bolt of lightning. The shockwave in thunder is sufficient to cause injury, such as internal contusion, to individuals nearby.

Inversion thunder results when lightning strikes between cloud and ground occur during a temperature inversion. In such an inversion, the air near the ground is cooler than the higher air. The sound energy is prevented from dispersing vertically as it would in a non-inversion and is thus concentrated in the near-ground layer. Inversions often occur when warm moist air passes above a cold front; the resulting thunder sound is significantly louder than it would be if heard at the same distance in a non-inversion condition

During Thunderstorms And Lightning

If thunderstorm and lightning are occurring in your area, you should:

Use your battery-operated NOAA Weather Radio for updates from local officials.
Avoid contact with corded phones and devices including those plugged into electric for recharging. Cordless and wireless phones not connected to wall outlets are OK to use.
Avoid contact with electrical equipment or cords. Unplug appliances and other electrical items such as computers and turn off air conditioners. Power surges from lightning can cause serious damage.
Avoid contact with plumbing. Do not wash your hands, do not take a shower, do not wash dishes, and do not do laundry. Plumbing and bathroom fixtures can conduct electricity.
Stay away from windows and doors, and stay off porches.
Do not lie on concrete floors and do not lean against concrete walls.
Avoid natural lightning rods such as a tall, isolated tree in an open area.
Avoid hilltops, open fields, the beach or a boat on the water.
Take shelter in a sturdy building. Avoid isolated sheds or other small structures in open areas.
Avoid contact with anything metal—tractors, farm equipment, motorcycles, golf carts, golf clubs, and bicycles.
If you are driving, try to safely exit the roadway and park. Stay in the vehicle and turn on the emergency flashers until the heavy rain ends. Avoid touching metal or other surfaces that conduct electricity in and outside the vehicle.

After a Thunderstorm or Lightning Strike

If lightning strikes you or someone you know, call your emergency number for medical help as soon as possible.

Check the items listed below when you try to give aid to a victim of lightning:
Breathing – if breathing has stopped, begin mouth-to-mouth resuscitation.
Heartbeat – if the heart has stopped, give CPR.
Pulse – if the victim has a pulse and is breathing, look for other possible injuries. Check for burns where the lightning came into and left the body. Be alert for nervous system damage, broken bones and loss of hearing and eyesight.
After the storm passes think to:
Never drive through a flooded roadway. Turn around, don’t drown!
Stay away from storm-damaged areas to keep from putting yourself at risk from the effects of bad thunderstorms.
Keep listening to a NOAA Weather Radio or local radio and television stations for news and orders, as right to use roads or some parts of the area(s) may be blocked.
Help people who may need special assistance, such as infants, children and the elderly or those with access or functional needs.
Stay away from downed power lines. Report these lines quickly to the power company.
Watch your animals closely. Keep them under your direct control.

Here are some videos from Youtube that show you how it looks like.

Credit to the owner of the video