![]() What is a “bolt from the blue”? A “bolt from the blue” is a cloud-to-ground flash which typically comes out of the side of the thunderstorm cloud, travels a relatively large distance in clear air away from the storm cloud, and then angles down and strikes the ground. The NOAA Storm Prediction Center routinely issues forecasts for dry lightning because this kind is more likely to cause forest fires. What is dry lightning? Dry lightning is lightning that occurs without rain nearby. Thunder can seem like it goes on and on because each point along the channel produces a shock wave and sound wave, so what you hear as thunder is actually an accumulation of multiple sound waves from the different portions of the lightning channel. ![]() The disturbance is a shock wave for the first 10 yards, after which it becomes an ordinary sound wave, or thunder. The high pressure air then expands outward into the surrounding air compressing it and causing a disturbance that propagates in all directions away from the stroke. This energy heats the air in the channel to above 50,000° F in only a few millionths of a second! The air that is now heated to such a high temperature had no time to expand, so it is now at a very high pressure. The bright light of the lightning flash caused by the return stroke mentioned above represents a great deal of energy. What causes thunder? Thunder is caused by lightning. Lightning can also be seen in volcanic eruptions surface nuclear detonations, and in heavy snowstorms (“thunder snow”). ![]() Is lightning always produced by a thunderstorm? Thunderstorms always have lightning (thunder is caused by lightning, and you can't have a thunderstorm without thunder!), but you can have lightning without a thunderstorm. Sometimes this is called “heat lightning” because it occurs most often in the summer. However, it is possible that you might see lightning and not hear the thunder because it was too far away. Thunder starts as a shockwave from the explosively expanding lightning channel when a large current causes rapid heating. Is it possible to have thunder without lightning? No, it is not possible to have thunder without lightning. Negatively-charged lightning channels in particular do not propagate continuously, but in relatively short “steps” where the air ahead is becoming ionized as multiple low-conductivity “streamers.” A streamer that develops more current and better conductivity can become the next step that connects to the “leader” channel. What is a “stepped leader?” A stepped leader is the development of the downward lightning channel. What are cloud flashes? A cloud flash is lightning that occurs inside the cloud, travels from one part of a cloud to another, and some channels may extend into clear air. It can be seen in volcanic eruptions, extremely intense forest fires (pyrocumulonimbus clouds), surface nuclear detonations, heavy snowstorms, in large hurricanes, and obviously, thunderstorms. Lightning is one of the oldest observed natural phenomena on earth. Lightning from thunderstorms begins in a strong electric field between opposite charges within the storm cloud, and can stay completely within the cloud (intra-cloud lightning) when the charge regions are similar strength (balanced) or can reach the ground (cloud-to-ground lightning) when one of the regions is much stronger than the other (unbalanced). This current flow temporarily equalizes the charged regions in the atmosphere until the opposite charges build up again. (The actual breakdown process is still poorly understood.) The air breakdown creates ions and free electrons that travel down the conducting channel. ![]() When the opposite charges build up enough, this insulating capacity of the air breaks down and there is a rapid discharge of electricity that we know as lightning. In the early stages of development, air acts as an insulator between the positive and negative charges in the cloud and between the cloud and the ground. Severe Weather 101 Frequently Asked Questions About Lightning What is lightning? Lightning is a giant spark of electricity in the atmosphere between clouds, the air, or the ground.
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