Heat lightning is a flash of lighting which appears near the horizon, typically without any accompanying thunder. It is caused by distant storms, and it is commonly associated with summer storms, when the temperatures are warm, hence the “heat” in the name. This type of lightning can be seen in many regions of the world, especially in areas where summer storms are common, and in mountainous regions. It is not the same thing as cloud lightning, also called sheet lightning, a form of lightning which discharges inside a cloud, causing the cloud to light up.
Sometimes, heat lightning isn't lightning at all, but a cloud reflection of lightning from a very distant storm. In other instances, it is regular lightning which has formed far off in the distance, making a flash instead of a recognizable lightning bolt. Especially when dark clouds are massed near the horizon, heat lightning can really stand out, and the absence of thunder can make it seem very eerie. It sometimes takes on a reddish cast, thanks to reflected and bent light.
There are a number of reasons why heat lighting is often unaccompanied by thunder. In the first place, sometimes the storm is simply too far away to hear thunder. In other instances, the thunder is muffled by weather conditions. Particles in the air and heavy clouds can muffle the sound of thunder, and thunder can also be disrupted by large land masses such as mountains. In mountain areas, heat lightning may come from a storm which is actually very close by, but the acoustics alter the sound waves, muffling or redirecting the thunder. Lightning without thunder can seem somewhat surreal and it can be very startling without the audible cue of thunder.
Sometimes, heat lightning indicates that a storm is on the way. If storms usually come from the south and lightning is spotted in the south, for example, it means that the storm may drift north, bringing lightning, thunder, and rain to the observer. In other cases, the distant storm will never reach the observer, although it may put on a show.
Like other forms of lightning, heat lightning can be dangerous, primarily to people who do not realize how close the storm is. While watching heat lightning can be interesting, it is a good idea to go inside or take shelter in case the storm moves into the area, and people who are visually impaired may appreciate a warning from friends and neighbors who have spotted the lighting.