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The Science of Thunder and Lightning
Episode Five: What is a dry thunderstorm?

   Written by: LEE Lap-shun

    Thunderstorms are usually accompanied by rain.  However, not all thunderstorms are wet and this seemingly weird weather phenomenon is called dry thunderstorm.  Let's see how this happens.

    Dry thunderstorm refers to thunder and lightning that occur without bringing rain to the ground.  In fact, the thunder-bearing clouds do produce rain but the rain droplets have evaporated in the air before reaching the ground.  This is possible if the clouds are sufficiently high and the humidity of air between the clouds and the ground is low enough.  One example is lightning originating from the spreading top of a cumulonimbus cloud, which is often called the anvil cloud and is distant from the main body of thunderstorm where rain pours heavily.  The anvil cloud is so high that rain coming from it evaporates before reaching the ground.  However, lightning from the anvil cloud is capable of traveling through the dry air and reaching the ground.

    Dry thunderstorm is very dangerous to people on the ground as one may not be aware of the sudden occurrence of lightning in a rain-free area.  It is also the culprit of many wildfires.  Without any rain to wet the grasses and trees, dry thunderstorm can trigger a fire more easily and the flames can spread more quickly.

    The person on the ground experiences lightning but no rain
    because the rain droplets have evaporated before reaching the ground (Objects in the diagram are not to scale)

Last revision date: <21 Dec 2012>