From: Steve (Primus) (Avatar) 30/10/2001 5:43:27 Subject: re: STEVE'S WEATHER FAQ post id: 476404 High humidity is a pre-requisite for lightning??? Perhaps, in a sense, but you are looking at it the wrong way. Thunder is the sound of the shock wave caused by lightning heating the air very quickly. Lightning is caused by the potential difference between different parts of a cumulonimbus cloud or between the cloud and the ground. Development of a cumulonimbus or thunderstorm cloud depends on three things. (1) An abundant supply of moisture (this is where the humid air comes in) (2) Instability through a great depth of the atmosphere - so that once the air is forced to rise it will continue to do so and (3) a trigger - something to get the air rising in the first place. A thunderstorm is a complex weather processor. When the rain starts falling out of the cloud it has defined edges because the cloud itself has defined edges. You will usually see the lightning and hear the thunder before the storm reaches you. When the lightning and thunder are close together, the storm is very close and the rain and/or hail are likely to start very quickly. You can now track thunderstorms on the Bureau of Meteorology website at http://www.bom.gov.au just click on radar images. Here is an example http://mirror.bom.gov.au/products/IDR283.shtml 128km Grafton Radar Raindrops vary in size from about 0.5 to 6mm. Smaller than 0.5mm they are considered to be drizzle. I do have a formula at work but I remember that terminal velocity is proportional to the square root of the diameter. Whether or not the raindrop will reach terminal velocity depends on the height from which the rain is falling in the cloud. The rain does not start falling at the cloud base but from above the freezing level. This varies according to the season and the latitude, but rain in the tropics will fall further than rain in Melbourne in winter.