There are different types of clouds that produce rain versus producing a thunderstorm. When it is raining, it is usually from stratus or cumulus clouds. Once the cumulus cloud grows higher into the atmosphere, it becomes a cumulonimbus; it is these clouds that produce lightning. Lightning occurs in what we call thunderstorms. To develop a thunderstorm, you need three ingredients: moisture, instability and lift. The storm cloud grows upward from continuous rising motion in the clouds and eventually can be 6 to 10 miles above the earth! Since air is cooler in the upper atmosphere, ice crystals will begin to form in the top of the cloud.
Ice is one of the main factors in whether or not a cloud will or will not produce lightning. Hence, the greater the height of the cloud, the more likely it is to produce lightning. The ice crystals will vary in size but they will continue to be in a rising and sinking motion within the cloud. This motion will eventually incite collision of the ice crystals. The positively charged crystals will rise to the top of the thunderstorm while the negatively charged crystals remain more in the middle and bottom of the cloud.
Now, there are a few different ways that we can produce the lightning from the charges in the cloud. The mostly commonly noticed form is cloud to ground lighting. Essentially, the negatively charged particles will send a charge to the ground (in less than a second) which is invisible to the human eye; this is called a step leader. When the step leader gets close to a positively charged object (i.e. house, tree, pole), a channel will develop. The transfer of electricity in this channel is what we call lightning!
Even on a clear day it is possible to see rain and lightning. Most lightning that occurs is negatively charged; however, there is such a thing as positively charged lightning. This lighting usually occurs in the cirrus anvil of a cloud, usually is 5 to 10 miles away from the rain core of the storm. Positively charged lightning is more dangerous for a few reasons. First, away from the main body of the storm it could be sprinkling or barely raining at all, giving a perception that lightning cannot occur. Secondly, positively charged lightning typically has a longer duration and has a higher electrical current.
There inevitably has to be a cloud to have rain. This is a fact, but sometimes, the cloud may not be over you, giving the observation that the rain is coming from nothing. If there are strong winds, the rain could be falling from the cloud near you and be blowing in your direction, but if you look up the sky above you could be clear.
In closing, only cumulonimbus clouds produce lightning and the main factor in whether or not lightning occurs is ice. There are two types of lightning, negatively charged and positively charged. Positively charged lightning is more dangerous because it branches from the anvil which in some storms produces little or no rain giving the perception that rain is coming from a clear sky.
Meteorologist Kristen Connolly