Now fixed (July 2018), both when I gave my answer and in the translation shown at the top of this page. (It may still be incorrectly spelled "lightening" elsewhere though.)
"Radi" is one of those many words of Arab origin. It's a N-group Class 9/10 word (i-/zi-), so singular and plural are the same word. It can be used for both a clap of thunder or lightning. Lightning might also be translated as "Umeme" (electricity). Lightening (becoming lighter in colour or weight) might come afterwards!
Asante sana BwanaSimba :) The dictionaries I am using are about only swahili and they don't take the passwords out of swahili also not sheng versions even if they complete the explanations by sheng words too. This is why sometimes I can not to find the informations as in case of " radi". Looking at the " quality" of this course the surching it is my daily duty what can make really tired.l =D
Mpendwa wangu, I know a few good resources that might be helpful to you: for a good free course google scribd.com kiswahili mwanasimba (also in french); for dictionaries google: somabiblia.files.wordpress.com kamusi swahili english pdf (both sw-en and en-sw); also TUKI Kamusi at elimuyetu.co.tz/subjects/arts/swa-eng/a.html (both sw-en and en-sw); and weltladen-moemlingen.de reichenbach swahili english pdf. This last one is very old but is like a bible for Swahili. Succes!
Asante sana BwanaSimba :) I will look every of materials you propose. Learning Swahili it is a little bit like an adventure - something really different but it makes me fun.
Is well that BwanaSimba writes about mistake. Not everybody are perfect english speakers and some can be confused. By the way, somebody knows about " radi" what is the class and if it has plural ?
This really needs to change. Lightening does not work when you are talking about weather. Some of the questions, "lightning" works, and some it doesn't work.