it’s on izzy :

https://apt.izzysoft.de/fdroid/index/apk/com.yoshi.rain

you can enable their repo on f-droid, see :

https://apt.izzysoft.de/fdroid/index.php

btw, omWeather is also a cool light one :

https://f-droid.org/en/packages/org.woheller69.omweather/

yeah, one of my fav

there is also “rain”, also lightweight an nice

512 m ram dunno, but I had an old 10 yo+ dual core / 4 gb ram laptop on arch working well, except for gaming. Just replaced it with ghostbsd, also fluid.