When we say " We have heavy rains in winter.", why wouldn't we say " We have so much or too much or a lot of sun in June etc."? Similarly, when we can say " There's a lot of rain in winter.", why can't we say " There is a lot of sun today etc."?
Let me see if I can work out the slight difference. It's true that we might say "There's a lot of sun in the summer" but to me that means "There are a lot of sunny days in the summer" which is a little different. I think it's just the use of "today".
You're right - I might say "There was a lot of rain today" because rain is sort of measurable! I'll be honest, I can't really explain why I probably wouldn't say "There's a lot of sun today" - maybe it's just an anomaly with sun.
There's a lot of snow today. (It is snowing a lot today)
There's a lot of rain today. (It's raining a lot today)
There's a lot of cloud today. (It's very cloudy today)
There's a lot of fog today. (It's very foggy today)
BUT... for some reason, I would say "It's very sunny today" and not "There's a lot of sun". The only thing I can think is that the other weather types are sort of "tangible" (and also changeable), whereas the sun is just always there and it's either visible or it's not.
I don't think that was a very good explanation, for which I apologise!!!