Heavily raining or Raining heavily?

It seems that fewer adverbs can precede the verb to rain. When the adverb "describes" the way in which the rain falls or its quantity, it almost always follows the verb (even if it is a metaphor - see e.g. 3):

• In the year 553 it rained violently in Scotland for five months. (1)
• It rained steadily, clouds resting upon the very treetops. (2)
• The blows rained thick on the chest of the prostrate man... (3)

Short adverbs referring to the frequency of the rain are more commonly used before the verb:

• Here are boundless plains where it seldom rains, and you’ll maybe die of thirst (4)(inspirassion)

• It rarely rains in Hollywood - and I love the rain. (Danielle Campbell)(5)

However rain + frequently seems to be more common than the other option:

• The air is on that account moist and unhealthy, and it rains very frequently. (6)

It was very interesting to find that this was the case with other "meteorological" verbs like snow (heavily), blow (heavily).

Looking up heavily, I found that it is almost always used after active verbs. However, heavily always goes before the past participle of verbs in the passive voice, and before participial adjectives. See these examples from Cambridge

• The compound is heavily guarded. (passive - heavily + past participle)
• She's heavily involved in the project. (passive - heavily + past participle)
• Estimates of the amount of money needed to decontaminate the heavily polluted chemical installations vary. (heavily + participial adj)
• The country depends heavily on foreign aid. (active verb + heavily)
• It had snowed heavily during the night and in the morning the garden was a white fairyland. (active verb + heavily)

It may be that the "meteorological" factor might not play a big role in the position of adverbs, but there seems to be some kind of pattern. As for the use of "heavily" with verbs, we can also discern a pattern. It is only a pattern of use, I have not found any grammar rules that stipulate restrictions in the placing of this adverb before or after a verb.

Aside: I was also intrigued by the reverse situation of gradually rising/rising gradually. Yet I was fascinated to see that the Ngram discrepancy between the two variants diminished when I searched gradually rise/rise gradually (actually,"rise gradually" was preferred for almost a century! It declined in favor to "gradually rise" only around 2010) or gradually rose/rose gradually. This does not occur in the case of heavily raining/rains/rained vs raining/rains/rained heavily: the position of "heavily" after the verb "to rain" is uniformly and constantly more common throughout the last 3 centuries.

Addition: I found this example in the comments worth looking at:

Heavily raining down insults on the assembled guests from the railing over the dance floor, Terry stomped out somewhat unsteadily and fell flat on his face.

Here, heavily precedes the verb, and this may be determined by the fact that rain down is used transitively with the direct object insults. I can't see how you can have heavily after the verb here:

*Raining down heavily insults on the assembled guests OR *Raining down insults heavily on the assembled guests

looks clumsy. So, thanks to that comment, we've found another factor that influences the place of "heavily" in a sentence: the presence of a direct object.