Question of the week #2

In days gone by sailors and, the public in general, had a lot of “weather proverbs” they used to forecast the weather many of which are pretty accurate and useful even in this age of technology.  Here is one of my favorites that in certain conditions I’ve found to be pretty accurate.

When the wind before the rain

Let your topsails draw again

When the rain before the wind

 Topsail sheets and halyards mind.

Question (It is another two-part question)

1. What type of approaching weather might this weather proverb be true?

2. What is the scientific reason behind the need to pay attention to this weather proverb?