Soft winter OR mild winter?

I was doing some online tests that are supposed to be useful for the TOEFL and I was confused (although I gave a correct answer) with the following multiple-choice question:

The meteorologists say we're likely to have a _____ winter.

Given answers are: calm, soft, mild, smooth.

The right answer was MILD but however long I searched the web and dictionaries, I couldn't find the difference between soft and mild speaking of weather. I saw both soft weather and mild weather as well as winter in particular.

Question:

Weather-wise, what is the difference between soft and mild?

Solution 1:

Adding to Josh61's great answer, the adjective soft is more idiomatic when we describe rain, snow and wind as an antonym of strong and heavy. According to Oxford Online Dictionary it means:

Not strong or violent: 'a soft breeze rustled the trees'

Rain and snow could be heavy and wind could be strong or violent respectively. However, winter generally can't be heavy or strong and that's why soft winter is not as idiomatic as soft rain, soft snow or soft wind.

If the meteorologists are forecasting weather for a region known for frequent violent winter storms, calm winter would mean winter without as many frequent winter storms. But, generally speaking mild or warm is more idiomatic when describing winter weather which is warmer than previous years.

As commented below, light and gentle are also broadly used for rain and wind.

Solution 2:

Ngram: mild, soft, smooth, calm winter.

Mild is the more appropriate term, it is often used to refer to weather conditions:

• (Of weather) moderately warm, especially less cold than expected: mild winters (ODO)

Solution 3:

Mild is correct ...

In parts of Scotland, if you hear the word "soft" connected to weather, as in "it's a soft day today", put your raincoat on. And a waterproof hat. And wellington boots. You're certain to need them! Or stay inside and find a good book...

It's a regional thing, and might be the same in Ireland.