What is the meaning of "April smile?"

I was reading Rebecca of Sunnybrook Farm, and I came across the phrase "April smile." I was unable to find a meaning of this phrase using Google, so I thought I'd ask here.

Here is a quote where the phrase is found:

"Give me some more examples of the subjunctive, Rebecca, and that will do for this afternoon," she said.

"If I had not loved mackerel I should not have been thirsty;" said Rebecca with an April smile, as she closed her grammar. "If thou hadst loved me truly thou wouldst not have stood me up in the corner. If Samuel had not loved wickedness he would not have followed me to the water pail."

— from Rebecca of Sunnybrook Farm by Kate Douglas Wiggin, 1903 edition, pages 61-62.

What exactly does the author mean by "April smile" in this context? And how does it differ from a regular smile — eg. said Rebecca with a smle, as she closed her grammar. —?

Solution 1:

Romantic poet Robert Southey used the phrase in one of his letters, as a postscript to a somewhat melancholy letter he writes :

A momentary recollection has just brought an April smile upon my cheek,...

The editor gives that definition of the phrase in a footnote:

A smile that comes after great distress, like sunshine after a storm