That was the question on our Travel Trivia game card that sparked the conversation behind Morton Salt’s slogan. My husband knew the meaning behind the expression "When it rains it pours" – mainly that when something bad happens it seems like more bad things follow. Kind of like bad luck happens in groups of three. I explained that salt has a tendency to clump when it’s humid, or raining, and therefore won’t pour (sometimes restaurants put rice in the salt shaker to keep it pouring – maybe they are unfamilar with the slogan or don’t use Morton Salt?) However, Morton Salt has an ingredient that allows it to pour even when it rains, thus "When it rains it pours".
Upon further study I discovered that according to http://www.phrases.org.uk/bulletin_board/32/messages/265.html the correct meaning of the expression is as follows:
IT NEVER RAINS BUT IT POURS – "One stroke of good (or ill) fortune is often followed by many other instances of luck (or misfortune) when you least expect them. The proverb dates back to the eighteenth century. In 1726, English physician John Arbuthnot (1667-1735), published a book entitled ‘It Cannot Rain But It Pours.’ Jonathan Swift (1667-1745) and Alexander Pope (1688-1744) collaborated on an essay entitled ‘It Cannot Rain But It Pours.’ The saying has been use ever since." From "Random House Dictionary of Popular Proverbs and Sayings" by Gregory Y. Titelman (Random House, New York, 1996).
The saying, in a slightly different form, is the slogan for Morton Salt: ".The company developed a salt that would be free-running even in damp weather. In 1911, a little girl with an umbrella and her now-famous slogan, ‘When It Rains It Pours,’ were created to promote this new product in a national consumer advertising campaign. The Morton Umbrella Girl and slogan first appeared on the blue package of table salt in 1914. Throughout the years the ageless girl has changed dresses and hairstyles to stay fashionable. She was updated in 1921, 1933, 1941, 1956 and 1968. Together they have symbolized the growth and progress of the company through the years." From http://www.mortonsalt.com/consumer/about_us/history/index.htm
The ingredient that makes it flow according to Wikipedia – http://en.wikipedia.org/wiki/Morton_Salt –
Morton Salt’s hand-drawn logo features the "Morton Salt Girl," a young girl walking in the rain with an opened umbrella, and scattering salt behind her from a cylindrical container of table salt. The company’s logo, developed in 1914, and its motto, "When it rains, it pours" (developed in 1911), were developed to illustrate the point that Morton Salt was free flowing, even in rainy weather. Originally, the company had added magnesium carbonate as an absorbing agent to ensure that its table salt was free flowing; calcium silicate is now used instead for the same purpose.