How does the weather outside the antique shop match Rosemary's feelings as she stands on the step?

Despite that she is very wealthy and has everything material she could want in life, including a fine husband, Rosemary is discontented for reasons she doesn't understand. She comes out of an antique shop where she has been examining an exquisite box and feels strangely sad. She thinks as she sees her car and driver waiting for her that:

There are moments, horrible moments in life, when one emerges from shelter and looks out, and it's awful.

The weather matches her dark, grim, sad mood. In literature, when weather matches a person's mood, it is called the pathetic fallacy:

Rain was falling, and with the rain it seemed the dark came too, spinning down like ashes. There was a cold bitter taste in the air, and the new lighted lamps looked sad.

Rosemary doesn't know what she wants in life or why she feels unhappy. She gloms onto a young woman who is poor and asks her for money to buy a cup of tea. The young woman is another object she can use as a toy to amuse herself with—however, when her husband says the girl is beautiful, Rosemary quickly gets rid of her.

In "A Cup of Tea" a wealthy lady by the name of Rosemary is out shopping. During her latest retail therapy session she visits an antique shop, where she examines a charming little box that particularly tickles her fancy. Although Rosemary doesn't actually buy the box—she asks the man in the shop to keep it for her—she seems perfectly happy, nonetheless.
But her mood soon changes when she steps outside into the winter afternoon. The rain is starting to fall. Not only that, but it seems quite dark too. There's an air of sadness and depression about Curzon Street as Rosemary stands outside the antique shop. The unpleasant atmosphere is reflected in Rosemary's mood. All of a sudden, she's seized with a horrible pang; everything just seems so awful. Such moments happen, of course, but when they do it's best not to think about them. Far better to go home to a lovely, warm house and have a nice cup of tea, which is precisely what Rosemary does, albeit with an unexpected guest.