One of the greatest things about doing street fairs in NYC is also one of its biggest drawbacks. You get to be outdoors the entire day, which also means if the weather isn't too nice, you get the worst of it.
You can easily separate new vendors from veterans based on the equipment they bring to the shows. Merchants who have done this long enough know to bring tents with sidings, some type of weights (more important than you think), and bins or other kinds of weatherproof storage for your valuable goods. There's nothing worse than being unprepared for the elements.
Rainy days don't necessarily mean a show is a bust. In fact, sometimes, your shelter will bring potential customers into your tent. This can lead to purchases that otherwise wouldn't have happened. But, more often than not, rain makes what could have been a great street fair simply mediocre, especially when it comes to breaking down at the end of the day.
The lesson to be learned here is this. If you're enjoying a rainy day stroll, and happen upon a street fair, stop by and chat with some of us. Most likely we're not having the best of days, and could use a little cheering up :)