There's No Such Things as Bad Weather -- only bad clothes

 "Play in and of itself has a therapeutic effect on children. And they play differently outside. The games are more open and flexible, and it's easier for them to organize the situation in a way that is beneficial to them physically, socially, and psychologically." (Mckurk p. 91, quoting Professor Martensson)

"Streets and parks that used to teem with children are now empty. Simultaneous with this development, obesity, diabetes, and ADHD, and other behavioral problems have become rampant, with American children now being three times more likely to be medicated with stimulants  and antidepressants than their European peers." (McGurk, p 5)

I have recently been inspired by this book:

It is written by a wonderful Scandinavian mom who married an American and was shocked by how few kids she saw playing outside here in the US. She describes the contrast with a nature-centric culture in Scandinavia and uses both personal experiences and excellent research data to urge parents and schools to help kids get outside more... Not just on a few "good weather days". And not just for an occasional 20 minute recess.
I love being outside. I automatically feel better out in nature. It's one of the reasons that I love gardening and farming. The farm makes me get outside and work on things every day and the kids too. Usually what happens is that they go out to do their morning or evening farm chore and end up staying out much longer playing imaginative games. 
She is recommending 4-5 hours outside every day for kids of elementary school ages like mine. We are blessed to have made some good friends who also believe that learning and playing outside is essential for developing happy, healthy, creative, and intelligent minds. That is helping me in my quest to expand our outdoor time.
They have been using their imaginations playing with the wading pool that they pulled out of the barn this week.  Both in snowy weather --

And on a random warm 70 degree day:

  "One cross sectional study representing 4 million children in the US showed that roughly half of all preschoolers don't have any daily outdoor playtime, even though the American Academy of Pediatrics recommends encouraging 'children to play outside as much as possible'. Older children don't fare much better, with digital entertainment on average now eating up nearly 53 hours of their time every week. By the time they reach their teens, only 10 percent of American children report spending time outside every day, according to Nature Conservancy" (McGurk, p. 5)

Their is a lot more excellent information and funny anecdotes in this book. I highly recommend it and her blog: Rain or Shine Mamma