A fogger is an automatic misting system that fan-forces very very fine water into the greenhouse. It's one way to distribute humidity into your GH. Without a fogger or misting system, you can manually raise your humidity with your water-hose (water the floor and even your GH sides and roof periodically). Other ways to raise your internal humidity is to keep contained water vessels in your GH. The greater the vessel surface-area for water, the more you can add to your GH humidity. Of course, you can also grow your plants on trays that contain water, making sure that your plant containers are not in the water but on top. Most people doing humidity trays will add pea-size stones so that plant containers sit on the stone surface.
Unless you have water in your GH, you can't have an automatic fogger because it requires water on-demand for the system to release the water. In your zone, your water pipe would need to be placed below your freeze-line (about 18 to 24 inches deep) to avoid a frozen pipe.
A misting system can help your greenhouse stay cooler in warmer weather, too. It probably wouldn't be sufficient to cool your GH in the summertime in your zone, but through the month of May and into June, it would help reduce your internal heat (that and opened vents, door, etc).
Here's a link to some misting equipment from a business I use (no affiliation, just a customer of theirs): http://www.littlegreenhouse.com/accessory/mist.shtml They do have an assortment of misting equipment to give you an idea of the type of equipment that can be purchased.
