Free space optics are heavily affected by the atmospheric conditions you mention, especially at longer distances. In your environment you also have to take into account such things as exhaust stacks and flares. These also cause atmospheric distortions that can disrupt optical transmissions.
My suggestions in order of preference:
1) Call your local service provider, (Sprint, Time Warner, etc,) and ask about direct IP connectivity. The larger carriers are offering high bandwidth IP services now. You may be able to get a business class Cable or DSL circuit and do a VPN connection between offices.
2) Look into point to point services, (T1’s, etc.). You can get a couple point to point T1’a, bind the channels on the router to load share, and it offers some redundancy for T1 failure.
3) Point to point wireless. Motorola offers a Canopy backhaul radio that will do 20 Mbit full duplex, built in security, and will operate over 10 miles. We use these in several applications and they work well. There are many unlicensed, and licensed alternatives for short haul, high data rate communications.
For redundancy, I would recommend a combination of methods. In several locations we use the common carrier, (Sprint, Verizon, etc.) for primary, with a wireless backhaul as backup. At other locations we use the wireless as primary with older in-plant cabling as backup.