You're confusing Phoenix with the Colorado Plateau, which is where the bulk of the Navajo Nation rests. Huge difference temperature wise. For instance, Window Rock, AZ has an USDA Hardiness Zone and Sunset Zone of 1. Another US city that has this same gardening climate zone is Fairbanks, Alaska, hee hee. Most of the reservation is 1 or 2 (2 being slightly warmer).
The warmest part of the year is during the summer, which is fairly mild. It may reach around 100 a couple days during the summer but usually ranges between 80-90 during the day. At night, temperatures drop severely, sometimes as much as 30-40 degrees. Basically, there is a risk of frost all year long. It's a desert out here but it's a high desert tundra. The elevation ranges from 6000-8000 ft out here.

I used to know an equation where you can tell the difference between what a climate would be like based on its altitude. It went something like for a certain number of feet upward was the equivalent of going north around that amount. It's the same concept as to why mountains tend to have snowy tops. Phoenix will definitely pass 100F during the summer (and even reach the 120s), winters there usually have a low of the high 50's to low 60's. Winters on the Navajo reservation are extremely cold. Kind of explains why Pendleton blankets are so popular out here as shawls and just in general--anything to keep warm!

Stephanie

In every man there is something wherein I may learn of him, and in that I am his pupil.--Ralph Waldo Emerson