It’s all about the sun!
The best piece of information we can provide to anyone who plans to hike in the Colorado wilderness is simple advice on what to look for in the terrain itself. There are a couple of ways to discern the difference between north and south facing slopes. Being able to differentiate between the cardinal directions is important when you get lost or disoriented.
Take a close look at Figure 1. As you can see, some of the slopes are nearly void of trees. The absence of trees indicates that the slope receives a greater amount of direct sunlight. Thus, a hiker can determine that these slopes most likely face south. On the other hand, slopes containing dense stands of trees generally face north. Figure 2 (below) is another striking example of how north facing slopes tend to contain thick stands of trees, whereas south facing slopes do not.
The reason south facing slopes do not host as many trees is a result of heat and lack of moisture. Because the sun directly strikes south facing slopes, they tend to exhibit a much drier microclimate. At the right elevation, pinon pine and juniper trees can sometimes thrive on south facing slopes (due to the fact they need less moisture than other tree species). The few trees on the south facing slope in Figure 2 are actually juniper trees.
As you move higher up in elevation, there is another relatively easy way to tell the difference between north and south facing slopes. Timberline can be a good indicator of whether you are looking at a south facing slope or at a north facing slope.
For example, in and around Rocky Mountain National Park, on warm, southwest facing slopes, timberline is generally located at 11,500′. At that same latitude in Colorado, approximately 40 degrees north, timberline on northeast facing slopes can be as low as 10,800′. The 700′ difference in timberline amongst the two directional slopes is a result of a variation in the length of the growing season, which on northeast facing slopes is actually shorter than the growing season on southwest facing slopes. Because northeast facing slopes hold snowpack for a greater portion of the year than southwest facing slopes, they provide less viable time for trees to grow.