http://extension.colostate.edu/  Fact sheet 7.403 Evergreen trees

This pine is showing needle cast. Notice the brown needles are lower on the branch while the healthy green needles are closer to the tip.

Needle Cast: If your conifer (pine, spruce, fir, or juniper) is dropping needles, it may be a perfectly normal and healthy occurrence. If the needles that are dropping are only on the interior part of the tree while the needles toward the ends of the branches are still flexible, green, and firmly attached, then your tree is going through a process called “needle cast.” This process is kind of like deciduous trees casting off their leaves every fall – the needles deepest inside the tree no longer receive much in the way of sunlight as they are shaded by the newer exterior needles, so the tree drops them. This is totally normal and you should not be alarmed.

Sun Scald: If the needles on one side of the tree are showing yellow or brown coloration, but the other side of the tree still looks healthy, it could be suffering from sun scald. The exceptionally dry winter air combined with low soil moisture and intense sun causes the needles to dry out. The damage is often only present on the most exposed parts of the tree where prevailing winds or southern sun can have the greatest impact. Often, only the tip of the needle will be discolored while the base of the needle remains green.