I wanted to go over an environmental factor that affects aircraft performance that probably does not get talked about enough, and that would be icing.  I thought that it was significant to talk about since this year has been extremely cold for most of the United States this winter.  As we have learned from the principles of flight that there are four forces that act on an aircraft; Lift, drag, weight, and thrust.  Icing has an effect on all four forces. 
         When Ice builds on the wings of an aircraft due to moisture and freezing temperatures it changes the airfoil due the ice accumulating on the wing thus causing the lift to be reduced.  As we learned the lift equation L = Cl * A * .5 * r * V^2 we can see that as ice builds on the airfoil it changes the coefficient of lift and the surface area to change making the overall lift decrease. 
          We also know that with Ice building on the surfaces of not only the wings but the other parts of the aircraft that it increases the parasite drag which also decreases lift and increases drag. 
          As ice builds on any portion of the aircraft the weight of the aircraft will increase as well.  This can have an effect on the overall CG of the aircraft as well.  The very last thing that icing has an affect on is thrust.  To produce thrust we tend to rely on the power plant of any aircraft to produce the thrust.  Power plants rely on an airfoil to produce the thrust, and if they don't you will require more output to maintain the desired flight you want due to the previous points I brought up earlier.
         There are a few ways to mitigate Icing.  One is to check the weather, and know whether or not your aircraft is able to fly in icing conditions or not.  If so what icing conditions, and what affect it will have on the aircraft when it is encountered.  Also ensure your aircraft has anti icing equipment and that it is fully functional.  Do not fly in any conditions the aircraft is not capable of handling.  If it is capable, know what your capabilities are as a pilot and determine whether or not the risk is worth it.

 RESOURCES:
http://hartzellprop.com/icing-affect-aircraft/
Pilot Handbook Chapter 4, and 5
https://www.youtube.com/watch?v=5O-j0w-h7v0&index=1&list=PLzW-Ub1FWeZzdOHQhNK0U0Ci1a-VRH8IO
AC-006A Chapter 10