If you know the temperature and elevation at the starting point and the elevation of the summit you can predict (within certain bounds) the temperature of the summit using the adiabatic lapse rate.
For unsaturated air, the rate is about 10 degrees centigrade per Km. Portal to Mt. Whitney summit happens to be very close to 2 Km gain and 20 degrees centigrade change is 36 degrees Fahrenheit. Don't forget to subtract!
So for metric-challenged hikers, it's about 5.5 degrees F. per thousand feet. That's a good rule of thumb, but there are many variables. YMMV.
That predicted temperature is before the wind-chill factor is calculated. That's a whole other post.
