Determine the altitude for a given area. Use online resources or consult reference materials to find out the altitude, or height above sea level. Write the value down.
Determine the barometric pressure of the area. You can use a barometer, or use the Internet to check values for the area.
Again, using the internet enter keywords to get results for a calculator that will give you values for the freezing point of water using figures that relate to altitude and atmospheric pressure.
Enter your noted figures in the calculator. Click enter and jot down the resulting freezing point in your notebook.
Now change the figure for the altitude and barometric pressure by increasing or decreasing them by a multiple of around 500. Using the same calculator, enter these new figures and note the results.
Subtract the two resulting figures that you got from both the exercises and you will get an answer for the change in freezing point with altitude.