The atmospheric conditions play an important role in firearm ballistics. The order in which they affect the trajectory from the greatest change the least is; altitude, pressure, temperature, humidity. The International Standard Atmosphere is generally used by ammunition and bullet manufacturers to publish the data about their ammunition or bullets.

International Standard Atmosphere

If you do not check the 'Correct for Atmosphere' checkbox then the International Standard Atmosphere will be used as your atmospheric conditions. Here is a map that shows roughly your altitude if you live in the United States.

The Weather Underground is a good website to use to find out the altitude, pressure, temperature, and humidity for where you live. Plug those numbers into the ballistic trajectory calculator in order to have more accurate results.