As you fly into an area of lower pressure (without adjusting the altimeter setting) the altimeter will indicate higher than you really are, because the pressure at a given altitude is lower, the same as it would be if you were actually flying higher. Also, as you fly into an area of colder air, the actual pressure is higher because the air is colder, not vice versa.
There is a great tool you can play with that you can find at http://www.luizmonteiro.com/Learning_Alt_Errors_Sim.aspx
You can play with this to adjust the pressure and temperature and see what it does to the altitude readout.
According to FAR 91.121, when operating below 18,000 feet, the altimeter must set to a current ground station within 100 NM of the aircraft position.
Part 1, Definitions and Abbreviations
Part 61, Certification: Pilots and Flight Instructors
see Private Pilot Training Requirements for a checklist and simulator restrictions
Part 67, Medical Standards and Certification
Part 91, General Operating and Flight Rules
Part 830, National Transportation Safety Board (NTSB) Rules Pertaining to the Reporting of Aircraft Accidents or Incidents
Chapter 12: Weather Theory
Chapter 13: Aviation Weather Services
For fixed wing pilots looking to do the rotorcraft add-on, you can add the category to your license by completing the following requirements: hold a PPL certificate, hold a current FAA medical certificate, obtain 30hrs in a helicopter (including 10hrs of solo time) and pass an FAA oral and check ride.
For commercial fixed wing pilots looking to do the rotorcraft add-on, you can add the category to your license by completing the following requirements: hold a CPL certificate, hold a current 2nd class FAA medical certificate, obtain 50hrs in helicopters (including 35hrs PIC), 10hrs cross country time, 10hrs instrument in an aircraft, and pass an FAA oral and check ride.
see listing here