As Fred and OpenCirrus say, when flying in the Northern Hemisphere and the wind is coming from the port side (i.e. starboard drift) then you are flying from a relatively high pressure to a relatively low pressure area. This is based on Buys Ballot's law and is the opposite in the Southern Hemisphere. Therefore, flying E-W with a wind from the North gives a port drift..as such you are flying from a low pressure to a high pressure.
If you are flying from a low pressure to a high pressure at a constant pressure level of 500m, in the high pressure area the 500mb level will be at a higher altitude than in the low pressure area. Thus flying at the 500mb level will give an increase in true altitude (but a constant indicated altitude..since your altimeter is merely a glorified barometer)
Best way I've remember these things is using Hi-Lo-Hi and the convese, Lo-Hi-Lo. If you are flying from a Hi-press to Lo-press then your altimeter will overread - Hi - decreasing true altitude. If you are flying from a Lo-pres to a Hi-pres, then your altimeter will underread - Lo - increasing true altitude. The same works for flying from high temperatures to low temperatures but in the opposite sense to the pressure..i.e. High temp/Low Press, Low temp/High Press.
Hope that helps!
