The northern hemisphere is warmer than the southern hemisphere, it warms up earlier in the spring, and by the time there is sunlight available, the PSCs have already disappeared.
(the PSCs; have disappeared; T:already) Context(the PSCs have disappeared,List([144, 177))):(it; warms up; T:earlier in the spring) Context(it warms up the PSCs have disappeared,List([64, 177))):(The northern hemisphere; is; warmer than the southern hemisphere)
the PSCs have disappeared,List([144, 177)) it warms up the PSCs have disappeared,List([64, 177))
the PSCs it The northern hemisphere
have disappeared warms up is
T:already
T:earlier in the spring
warmer than the southern hemisphere