Why is there such a time lag...

From our point of view in the Northern Hemisphere, this is a timely question for today, the winter solstice for 2001. This is the shortest day of the year — the time when the Sun reaches its southernmost point in the sky. Although this part of Earth is cooling, its great thermal mass still retains some heat from the summer and fall. As the gradual cooling process continues over the next two months, temperatures will continue to fall, and the coldest temperatures will be recorded. The same pattern holds true for the summer solstice in June, as the year’s highest temperatures are recorded later, in July and August.