In contrast to meteorology,
which studies short term weather systems lasting up to a few weeks,
climatology studies the frequency with which these weather systems occurred in the past. It studies the periodicity of weather events over years to millennia, as well as changes in long-term average weather patterns, in relation to atmospheric conditions.
Climatologists, those who practice climatology, study both the nature of climates - local, regional or global - and the natural or human-induced factors that cause climates to change. Climatology considers the past and can help predict future climate change.