Climate sensu stricto is defined as the average weather over a bigger time interval of thousands or millions of years. Meteorologists use an interval of 30 years to average the variables like temperature, precipitation and wind. The climate changed very much over time. The dinosaurs lived in a world much warmer than today. During the last ice age about 20.000 years ago it was so cold, that thick ice layers covered large parts of the landmasses on the northern hemisphere. The oldest systematic collection of surface weather observations began 1781 at an observatory in Germany. For the analysis of the climate before that time scientists have developed various methods, e.g. the sampling of ice cores or shells of foraminifers (small shell-building organisms that live in the oceans).