Phenology is the study of recurring plant and animal life cycle stages, or phenophases, such as leafing and flowering of plants, maturation of agricultural crops, emergence of insects, and migration of birds. Many of these events are sensitive to climatic variation and change, and are simple to observe and record.
Polychlorinated biphenyls (PCBs) are a group of manmade chemicals. They are oily liquids or solids, clear to yellow in color, with no smell or taste. PCBs are very stable mixtures that are resistant to extreme temperature and pressure.
1.2 All PCBs are man-made and have a similar basic structure. They are made of carbon, hydrogen and chlorine atoms. Because these atoms can be combined in many different ways, a total of 209 different PCB molecules can be formed. Some PCBs are more harmful than others.