PHENOLOGY

is the science dealing with the relationships between climate and periodic biological phenomena that are related to or caused by climatic conditions, such as the seasonal budding of trees and migration of birds. Photochemical Smog, present in many large cities, :is formed by chemical reactions involving nitrogen oxides and hydrocarbons (from human activities including automobile use) taking place in the presence of sunlight. The principle component of photochemical smog :is tropospheric or ground-level ozone.