A seasonal variation in rate of height growth has long been recognised in the temperate zones. Children tend to grow faster in height in the spring and summer and faster in weight in the autumn and winter. Marshall (1971) in a longitudinal study found that in children 7 to 10 years of age maximum height velocities were reached in 3 month, periods ending between March and July, and minimum height velocities in the periods ending between September and February. During the 3 months of fastest growth, a child on average grew three times as much as during the 3 months of slowest growth. The cause of these differences is unknown. Totally blind children showed similar variations in rate to sighted children but failed to synchronize them with season of the year. Length of daylight seems to play only a minor part.
Seasonal variation in height and weight growth is governed by the rainy and dry periods in tropical areas. The rainy season is marked with food supply running low and the frequency of infections is highest. In Gambia, children under 3 years had good weight gains during the dry season but experienced low gains and even weight loss during the wet period. In Tanzania, both African and European school children displayed seasonal changes in height and weight gain, however the differences in weight were not constant in both the groups. Change of season has an impact on diet of an African child whereas that of the Europeans remains relatively constant. Thus, it can be concluded that different factors may be responsible for seasonal variations in the two areas.
Moreover, the effect of altitude on growth has also been studied. In general, people living at high altitude are smaller than those living at sea level or plains. The principal environmental factor affecting human physiology at altitude 3000 meters is the low atmospheric pressure which reduces the partial pressure of oxygen in the inspired air. It results in hypoxia, a condition in which haemoglobin carries less oxygen than at sea level and oxygen tension in the plasma is reduced. Other factors which affect growth of man at higher altitude are cold, lower air density and nutrition. As a result of adaptation to high altitude, the period of growth is slow and prolonged; Individuals have larger chest circumference, higher average lung size and higher haemoglobin concentrations as compared to individuals living in plains and at sea level.