Hair type is curly or straight, largely depending upon the number of disulfide bonds existing between hair proteins that are found in the hair shaft. The bigger the number of links, the curlier your hair, and vice versa (the lesser the number of links, the straighter your hair)!
Human hair is primarily composed or formed of keratin. It is a protein that grows from a sac termed the follicle. Cells existing in the hair follicle make keratin as well as various other types of proteins. They become a part of our hair shaft. The proteins consist of sulfur atoms. If two of these sulfur atoms happen to pair up and bond, the action results in formation of a disulfide bond. If two of the sulfur atoms at a distance (in the same protein) join, they will form the disulfide bond, and the protein will bend.
The amount of humidity present in the air not only makes for what we generally label a â€˜bad hair dayâ€™ but also alters the degree or extent of curliness or of straightness of hair.