Fifty percent of the total mass of the atmosphere is located in the lower 5.6 km (18,000 ft) of the troposphere. It is primarily composed of nitrogen (78%) and oxygen (21%) with only small concentrations of other trace gases.
Having irregular periods is very common. I see them in my practice all the time. The causes can range from something insignificant to something that requires treatment. For example, if you're a teen, your body's hormones can fluctuate and take some time until they find a balance.
Answers to your questions. To begin with, let me reassure you that most irregular periods are benign. Missed periods, too frequent periods, spotting, or bouts of heavy clotting and bleeding are usually caused by an underlying hormonal imbalance that is easily treated.