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.
When a woman's period deviates from a fairly regular cycle - for instance, having a period every two weeks or more than one period in a month - her periods are said to be irregular. Irregular periods can encompass a broad range of issues concerning a woman's menstrual cycle.