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.
The menstrual period lasts between two days and seven days, usually occurring once every 28 days, although anything from once every 24 days to once every 35 days is considered normal. If you are having a period more than once every 24 days, it is considered an irregular cycle.