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.
Updated July 05, 2016. It is quite common for some girls to have irregular periods during their first year of menstruation. However, it's not normal if she misses her period for 3 to 5 months in a row, especially if she has had a period regularly for some months before that.