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 someone says they have an irregular period, they are typically referring to the number of days between cycles or the variation between periods. Your period is considered irregular if the number of days between periods is either shorter or longer than the normal range. So, your period is irregular if... For that last one, for example, if one cycle is 25 days, but another is 33 days, that would be considered irregular. This would be irregular even though a 25 or 33-day cycle is normal otherwise. It is normal if your cycles vary by just a few days from month to month.