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.
On the other hand, an irregular period is defined as a menstrual cycle that is consistently shorter than 21 days or consistently longer than 36 days. You can also be considered irregular if your cycle length varies significantly from cycle to cycle, sometimes lasting 28 days, then 40 days, then 32 days.