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.
If you have just started your period, it's not uncommon for your period to be irregular for the first few years. Irregular periods can be caused by a variety of health conditions, such as ovarian cysts, pelvic inflammatory disease, and habits like exercise and weight loss.