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.
Do irregular periods mean you have Polycystic Ovarian Syndrome (PCOS)? The truth is that there are many causes of irregular periods but PCOS is the most common cause. About 7% of the overall population, about 25% of young fertile women and over 50% of women with irregular periods have PCOS. So most women with irregular menses will turn out to have PCOS.