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.
Many things can cause irregular periods. Changes in your body's level of the hormones estrogen and progesterone can disrupt the normal pattern of your period. That's why young girls going through puberty and women approaching menopause commonly have irregular periods.