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.
For example, if one month your cycle is 28 days and another cycle is 30 days, this would be within the normal range. A large variation is not considered to be normal. For example, if some cycles were 21 days long, and others 33 days long, that would be considered an abnormal variation. If you have menstrual cycles that vary this much, you are experiencing irregular periods. Sometimes, due to stress or illness, your cycle may be delayed.