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.
Short answer: Irregular periods are any deviation from a woman's normal cycle. Long answer: The average menstrual cycle is approximately 28 days, with bleeding days averaging five, and blood flow between two and eight tablespoons. Thus, anything that differs from this can be considered irregular. However, in reality, not all women experience normal menstruation.