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.
You can get a monthly period and still not be ~regular~. First of all, a normal period totally depends on the person. The average cycle is 28 days, but if you get your period every 23 days or every 30 days, that's normal, too. What's NOT regular is if the length of your cycle changes from month to month. So if you sometimes have 23 days between periods and sometimes have 30 days, that's considered irregular â even though you're still getting a period once a month.