What are Braxton Hicks contractions? Braxton Hicks contractions can begin as early as the second trimester. However, they are most commonly experienced in the third trimester. When this happens, the muscles of the uterus tighten for approximately 30 to 60 seconds, and sometimes as long as two minutes.
Main articles: Hydrology and Water distribution on Earth. Water covers 71% of the Earth's surface; the oceans contain 96.5% of the Earth's water. The Antarctic ice sheet, which contains 61% of all fresh water on Earth, is visible at the bottom. Condensed atmospheric water can be seen as clouds, contributing to the Earth's albedo.
Water is a finite resource: there are some 1 400 million cubic kilometres on earth and circulating through the hydrological cycle. Nearly all of this is salt water and most of the rest is frozen or under ground. Only one-hundredth of 1 percent of the world's water is readily available for human use. This would be enough to meet humanity's needs-if it were evenly distributed.