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 distribution on Earth - Distribution of saline and fresh water. 1  The total volume of water on Earth is estimated at 1.386 billion kmÂ³ (333 million cubic miles), with 97.5% being salt water and 2.5% being fresh water. 2  The total volume of water in rivers is estimated at 2,120 kmÂ³ (510 cubic miles), or 2% of the surface fresh water on Earth. Rivers and basins are often compared not according to their static volume, but to their flow of water, or surface runoff. The distribution of river runoff across the Earth's surface is very uneven.