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.
The Earth contains four spheres: hydrosphere, atmosphere, geosphere, and biosphere . The hydrosphere is a dynamic mass of water that is continually on the move. It represents â¦water that is at or near Earth's surface. The hydrosphere comprises 71 percent of Earth's surface and 97 percent of Earth's water. Freshwater, underground water, streams, lakes, ice, and glaciers are included in the hydrosphere. The atmosphere is the Earth's gaseous envelope. It is thin and tenuous and it protects us from the sun's heat and ultraviolent radiation.