The water quantity on earth is constant. There are approx. 1,386 m. km³ water, which are permanently in motion. By solar radiation, which converts liquid water into water vapour, it evaporates from the earth's and water's surface. Also humidity stored in vegetation is given off to the air (evapotranspiration).
With increasing temperatures the air can take up increasing quantities of water vapour, which is the basis for the water cycle.
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0°C
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4.8 g/m³
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10°C
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9.4 g/m³
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20°C
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17.1 g/m³
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30°C
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30.4 g/m³
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45°C
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65 g/m³
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In the atmosphere the water vapour condensates to water drops, from which clouds are formed and eventually rain is falling. For our survival it is essential that part of the water evaporating from the oceans falls on the land, feeds the rivers, waters the soil and fills up groundwater aquifers.