|
पानीको ठोस रूप बरफ तरल पदार्थमा तैरिने गर्दछ। यो साधारण गुण भएपनि यसको वातावरणीय महत्व धेरै छ। ठोस अवस्थामा बरफको घनत्व पानीको घनत्व भन्दा कम हुने भएकोले बरफ पानीमा तैरिन्छ। तैरिएको बरफले वातवरणीय चिसोपनालाई तल रहेको पानीसम्म पुग्न दिदैन फलस्वरूप तल्लो सतहको पानी तरल अवस्थामै रहन्छ। पानीको यही अनौठो गुणका कारण धेरै चिसा ठउहरूमा माथिल्लो सतहमा बरफ जमे पनि तल रहेको पानीमा जलचरहरू जीवित रहन सक्छन।
== दैनिक जीवनमा जलपानी == ▼== खगोलीय स्थितिको महत्व ==
{{eng}}
[[चित्र:Water droplet.jpg|thumb|right|250px|Impact of a water droplet.]]
The coexistence of the solid, liquid, and gaseous phases of water on Earth is perhaps vital to the origin and evolution of life on Earth as we know it. However, if the Earth's location in the [[solar system]] was marginally closer or further from the [[Sun]], the conditions which allow the three forms to be present simultaneously would have been more unlikely.
Earth's mass allows [[gravity]] to hold an [[atmosphere]]. [[Water vapor]] and [[carbon dioxide]] in the atmosphere provides a [[greenhouse effect]] which helps maintain a relatively steady surface temperature. If Earth were less massive, a thinner atmosphere would cause temperature extremes preventing the accumulation of water except in [[polar ice cap]]s (as on [[Mars (planet)|Mars]]). According to the [[solar nebula]] model of the solar system's formation, Earth's mass may be largely due to its distance from the Sun.
The distance between Earth and the Sun and the combination of solar radiation received and the greenhouse effect of an atmosphere ensures that its surface is neither too cold nor too hot for liquid water. If Earth were more distant, most water would be frozen. If Earth was nearer to the Sun, its higher surface temperature would limit the formation of ice caps, or cause water to exist only as vapor. In the former case, the low [[albedo]] of [[ocean]]s would cause Earth to absorb more solar energy. In the second a runaway [[greenhouse effect]] and inhospitable conditions similar to [[Venus (planet)|Venus]] would result.
It has been proposed that life itself may maintain the conditions that have allowed its continued existence. The surface temperature पृथ्वीको has been relatively constant through [[geologic time]] despite varying solar flux, indicating that a dynamic process governs Earth's temperature via a combination of greenhouse gases and surface or atmospheric albedo. See ''[[Gaia hypothesis]]''.
[[चित्र:Sprinkler03.jpg|thumb|250px|left|पानी छर्कने उपकरणमा पानीको चाप]]
[[चित्र:Humanitarian aid OCPA-2005-10-28-090517a.jpg|thumb|250px|right| बोतलको पानी पिउदै एक बालिका]]
| 