7 th Grade Science - Weather

 Water is constantly cycled through ecosystems  Oceans contain about 97% of the world’s water  The remainder is freshwater that is in the form of gas (vapor), liquid, or ice 75% of this water is frozen in glaciers and in ice in polar regions Only about 1% of the earth’s water is available as fresh, liquid water

 Most of the fresh, liquid water is in the ground in rock and soil layers called aquifers  The rest of our freshwater is in the surface water of lakes, rivers, and ponds, and in the bodies of organisms

A Simplified Model of the Water Cycle

Evaporation is when the sun heats up water in rivers, lakes, or in the ocean and turns it into vapor or steam. The water vapor or steam leaves the river, lake, or ocean and goes into the air.

Water vapor in the air gets colds and changes back into liquid, forming clouds. This is called condensation.

Precipitation occurs when so much condensation has occurred that the air cannot hold it anymore. The clouds get heavy and water falls back to the earth in the form of rain, hail, or snow.

When water falls back to earth as precipitation, it may fall back in the ocean, lakes, and rivers or it may end up on land. When it ends up on land, it will either soak into the earth and become part of the “ground water” that plants and animals use to drink or it may run over the soil and collect in the oceans, lakes, and rivers where the cycle starts all over again.

Water is essential for life on earth. It is recycled through the water or hydrologic cycle, which involves the following processes: Evaporation Condensation Sublimation Precipitation Transpiration Surface Runoff Infiltration Percolation

Evaporation, the changing of water from a liquid to a gas Condensation, the changing of water from a gas to a liquid Sublimation, the changing of water from a solid to a gas Precipitation, the process by which water molecules condense to form drops heavy enough to fall to the earth's surface Transpiration, the process by which moisture is carried through plants from roots to leaves, where it changes to vapor and is released to the atmosphere Surface runoff, the flowing of water over the land from higher to lower ground Infiltration, the process of water filling the porous spaces of soil Percolation, groundwater moving in the saturated zone below the earth's surface

 Through these processes, the amount of water on earth remains nearly constant and is continually recycled through time.  Water molecules may remain in one form for a very long period of time (for example, water molecules can be locked in Antarctic ice for thousands of years) and in other forms for very short times (for example, water molecules in desert rainstorms spend mere minutes as surface water before evaporating into vapor again).

 What are clouds? Clouds are aggregates of many, many ice crystals or droplets of liquid water, suspended in the air.

 There are three necessary ingredients for cloud formation: Water Vapor Condensation Nuclei And Cooling of Air

 Water Vapor This is regular water in the form of an invisible, odorless gas, which is what liquid water becomes when it evaporates. Water vapor is always in the air, just in varying amounts.

 Condensation Nuclei These are tiny, solid particles of dust, smoke, ash, salt, pollen, etc. suspended in the air There is usually lots of these around, even when we can’t see them.

 Cooling of Air When the air is cooled, some of the water vapor begins to condense onto some the condensation nuclei, forming lots of tiny droplets of liquid water. If it’s cold enough in the cloud, the droplets can freeze into tiny ice crystals.