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Water is the fundamental
ingredient for life on Earth.
Looking at our Earth from|space,
with its vast and deep ocean,|it appears
as though there is an abundance|of water for our use.
However, only a small portion
of Earth's water is accessible|for our needs.
How much fresh water exists and|where it is
stored affects us all.
Nearly two-thirds of this fresh|water is stored
in the polar ice caps, snow|packs, and glaciers,
making it inaccessible for long
periods of time.
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The water cycle is dynamic
- it describes the continuous|movement of water
on, above and below the|surface of the Earth
and the transitions from one|state
to another.
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Sea surface temperature,|surface winds, and air
temperature influence the rate|of evaporation at the
ocean surface. In the tropics,
warm ocean surface temperatures|support high rates
of evaporation. Wind
also increases evaporation.
When the air's temperature
is warmer, it can hold more|water.
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While the atmosphere does not
store a large quantity of water|compared to the ocean,
rivers and lakes, it can|transport water
quickly from one place to|another.
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Low-lying regions of the|atmosphere with high
moisture and strong winds, can|form "atmospheric
rivers" to transport water|horizontally.
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Clouds are formed as water|vapor cools
and condenses into droplets and|ice crystals.
Clouds and water vapor act
as insulators in the atmosphere.
Clouds help shield the Earth|from the Sun and trap
heat from below.
When cloud particles grow large|enough,
they may fall out as rain or|snow.
Under the right conditions,|areas of precipitation
can grow into large storms.
As storms grow, they transfer
heat vertically into the upper|atmosphere.
The migration of storms helps|to distribute heat
between the equator and poles -|shaping
winds patterns globally.
How storms grow and intensify
depends upon atmospheric|mositure, surface temperatures
and wind patterns. Precipitation
is concentrated in some parts|of the world
and scarce in others. It can|vary substantially
from season-to-season and from|year-to-year.
Water that falls on the land|surface as
precipitation is stored within|snow packs,
lakes, reservoirs, soils
and underground aquifers.
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Water availability varies
from place-to-place and over|time.
The availability of water|affects the
and abundance of vegetation.
The primary source of food for|animals and people.
Extreme water
cycle variability, unusually dry
or wet conditions, impacts|humans
worldwide.
Each year, hazards such as|floods exact costly
economic and human toll.
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Most of the water that
falls on to land remains stored|there for weeks
or more.
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Snowfall is stored as snow|pack or ice.
Melt water and rainfall
are stored in lakes and soils.|Some of the water
is absorbed by plant roots or|drains into the water table.
Eventually, the water will|evaporate
to the atmosphere or return to|the ocean
in streams and rivers,|providing a source
of nutrient rich water that|supports ocean life.
Sensors on a suite of NASA|satellites observe and measure
water on land, in the ocean and|in the atmosphere.
These measurements are
important to understanding the|availability and distribution
of Earth's water, which is both|vital
to life and vulnerable to the|impacts
of climate change and a growing|world population.
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