Download 5.8B Key Concepts

5.8B Sun & Water Cycle
Key Concept 1: The Sun provides the energy that evaporates water on
Earth.
Radiant energy from the Sun heats the surface of the Earth. Water in
ponds, lakes, rivers, seas, and oceans is heated by the Sun’s energy, which
breaks the bonds of the surface water molecules allowing their escape
into the air as a gas, water vapor. The process of changing from a liquid
to a gas is called evaporation.
The water cycle begins with the Sun heating the oceans, causing the
surface water to change from liquid to gas releasing tremendous amounts
of water vapor into the atmosphere as part of the water cycle. As the
water vapor rises, it cools at higher altitudes, and condenses (changes
from gas to liquid) onto dust particles in the air and forms clouds. When
the clouds become saturated with water vapor and the molecules become
too heavy, precipitation occurs in the form of rain, hail, sleet, or snow.
This moisture falls back to the ocean or onto land where gravity pulls it
downward as groundwater (moisture in the soil or underground rivers) or
as runoff that eventually flows into rivers that carry the water back to the
ocean, and the water cycle begins anew.
Sometimes, the Sun heats the Pacific Ocean, causing unusually warm
currents that release heavy amounts of water vapor and bring torrential
rainstorms and flooding (called El Niño) to the Western Hemisphere. The
reverse also happens when the Sun does not heat the Pacific Ocean as
much, and unusually cold currents push warmer currents too far west and
a severe drought with dry air with no rain occurs (called La Niña).
Although El Niño and La Niña are not terms required in this module, their
effects emphasize the impact of the Sun and oceans interacting.
Key Concept 2: As evaporation can be used to separate solutions,
water that evaporates from the ocean does not contain salt.
Water from ponds, lakes, rivers, and streams is fresh water. However,
water from oceans contains 3-4% salt and is considered salt water.
Students have learned in previous modules that salt water is a special
mixture called a solution, which can be separated through evaporation.
The same is true for ocean water as it evaporates. The salt remains
behind, so the ocean water vapor that rises into the atmosphere is not
salty. Students will explore separating salt from salt water using
evaporation in this module.
Key Concept 3: Much of the water cycle begins when the Sun’s energy
evaporates water from oceans, which collectively cover approximately
71% of the Earth’s surface.
Although surface water evaporates from all bodies of water, the oceans
provide the largest amount of evaporation contributed to the water cycle.
The five oceans (Atlantic, Pacific, Indian, Arctic, and the Southern, which
surrounds Antarctica) cover more surface of Earth than continents.
Students have the opportunity to calculate the percentage of that ocean
area. With that enormous amount of surface water heated and evaporated
by the Sun, ocean water is the major contributor to the water cycle.
Special Note
The current trend in the scientific community is to refer to the water cycle as the hydrologic cycle.
This change in terminology is based on the concept that the molecules of H20 change state
between liquid (water) and solid (ice) and gas (water vapor) depending on the conditions and
stage of the cycle. Students need to be aware the cycle may be referred to as either the water
cycle or the hydrologic cycle in literature and on standardized tests.