Water and Heat Budget
- Earth's climate has remained essentially unchanged for centuries.
- The amounts of heat and water from all sources that
enter (input) the atmosphere must equal the amounts removed (output)
from the atmosphere for the climate to remain basically unchanged.
- Today, emissions of gases such as carbon dioxide into
the atmosphere (by humans) may be upsetting the temperature of the atmosphere
necessary to maintain the balance between heat gained and heat lost.
Hydrologic Cycle (Water Budget)
- The global water budget or hydrologic cycle is relatively simple:
- Water is evaporated mostly from the oceans. Much
less is evaporated from lakes, rivers, soils, and vegetation on land.
- The evaporated water eventually condenses and falls
as rain or snow over both land and ocean.
- Approximately 93% of the evaporated water comes
from the oceans.
- Only 71% of the rain or snow (global precipitation)
falls on the oceans.
- In other words, more water evaporates from the
ocean than reenters it as rain or snow. This is good for us as that
is where most of the freshwater supply for Earth comes from. Only
3% of the total water on Planet Earth is freshwater.
- The excess water flows back to the oceans through
rivers, streams, and groundwater where evaporation once again takes
place and the cycle repeats itself.
- Earth's heat budget is more complicated than its water
- An expression of the total solar energy received on
Earth during some period of time and the total heat lost from Earth
by reflection and radiation into space through the same period of time
is called the heat budget.
- The source of heat energy for the atmosphere is the
sun (solar energy).
- Solar energy that reaches Earth's surface is partially
reflected and partially absorbed and converted to heat energy by the
land and ocean waters.
- Less than 20 percent of the solar radiation reaching
Earth is absorbed by atmospheric gases and clouds, 50% is absorbed
by Earth's surface--mostly the ocean, and 31% is reflected back to
- The oceans effectively capture a major portion (about
50 percent) of the sun's radiated energy and transfer much of it to
the atmosphere as latent heat of vaporization and as radiation.
- Several factors influence (affect) solar energy reaching
- Shape of Earth - because Earth is spherical (shaped
like a ball) the sun does not shine equally on all parts.
- Tilt of Earth - since Earth's axis is tilted relative
to the plane of Earth's orbit, the amount of sunlight reaching the
poles varies with the seasons. Most sunlight reaches the tropics.
- This creates a temperature gradient (differences
by steps) between the poles and the equator.
- This gradient sets up convection currents in the
atmosphere between the poles and the equator.
- Winds - carry heat, mostly as water vapor, toward
the poles. Clouds influence the amount of sunlight reaching Earth's
- Aerosols - aerosols in the air may scatter sunlight.
- Water Vapor & Dust - absorption of sunlight
by atmospheric dust and water vapor diminishes (reduces) the amount
of sunlight reaching the surface--but not by much most of the time.
Global Conveyor Belt
- In addition to providing water and heat to the atmosphere
as part of the hydrologic cycle, the oceans also distribute heat around
Earth through ocean currents.
- One major current subsystem that carries heat in the ocean is referred to as the
global conveyor belt.
- The oceans transport heat from the equatorial regions
to the polar regions in a global circuit.
- As heat is lost to the atmosphere in the north, the colder water sinks below in the warmer surface layer and
throughtout the depths of the global oceans.
- The entire circuit takes as long as 1,000 years to complete.
- Scientists believe that small changes in the
by the conveyor belt play an important role in controlling global climate.
- Water has one of the highest heat capacities of any
natural substance--including air and the rocks and soil of the land.
- The amount of heat required to raise the temperature
of 1 kilogram of a substance by 1 degree C (1.8 degrees F) is called
- All parts of Earth's surface are subjected to a daily
cycle of heating and cooling as night follows day.
- Land warms more quickly than the oceans under the
same conditions of solar radiation.
- Land cools and gives off heat to the atmosphere much
more quickly than the oceans.
- The surface water of the oceans absorbs energy from
the sun during the day with very little change in the temperature of
the ocean because:
- Water has a higher heat capacity,
- Much of the solar energy penetrates beneath the
surface before it is absorbed--especially clear water, and
- The heated water is distributed down many meters
by winds stirrng the water, so there is much more mass absorbing solar
- Interactions of landmasses with the atmosphere and
oceans exert substantial control over the climate characteristics of
most the Earth's land and much of its oceans.
- Some of the effects on climate may be localized
or limited to one small area (i.e. land and sea breezes or the "island
- Some may range across an entire continent or ocean
(i.e. monsoons or climate in the interior of continents).