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FLYING
LABORATORY: AQUA'S SCIENCE GOALS
HOW CLOUDS SHIELD AND INSULATE THE EARTH
A
dynamic fabric of suspended water floats above the Earth:
clouds. A portion of the sunlight that reaches the planet
bounces off the tops of clouds and gets reflected back into
space. Most of the rest reaches the Earth, warming it and
powering the systems of life all around us. But after sunlight
hits the surface, much of that energy then radiates back into
the sky. Clouds floating above act as a blanket, trapping
energy in the form of valuable heat. This helps maintain a
regular temperature range for the Earth.
These images show huge regions of the Earth mottled by clouds.
The pictures come from data collected by one of the GOES (Geostationary
Operational Environmental Satellite) satellites managed by
NOAA (National Oceanic and Atmospheric Administration). If
we regard these images as simply brief glimpses of the eternal
atmospheric dance, we start to see how changes in clouds can
dramatically alter the way the Earth not only looks but behaves.
A number of instruments on the Aqua satellite will study clouds
and how they interact with the rest of the world. Scientists
hope that these tools will enable them to gain a better understanding
about how a major part of the Earth's climate both affects
and is affected by the many complex forces that regulate the
planet.
WATER
VAPOR AND CLIMATE CHANGE
There
is no more important greenhouse gas than water vapor. As one
of the fundamental parts of Earth's atmosphere, water vapor
affects global warming in both positive and negative terms,
and offers a trail for scientists to follow towards a better
understanding about how the planet functions as a whole.
As water vapor moves through the hydrological cycle, it directly
affects precipitation and surface water. These different components
of the water cycle (discussed in the next section) themselves
play an important role in regulating climate, but we're getting
ahead of ourselves.
As sunlight hits the surface of the planet and consequently
raises atmospheric temperatures, the empty space between air
molecules increases, allowing the atmosphere to hold more
water vapor. This additional vapor absorbs energy, trapping
it in the form of heat like a blanket. As a result, the planet
continues to warm and a positive feedback loop ensues: higher
temperatures mean the atmosphere can hold more vapor, and
more atmospheric vapor creates conditions for the temperature
to climb.
But water vapor does more than simply trap heat. It circulates
heat, essentially helping to ensure that energy from the sun
gets distributed through the entire column of air stretching
from the surface of the planet to the boundary of space. The
process of evaporation and condensation stirs it up. Evaporating
water pulls energy from the heated surface of the Earth while
condensation puts it back into the atmosphere when that vapor
later condenses at high altitude, forming clouds.
Naturally, these climatological functions are far more complex.
Water vapor directly interacts with other atmospheric contaminants,
gasses, winds, clouds, and more. Much of what is so important
about water vapor in the atmosphere is still only understood
in the coarsest of terms. That's why water vapor is one of
the principal aspects of the Earth's climate targeted for
study by the Aqua satellite. By applying integrated analytic
tools to the study of climate and climate change, experts
hope to learn more specifically how water vapor and other
greenhouse gasses move and function throughout the atmosphere.
THE WATER CYCLE
Water
falling from summer storm clouds onto a field of wheat today
will someday fall again somewhere else. This is the essence
of the water cycle. On all parts of the Earth, from the jungles
of the Amazon to the deserts of North Africa, water moves
through the biosphere, changing its physical state and location
in a never-ending tour of the planet. This fundamental process
is imperative for life, as well as climatological stability
around the world. Although a common molecule made of three
ordinary atoms-two hydrogen, one oxygen-water is a complicated,
dynamic substance. It easily interacts and combines with other
materials and it can also change its physical state. Further,
water tends to migrate, passing from one location to another
as it changes form and interacts with other parts of the natural
world.
The first step in the cycle is evaporation. Heated by sunlight,
liquid water turns to vapor and enters the atmosphere.
Another source of atmospheric water vapor is the respiratory
process of plants. Vapor leaves plants through tiny pores
called stomata. This process is called transpiration.
Pressure and temperature conditions play an important role
in how much moisture the atmosphere can hold. As moist air
ascends into the atmosphere and encounters lower atmospheric
pressure, the invisible water vapor transforms back into liquid
water, and we see the next phase in the water cycle: condensation.
Droplets of water coalesce from traces of vapor, and as they
gain size by joining with other droplets, they yield the next
part of the water cycle. This is called precipitation.
Precipitation can be rain, snow, hail, or other means by which
water falls out of the sky. It's a process that tends to distribute
water over a wide area. But not all precipitation necessarily
reaches the ground. Some of it evaporates as it falls, returning
immediately to the atmosphere as vapor. Plants absorb and
use some of it. But of the precipitation that reaches the
ground, there are generally two ways it can return to larger
bodies of water. Infiltration is the process by which water
soaks into the Earth, either through soil, or cracks in rock,
or sand, or other materials. Gradually this water will migrate
and find its way to larger bodies like lakes and rivers and
oceans.
A more direct means of reaching bigger bodies of water is
through runoff. Water that simply cannot be absorbed travels
either across the surface or through the interstitial spaces
high in the surface layer and finds its way to other reservoirs.
The cycle is endless, with water traveling through the biosphere
via clouds and streams and rain every day. Considering how
vital it is for life to thrive, an understanding of its processes
and functions are important to people all over the world.
As it's name suggests, the Aqua spacecraft will be intensely
involved in studying the water cycle in its many forms. The
specific sensors flying onboard this major new Earth observing
platform will study water as it passes to and from the atmosphere,
collects in soil and sand, caps the poles in the form of snow
and ice, and falls from the air as precipitation.
In many ways, water drives life on Earth. In the following
series of animations we look at the various stages of the
water cycle.
Evaporation
Matter
can exist in three phases: solid, liquid, and gas. When considering
the primary components responsible for climate change, it's
the gaseous state of matter that often is most important.
And in the case of one of the most common substances on Earth
-- water-- it doesn't take much to become a gas.
Depending on total ambient temperature, relative humidity,
wind speed, and water temperature, some molecules of water
are almost always passing from liquid to gaseous state at
the surface. This is called evaporation.
Evaporation is what puts moisture into the air, pulling water
off the surface of lakes and streams and topsoil. Not only
does water vapor enter the atmosphere, but also evaporating
water pulls heat away from the surface. That heat will get
redistributed to a different part of the atmosphere when the
recently liberated water vapor re-condenses.
Transpiration
Related
to evaporation, this is the respiratory equivalent of breathing
in plants. Transpiration is how plants lose water to the surrounding
air. While some water directly evaporates through the walls
of cells on the surface of plants, the majority of water lost
happens through intercellular structures called stomata. These
are like tiny pores. Transpiration helps pull nutrients from
plant roots up to leaves. It's a natural process that's heavily
influenced by ambient temperature, humidity, and other factors.
Additionally, transpiration also helps properly circulate
carbon dioxide and oxygen, diffusing the first into plant
cells for growth, and carrying the second away from cells
as waste gas.
In terms of the water cycle, transpiration plays a significant
role. Vast quantities of water move through the jungles, forests,
and grasslands of the world as plants hold and release water
through their life cycles. As climatological conditions affect
the ability for plants to thrive, changes in large area transpiration
values can have a significant effect on overall weather and
climate conditions.
Condensation
The
process that describes the change in physical state of a gas
to a liquid is called condensation. Generally this is a phenomenon
brought about by either of two processes: cooling of air to
its dewpoint, or the addition of enough water vapor to bring
the air to the point of saturation. In terms of climate, condensation
is an important part of the water cycle. Moisture carried
into the atmosphere following evaporation and transpiration
circulates and travels. But as that moisture either reaches
high enough altitudes so that the air containing it is chilled
by lower temperatures found there, or affected by increasing
humidity from dynamic meteorological conditions, it condenses.
The water molecules start moving more slowly, and the state
of matter begins to change, as water molecules start hooking
up. Gas becomes a liquid.
Condensation can take many forms without necessarily falling
from the sky. Dew, fog, mist, and clouds are all examples
of condensed water.
Precipitation
In
terms of meteorology, precipitation is water in a solid or
liquid form that falls through the atmosphere. Although generally
associated with water that reaches the ground, not every particle
that coalesces and falls through the sky in fact makes all
the way to the surface.
Simply speaking, precipitation is a function of water changing
its material state from vapor to a liquid or a solid. But
more specifically, two fundamental steps must take place for
water to fall from the sky. The first is that basic precipitation
components must develop. These include ice crystals that form
around various minute particles in the atmosphere such as
dust or salts. These are called deposition nuclei. Similar
particles that facilitate water droplets to form are called
condensation nuclei.
The second step is for those ice crystals or condensed droplets
to grow. There are a number of ways this can happen. One common
process is called aggregation, a sequence of joining through
collision or freezing with other crystals or droplets. Because
of their increasing size, these larger droplets or ice crystals
are more apt to collide with other particles of water, and
thus more likely to fall or "precipitate" out of
a cloud.
MONITORING
FIRE FROM SPACE
In
many ways the new MODIS instrument onboard the Aqua spacecraft
will compliment its analogous twin already flying on the Terra
vehicle. One of its parallel uses comes in the form of advanced
fire detection capabilities. MODIS will collect information
about the Earth in 36 different spectral bands, or colors.
By doing so, it will be able to determine where hot spots
or active fires are on the ground, thus enabling fire managers
to better cope with potentially dangerous or destructive scenarios.
The instrument will be able to "see through" clouds
and smoke, helping officials keep track of what's changing
on the ground even when it's difficult to see by conventional
means.
Many images from MODIS will be available in real-time through
the Internet, in addition to what's known as "direct
broadcast", a system for providing near real time access
to data directly from the spacecraft. The United States Forest
Service, already a customer of data from the Terra platform,
will likely continue to use and expand its fire monitoring
capabilities with the new information expected from MODIS
on Aqua. These shots, taken by the Terra MODIS instrument
from August 13-18, 2001, show the progress and containment
of a wildfire in California.
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