Vapor : the
number one greenhouse gas
Water vapor is water in
its gaseous state-instead of liquid or solid (ice). Water vapor is
totally invisible. If you see a cloud, fog, or mist, these are all
liquid water, not water vapor.
with it energy—is carried around the globe by weather systems. This
satellite image shows the distribution of water vapor over Africa and
the Atlantic Ocean. White areas have high concentrations of water vapor,
while dark regions are relatively dry. The brightest white areas are
towering thunderclouds. The image was acquired on the morning of
September 2, 2010 by SEVIRI aboard METEOSAT-9. (Image ©2010 EUMETSAT.)
Water is a unique
substance. It can exist as a liquid, solid (ice), and gas (water vapor).
A primary way water vapor increases in the atmosphere is through
evaporation. Liquid water evaporates from oceans, lakes, rivers, plants,
the ground, and fallen rain. A lot or a little water vapor can be
present in the air. Winds in the atmosphere then transport the water
vapor from one place to another. Most of the water vapor in the
atmosphere is contained within the first 10,000 feet or so above the
earth's surface. Water vapor also is called moisture.
(expressed as grams of water vapor per cubic meter volume of air) is a
measure of the actual amount of water vapor (moisture) in the air,
regardless of the air's temperature. The higher the amount of water
vapor, the higher the absolute humidity. For example, a maximum of about
30 grams of water vapor can exist in a cubic meter volume of air with a
temperature in the middle 80s. SPECIFIC HUMIDITY refers to the weight
(amount) of water vapor contained in a unit weight (amount) of air
(expressed as grams of water vapor per kilogram of air). Absolute and
specific humidity are quite similar in concept.
Relative humidity (RH)
(expressed as a percent) also measures water vapor, but RELATIVE
to the temperature of the air. In other words, it is a measure of the
actual amount of water vapor in the air compared to the total amount of
vapor that can exist in the air at its current temperature. Warm air can
possess more water vapor (moisture) than cold air, so with the same
amount of absolute/specific humidity, air will have a HIGHER relative
humidity if the air is cooler, and a LOWER relative humidity if the air
is warmer. What we "feel" outside is the actual amount of
moisture (absolute humidity) in the air.
consider the "dewpoint" temperature (instead of, but analogous
to absolute humidity) to evaluate moisture, especially in the spring and
summer. The dewpoint temperature, which provides a measure of the actual
amount of water vapor in the air, is the temperature to which the air
must be cooled in order for that air to be saturated. Although weather
conditions affect people differently, in general in the spring and
summer, surface dewpoint temperatures in the 50s usually are comfortable
to most people, in the 60s are somewhat uncomfortable (humid), and in
the 70s are quite uncomfortable (very humid). In the Ohio Valley
(including Kentucky), common dewpoints during the summer range from the
middle 60s to middle 70s. Dewpoints as high as 80 or the lower 80s have
been recorded, which is very oppressive but fortunately relatively rare.
While dewpoint gives one a quick idea of moisture content in the air,
relative humidity does not since the humidity is relative to the air
temperature. In other words, relative humidity cannot be determined from
knowing the dewpoint alone, the actual air temperature must also be
known. If the air is totally saturated at a particular level (e.g., the
surface), then the dewpoint temperature is the same as the actual air
temperature, and the relative humidity is 100 percent.
Water vapor is added
to the atmosphere by two primary natural mechanisms:
Plants take in water in their root systems and release it into the
atmosphere through their leaves.
Water is converted to vapor phase from the ground or a body of water and
mixes with the atmosphere.
for approximately 90% of water vapor, and transpiration accounts for 10%
of water vapor.
Water vapor is a natural
and very important component of the Earth’s atmosphere. Its
distribution influences many physical and chemical properties of the
atmosphere, including weather, clouds, precipitation, lightning
generation, convective uplift, and the Antarctic ozone hole.
Water is constantly
cycling through the atmosphere. Water evaporates from the Earth’s
surface and rises on warm updrafts into the atmosphere. It condenses
into clouds, is blown by the wind, and then falls back to the Earth as
rain or snow. This cycle is one important way that heat and energy are
transferred from the surface of the Earth to the atmosphere, and
transported from one place to another on our planet.
The water vapor image show an infrared band which is
affected strongly by the presence of water vapor. Essentially, the image
shows the altitude of the highest moist layer in the atmosphere. Bright
areas reflect the location of high clouds either due to jet stream
cloudiness or due to thunderstorm activity. The dark areas reflect the
location of dry air at high altitudes. This is associated with dry air
intrusion and sinking motion associated with high pressure systems. This
image is a decent tracer of jet stream winds which will show up as
Water vapor is also the
most important greenhouse gas in the atmosphere. Heat radiated from
Earth’s surface is absorbed by water vapor molecules in the lower
atmosphere. The water vapor molecules, in turn, radiate heat in all
directions. Some of the heat returns to the Earth’'s surface. Thus,
water vapor is a second source of warmth (in addition to sunlight) at
the Earth’s surface.
Despite its importance to
atmospheric processes over a wide range of spatial and temporal scales,
water vapor is one of the least understood and poorly described
components of the earth's atmosphere.
As the temperature of the
atmosphere rises, more water is evaporated from ground storage (rivers,
oceans, reservoirs, soil). Because the air is warmer, the absolute
humidity can be higher (in essence, the air is able to 'hold' more water
when it's warmer), leading to more water vapor in the
atmosphere. As a greenhouse gas, the higher concentration of water vapor
is then able to absorb more thermal IR energy radiated from the Earth,
thus further warming the atmosphere. The warmer atmosphere can then hold
more water vapor and so on and so on. This is referred to as a 'positive
feedback loop'. As water vapor increases in the atmosphere, more
of it will eventually also condense into clouds, which are more able to
reflect incoming solar radiation (thus allowing less energy to reach the
Earth's surface and heat it up).
Water vapor accounts for the
largest percentage of the greenhouse effect.
The major greenhouse
water vapor, which causes about 36–70% of the greenhouse effect on
Earth (not including clouds)
dioxide, which causes 9–26%
which causes 4–9%
ozone, which causes 3–7%.
It is not possible to state
that a certain gas causes a certain percentage of the greenhouse effect, because
the influences of the various gases are not additive. (The higher ends of the
ranges quoted are for the gas alone; the lower ends, for the gas counting
Other greenhouse gases include, but are not limited to, nitrous
oxide, sulfur hexafluoride, hydrofluorocarbons, perfluorocarbons and
Vapor Confirmed as Major Player in Climate Change