Humidity and Condensation

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Humidity and Condensation
Water is unique because it is the only substance
that commonly exists in all three states of
matter. Depending upon temperature, water can
be a solid, a liquid or a gas.
Water is in a solid state at temperatures of 0oC
or below, appearing as ice, snow, hail and ice
crystals.
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Water is in a liquid state between 0oC and 100oC,
present as rain and cloud droplets.
At 100oC or above, water evaporates and enters
the atmosphere as water vapor, an invisible gas.
The bubbles in boiling water are an example of
water vapor. Clouds and steam are liquid
droplets, not gas.
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Although you can't see water vapor, sometimes you
can feel it. The more water vapor the air
contains, the more humid the air feels.
Water often changes state in the atmosphere.
Changing from one state to another requires
energy to either be absorbed or given off.
Condensation occurs when water vapor becomes a
liquid - this creates dew, fog and clouds.
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The opposite of condensation is evaporation.
While condensation releases heat, evaporation
absorbs heat.
So condensation slows down the rate at which air
cools. Evaporation, by contrast, is a cooling
process. After you get out of a swimming pool
you may feel chilly, because the water
molecules on your skin are stealing heat from
your body as they evaporate.
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Frost forms by deposition when water vapor
condenses as a solid. Snow often disappears
as much through sublimation as by evaporation.
Sublimation is when water changes directly from
a solid to a gas, without becoming a liquid.
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The amount of water vapor present in the air
varies widely. The actual amount of water vapor
in the air at a given time and place is called
the specific humidity.
It is expressed as the number of grams of water
vapor per kilogram of air. On a humid summer
day, for example, the humidity may be about 20
grams per kilogram.
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There is a limit to the amount of water vapor
that can be present in the air.
Imagine a fish tank with a glass lid. Some
water molecules have enough energy to to escape
from the surface and become water vapor. Other
water vapor molecules lose energy and return to
the liquid state though condensation.
When there is so much water vapor in the air that
the rate of condensation equals the rate of
evaporation, the air is saturated.
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If any more water evaporates into saturated air,
an equal amount will condense. This explains
why water droplets may form on the lid of the
fish tank. These drops confirm that as water
continues to evaporate, an equal amount of water
condenses from the saturated air.
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The amount of water vapor present in saturated
air depends upon the temperature of the air.
The warmer the air, the more water vapor it can
contain. The water vapor capacity of air roughly
doubles for every rise in air temperature of
about 11oC
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Relative Humidity
When meteorologists refer to the relative
humidity, they are telling us how near the air
is to its maximum capacity for holding
water. Relative humidity compares the actual
amount of water vapor that is present in air
with the maximum amount of water vapor that can
be present at a given temperature and pressure.
It is usually stated as a percentage. Saturated
air has a relative humidity of 100 air that
contains no water vapor has a relative humidity
of 0.
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To calculate the relative humidity of a kilogram
of air, divide its specific humidity by its
maximum capacity.
If the air holds 11 grams of water vapor per kg
of air and it can contain at most 22 grams per
kg, then it holds half of the water vapor it can
contain. Thus, its relative humidity is 50.
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Measuring humidity
Humidity is typically measured with a
psychrometer - an instrument that works on the
principle that evaporation causes cooling.
A psychrometer consists of two thermometers.
One is a dry-bulb thermometer that shows the
air temperature. The other is a wet-bulb
thermometer that has a water-soaked wick
wrapped around its bulb.
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The wet-bulb thermometer usually has a lower
temperature because water evaporating from the
wick cools the wet bulb. The drier the air, the
greater the difference in the readings. If both
the wet-bulb and the dry-bulb thermometers read
the same, this shows that no water is
evaporating from the wet bulb, and the air must
be saturated.
Relative humidity can be determined by using a
psychrometer along with a table like the one on
the next page.
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Condensation
At night, the air cools rapidly. Its ability to
contain water vapor diminishes. The air becomes
saturated. If the air continues to cool past
the point of saturation, condensation occurs.
The water vapor may condense into droplets,
forming clouds or fog. If the water vapor
condenses on a surface, such as grass, it's
called dew.
The temperature at which saturation occurs and
condensation begins is called the dew point.
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The dew point is a measure of the amount of water
vapor in the air. The more water vapor the air
contains, the less the air has to cool in order
for condensation to start, so the higher the dew
point.
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Cooling and Condensation
Two conditions are necessary for water vapor to
condense.
1 - There must be material for water vapor to
condense onto and,
2 - Air must cool to or below its dew point.
When fog or clouds form, the water vapor is
condensing on tiny particles called condensation
nuclei.
Condensation nuclei are usually substances such
as salt, sulfate particles or nitrate particles.
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The salts usually come from ocean water
evaporating. The sulfates and nitrates come
from natural sources and from the burning of
fossil fuels.
These particles are so small that one puff of
smoke contains millions of condensation nuclei.

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The cooling of the air occurs in the following
ways - contact with a colder surface. -
radiation of heat - mixing with colder air -
expansion as it rises
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Dew and frost form when moist air contacts a
colder surface. Fog forms when air cools
through contact and mixing.
Even when air is cooled below its dew point,
condensation into fog or clouds may not occur if
there are no condensation nuclei available.
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Formation of Dew and Frost
When air cools to its dew point through contact
with a colder surface, water vapor condenses
directly onto that surface.
If the temperature is above 0 oC, dew forms.
If the air temperature is below 0 oC, the water
vapor becomes frost through deposition. This
type of frost is often called a "killing frost"
because it causes liquid in the cells of some
plants to freeze. As liquid water freezes, it
expands, bursting the cell walls and killing
the plants.
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Formation of Fog
Fog forms when a cold surface cools the warmer
moist air above it. As water vapor condenses in
the air, tiny droplets fill the air and form
fog. Each droplet is centered around a
condensation nucleus. The droplets are so tiny
that they fall slowly. The slightest air
movement keeps them suspended in the air. At
very cold temperatures, the fog may consist of
tiny ice crystals.
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Radiation Fog
- forms when the night sky is clear and the
ground loses heat rapidly through radiation. As
the ground cools, light winds mix the cooled
bottom air with the warmer air just above it.
Eventually, the whole layer cools to its dew
point.
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The radiation fog occurs at ground level and is
cooler than the air above it. This arrangement
of cold air beneath warm air is called a
temperature inversion.
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Radiation fogs are common in humid valleys near
rivers or lakes. They are most frequent in the
late fall and in winter. These fogs are
thickest in the early morning and are "burned
away" by the later morning sunshine.
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Advection Fog
- forms when warm, moist air blows over a cool
surface.
In the northern US and southern Canada, advection
fogs form when warm, moist southerly winds blow
over snow-covered ground.
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