Transcript Water in the Air - lewisscientific7th

Water in the Air
Pgs. 36 - 43
Weather and Water
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Weather is the condition of the
atmosphere at a particular time and
place.
The water cycle is the continuous
movement of water from water
sources to the air, ground and back
again.
Water Terms
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Condensation – occurs when water vapor
cools and changes back into liquid droplets.
Evaporation – occurs when liquid water
changes into water vapor.
Transpiration – the process by which plants
release water vapor into the air through
their leaves.
Precipitation – occurs when rain, snow,
sleet, or hail falls from the clouds onto the
Earth’s surface.
Runoff – water that flows across land and
collects in rivers, streams and oceans
Humidity
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The amount of water vapor or moisture in
the air is called humidity.
As water evaporates, the humidity in the
air increases.
The air’s ability to hold water vapor
depends on air temperature.
As temperature increases so does the
air’s ability to hold water.
What does that mean?
Relative Humidity
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The amount of moisture the air contains
compared to the maximum amount it can
hold is relative humidity.
This amount is given as a percentage.
When air is holding 100% it is called
saturated.
If air is not saturated you can still
determine its relative humidity.
Calculating Relative Humidity
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Pretend we know the classroom can hold
50g of water vapor.
Also, we know that there is 36 grams of
water vapor in the air in this classroom.
If we take the amount of water vapor
present and divide by the water vapor
possible and multiply by 100, we get
relative humidity.
36g / 50g x 100= 72% relative humidity
Water Vapor vs. Temperature
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If temperature stays the same, relative
humidity changes as water vapor enters
or leaves the air.
More water vapor at a temperature
means a higher humidity.
If the water vapor in the air stays the
same, relative humidity can change based
upon the change in air temperature.
Warmer air can hold more water vapor
than cooler air.
Measuring Relative Humidity
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Psychrometer – an instrument used to
measure relative humidity.
It is made of two thermometers.
One is a wet bulb thermometer, and one is a
dry bulb thermometer.
The wet bulb thermometer has a damp cloth
on it.
When air passes over it the water
evaporates.
If humidity is high the water won’t
evaporate.
The amount that evaporates determines
relative humidity.
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The difference in bulb
readings will indicate the
amount of water vapor in the
air.
A large difference will mean
there is little water vapor in
the air.
The Process of Condensation
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Before condensation can occur, air
must be saturated, 100% relative
humidity.
When air cools once saturated,
condensation occurs.
Saturation can occur when water
vapor is added to air or when air is
cooled to its dew point.
Dew Point
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Dew point – the temperature at
which air must cool to be
completely saturated.
Dew will form once air is saturated
and has a surface to form on.
A glass of ice water is an example
of air meeting its dew point.
Clouds
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A collection of millions of tiny water
droplets or ice crystals is called a
cloud.
They form when warm rising air
cools.
The air is saturated, and water
vapor turns to liquid water.
Since condensation requires a
surface, the dust and smoke in the
atmosphere is the surface used to
form clouds and rain.
Cumulus Clouds
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Cumulus clouds look like big puffy
cotton balls.
They tend to have flat bottoms and
indicate fair weather.
Larger cumulus clouds can produce
thunderstorms.
These are cumulonimbus clouds.
Stratus Clouds
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Stratus clouds form in layers and
cover large areas in the sky.
They are caused by gentle lifting air.
A nimbostratus cloud is one that
produces light to heavy continuous
rain.
A stratus cloud on the ground is
called fog.
Cirrus Clouds
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Cirrus clouds are thin feathery
clouds found at high altitudes.
They form when wind is strong may
indicate bad weather approaching.
Classifying Clouds
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Clouds can be classified by both their type
and their altitude.
High clouds: these are made at low
temperature and made of ice crystals and
are given the prefix cirroMiddle Clouds: these are made of both ice
crystals and water vapor and are given
the prefix altoLow Clouds: these are made of water
droplets and are given the prefix strato-
Precipitation
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Precipitation is water, in solid or
liquid form, that falls from the air to
the Earth.
There are four major types of
precipitation: rain, snow, sleet, and
hail
Rain
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Rain is the most common type of
precipitation.
It is a liquid and raindrops fall when
they are so large that they cannot
be held up by the atmosphere.
Cloud droplets collide and join until
they are large enough to be
raindrops and fall to the Earth.
Snow and Sleet
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Snow is the most common solid
form of precipitation.
Snow forms when water vapor
changes directly into a solid.
Sleet (freezing rain) forms when
rain falls through a layer of freezing
air.
The rain can fall as a solid or turn to
a solid upon hitting a surface.
Hail
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Hail is solid precipitation that falls as balls
or lumps of ice.
Cumulonimbus clouds are typically the
only producers of hail.
Sometimes large updrafts of air cause
raindrops to go up to cooler temperatures
and freeze.
They fall back down and then a larger
updraft will force it back up.
The raindrop picks up moisture and
particles and gets bigger and bigger until
the updrafts can not support it.
Measuring precipitation
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A rain gauge is an instrument used
to measure rain fall.
It consists of a funnel and a cylinder
for measuring.
Snow can be measured by both
depth and water content.
A stick can show the depth and
melting the snow will show water
content.