Weather -

Chapter 16 Section 1
Study Guide
What is Weather?
Weather refers to the state of the atmosphere at
a specific time and place.
Weather describes conditions such as air
pressure, wind, temperature, and the amount of
moisture in the air.
Weather is the result of heat transfer and
Earth’s air and water.
Air Temperature:
Air is made up of molecules that are always
moving, even if there is no wind.
Temperature is the measure of the average
amount of motion of molecules.
When the temperature is high, molecules in
the air move rapidly and it feels warm.
When the temperature is low, molecules in
the air move less rapidly and it feels cold.
Air moving in a specific direction is called wind.
Wind moves from regions of high pressure to
regions of low pressure.
Wind direction can be measured using a wind
vane or a wind sock, which point in the
direction towards where the wind is blowing.
Wind speed can be measured using an
anemometer, which has rotating cups that spin
faster when the wind is strong.
The amount of water vapor present in the air is called
More water vapor can be present when the air is warm
than when it is cool.
When air is warm, the water vapor molecules in the air
move quickly and don’t come together easily.
When the air is cool, water vapor molecules move
slower, stick together, and form liquid water droplets
through condensation.
When enough water vapor is present in the air for
condensation to take place, the air is saturated
Relative Humidity:
Relative humidity is the measure of the amount
of water vapor present in the air compared to
the amount needed for saturation at a specific
Air at 25O C is saturated when it contains 22g of
water vapor per cubic meter of air, so the
relative humidity is 100%. If air at 25O C
contains only 11g of water vapor, the relative
humidity is 50%.
Dew Point:
When the temperature drops, less water vapor
can be present in the air. The water vapor will
condense or form ice crystals.
The temperature at which the air is saturated
and condensation forms is the dew point.
The dew point changes with the amount of
water vapor in the air.
When air near the ground cools to its dew
point, water vapor condenses and forms dew.
Forming Clouds:
Clouds form as warm air is forced upwards,
expands, and cools.
When the relative humidity reaches a 100% in
the atmosphere, the water vapor begins to
condense in tiny droplets around small particles
such as dust and salt. These particles are called
condensation nuclei.
The droplets of water are so small that they stay
suspended in the air, and billions of the
droplets form a cloud.
Classifying Clouds:
 Clouds are classified mainly by shape and height, which vary
with temperature, pressure, and the amount of water vapor in
the atmosphere.
 The three main cloud types by shape are stratus, cumulus, and
1. Stratus clouds form layers or smooth, even sheets in the sky.
They form at low altitudes, and are associated with fair
weather, rain and snow.
o Fog is a type of stratus cloud formed when air near the
ground reaches its dew point.
2. Cumulus Clouds are puffy, white clouds with flat bases. They
can grow to great hights and can be associated with fair weather
or thunderstorms.
3. Cirrus Clouds are fibrous or curly. They are high, thin, white,
feathery clouds made from ice crystals. They are associated
with fair weather, but can indicate approaching storms.
Prefixes of cloud names describe the height of the
cloud base.
o Cirro = high clouds
o Alto = middle elevation clouds
o Strato = low elevation clouds
o Example: Cirrostratus clouds are high clouds. They
signal fair weather or an approaching storm.
Altostratus clouds form at middle levels.
Clouds associated with rain or snow have the word
nimbus attached to them. (Latin for dark rain cloud)
o When a cumulus cloud grows into a thunderstorm,
it is called a cumulonimbus.
o Nimbostratus clouds bring long, steady rain or
Water falling from clouds is called precipitation.
When cloud droplets combine and grow large enough
to fall to the Earth, precipitation occurs.
The size of a raindrop depends on:
o the strength of the updraft in clouds- strong updrafts
can keep drops suspended where they can combine
and grow very large.
o the rate of evaporation as a drop falls to earth – if
the air is dry, the rain drops can shrink or completely
evaporate before they hit the ground.
The four main types of precipitation are determined
by air temperature.
1. Rain: falls in temperatures above freezing.
2. Snow: forms when the air temperature is so cold
that water vapor changes directly to a solid.
3. Sleet: forms when raindrops pass through a layer of
freezing air near the Earth’s surface, forming ice
4. Hail: lumps of ice that form in cumulonimbus
clouds when water freezes in layers around a small
nucleus of ice. Hailstones grow larger as they are
tossed up and down by rising and falling air.
All images and text © copyright Gene
unless otherwise indicated.
Batter Up!
Hail Damage
Hail about an inch larger than these stones broke this
windshield. Chasers have witnessed golf ball sized
stones about this diameter not break glass. It depends
on the hardness of the stones and the angle of the
impact. Occasionally a motorist can get lucky and get
hit with a large soft stone when temperatures aloft are
warmer. The soft stones crack apart absorbing the
impact. In this case the stones were rock hard.
Note the irregular shape of some of the stones. One on the left is elongated with
small spikes, and others that fell that day were jagged. Near this location a
particularly large stone was dug out of the mud; it measured five inches in
diameter; although, that is a big chunk of ice the largest stone fell at Coffeyville,
Kansas in 1970 and measured just under seven inches in diameter. I was there,
and had an old Polaroid camera, but was so busy watching the storm I forgot
about taking pictures.
Hailstorms, Do They Look Different?
Typical Hailstorm...Or Is It?
A severe thunderstorm rolling across the western prairie. This storm blasted windows
out of a town in western Oklahoma with golf ball size hail and 70 m.p.h. winds. The
worst of the hail was under the dark shelf of clouds close to the ground.
An early spring hailstorm
turns the green grass
white over the fields of
west Texas.
Note the lack of dark low
clouds. We can see
completely under this
storm which contains
almost all hail and little
Such pure hailstorms
are more transparent then
classic thunderstorms.
Travelers may be fooled
by storms with this
Without the hail in the
foreground this cloud
would not be that
Storm across the high plains may
have deceptively high cloud bases.
This feature allows more light to
filter in under the storm clouds.
Since hail does not occur in the
density of heavy rain, this too lets in
light making for a bright storm. This
particular cell was dumping very
large hail across northern Kansas in
the yellow of the late evening
sunlight. Long streamers and
striations are visible in the clouds as
evidence of the hail falling through
them, but nothing is seen below
cloud base.
Storms do give away their secrets
to careful observers. Note the thick
streamers in the clouds. This a
signature for very large hail when
nothing is seen below cloud base.
These streamers may be
distinguished from typical virga
streamers, which have a thinner
appearance and carry the cloud
base lower with them. This storm
battered the area west of Abiline,
Texas with softball size (4 inch
plus) hailstones. It's quite likely
this hail was coming from the anvil
well above these clouds, and
falling through the lower layers
making the streamers evident.
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