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Weather
What is the connection between
the water cycle and tornadoes,
floods, droughts, and hurricanes?
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States of Water
Water is known to exist in three different
states; as a solid, liquid or gas.
Clouds, snow, and rain are all made of up of
some form of water. A cloud is comprised of
tiny water droplets and/or ice crystals, a
snowflake is an aggregate of many ice
crystals, and rain is just liquid water.
Image/Text/Data from the University of Illinois WW2010 Project."
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Water on the Earth
 Water covers 70% of the
earth's surface, but it is
difficult to comprehend the
total amount of water when
we only see a small
portion of it.
 Look at the volumes of
water contained on land, in
oceans, and in the
atmosphere. Arrows
indicate the annual
exchange of water
between these storages.
Image/Text/Data from the University of Illinois WW2010 Project."
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Water on the Earth
 The oceans contain 97.5% of the earth's water,
land 2.4%, and the atmosphere holds less than
.001%, which may seem surprising because water
plays such an important role in weather.
 The annual precipitation for the earth is more than
30 times the atmosphere's total capacity to hold
water.
 This fact indicates the rapid recycling of water that
must occur between the earth's surface and the
atmosphere.
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Water Cycle
 Water in the different states move through
the water cycle.
– Liquid
– Solid
– Gas in the form of water vapor
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Evaporation
 Water is transferred from the surface to the
atmosphere through evaporation, the process by
which water changes from a liquid to a gas.
 Approximately 80% of all evaporation is from the
oceans, with the remaining 20% coming from
inland water and vegetation.
 Winds transport the evaporated water around the
globe, influencing the humidity of the air
throughout the world.
Image/Text/Data from the University of Illinois WW2010 Project."
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Evaporation
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 Water existing as a gas is called water vapor.
 When referring to the amount of moisture in the air,
we are actually referring to the amount of water
vapor.
 Common sources of moisture for the United States
are the warm moist air masses that flow northward
from the Gulf of Mexico and western Atlantic Ocean
as well as the moist Pacific air masses brought
onshore by the westerlies.
Image/Text/Data from the University of Illinois WW2010 Project."
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Rising Air
 Key process in the
production of clouds and
precipitation
 Imagine a block of air, or
air parcel, rising upward
through the atmosphere.
 The air parcel expands
as it rises and this
expansion causes the
temperature of the air
parcel to decrease.
Image/Text/Data from the University of Illinois WW2010 Project."
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Condensation
 Condensation is the change of water from its
gaseous form (water vapor) into liquid water.
 Condensation generally occurs in the atmosphere
when warm air rises, cools and looses its capacity
to hold water vapor.
 As a result, excess water vapor condenses to form
cloud droplets.
 The upward motions that generate clouds can be
produced by convection in unstable air,
convergence associated with cyclones, lifting of air
by fronts and lifting over elevated topography such
as mountains.
Image/Text/Data from the University of Illinois WW2010 Project."
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Condensation
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Condensation
 The upward motions that
generate clouds and lead
to precipitation can be
produced by convection in
unstable air, convergence
of air near cloud base,
lifting of air by fronts and
lifting over elevated
topography such as
mountains.
Image/Text/Data from the University of Illinois WW2010 Project."
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Lifting By Convection
 As the earth is heated by
the sun, bubbles of hot air
(called thermals) rise
upward from the warm
surface.
 A thermal cools as it rises
and becomes diluted as it
mixes with the surrounding
air, losing some of its
buoyancy (its ability to
rise).
Image/Text/Data from the University of Illinois WW2010 Project."
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 When a deep stable layer
exists just above the cloud
base, continued vertical growth
is restricted and only fair
weather cumulus are able to
form.
 However, if a deep unstable
layer (cold air aloft) is present,
continued vertical growth is
likely, leading to the
development of a
cumulonimbus cloud, which
contains raindrops.
Image/Text/Data from the University of Illinois WW2010 Project."
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Relative Humidity
 Indicates how moist the air is
 Relative humidity may be defined as the
ratio of the water vapor density (mass per
unit volume) to the saturation water vapor
density, usually expressed in percent:
Relative Humidity =
(Actual Vapor
Density)
-------------------(Saturation
Vapor Density)
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Precipitation
 If the cloud is sufficiently deep or long lived,
precipitation will develop.
 As the parcel rises, its humidity increases
until it reaches 100%. When this occurs,
cloud droplets begin forming.
Image/Text/Data from the University of Illinois WW2010 Project."
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Rain or Snow
 Dependent upon temperature
 Typically forms high in the atmosphere where
the temperature is below freezing.
 As ice crystals form aloft and fall toward the
surface, they collect each other to form large
snowflakes.
 If ground temperature is above 32 F, the freezing
level must be located somewhere above the
ground.
 As the falling snow passes through the freezing
level into the warmer air, the flakes melt and
collapse into raindrops.
Image/Text/Data from the University of Illinois WW2010 Project."
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Image/Text/Data from the University of Illinois WW2010 Project."
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Snow
 When the air
temperature at the
ground is less than
32 F, the snowflakes
do not melt on the
way down and
therefore reach the
ground as snow.
Image/Text/Data from the University of Illinois WW2010 Project."
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Thunderstorm Formation
 First Condition: Must have moisture in the
lower to mid levels of the atmosphere.
– As air rises in a thunderstorm updraft, moisture
condenses into small water drops which form
clouds and eventually precipitation.
– When the moisture condenses, heat is released
into the air, making it warmer and less dense
than its surroundings.
– The added heat allows the air in the updraft to
continue rising.
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Thunderstorm Formation
 Second condition: Air mass instability
– Air which is pushed upward by some force will continue
upward.
– An unstable air mass usually contains relatively warm
(usually moist) air near the earth's surface and relatively
cold (usually dry) air in the mid and upper levels of the
atmosphere.
– As the low-level air rises in an updraft, it becomes less
dense than the surrounding air and continues to rise.
– The air will continue to move upward until it becomes
colder and more dense than its surroundings.
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Thunderstorm Formation
 Third condition: A Source of lift.
– Lift is a mechanism for starting an updraft in a
moist, unstable air mass.
– As the sun heats the earth's surface, portions of
the surface (and the air just above the surface)
will warm more readily than nearby areas.
– These "warm pockets" are less dense than the
surrounding air and will rise.
– If the air has sufficient moisture and is unstable,
a thunderstorm may form.
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What is the connection between
the water cycle and
thunderstorms?
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SuperCells
 Supercells are large thunderstorms with
deep rotating updrafts and can have a
lifetime of several hours.
 Supercells can produce frequent lightning,
large hail, damaging winds, and tornadoes.
 These storms tend to develop during the
afternoon and early evening when the
effects of heating by the sun are strongest.
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Tornados
 Thunderstorms develop in warm, moist air in
advance of eastward-moving cold fronts.
 These thunderstorms often produce large
hail, strong winds, and tornadoes.
 Tornadoes in the winter and early spring are
often associated with strong, frontal systems
that form in the Central States and move
east.
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Tornado Formation
 Before thunderstorms
develop, a change in
wind direction and an
increase in wind speed
with increasing height
creates an invisible,
horizontal spinning
effect in the lower
atmosphere.
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Tornado Formation
 Rising air within the
thunderstorm updraft
tilts the rotating air
from horizontal to
vertical.
 The air is cooled as it
rises and water vapor
condenses to form the
familiar funnel shaped
cloud.
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Tornado Formation
 An area of rotation, 2-6
miles wide, now extends
through much of the
storm. Most strong and
violent tornadoes form
within this area of strong
rotation.
 As the rotating winds
begin to pick up dirt and
debris from the ground,
the funnel will darken.
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Tornadoes
 The strongest tornadoes occur in supercell
thunderstorms which can also produce large
hail and strong downbursts.
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What is the connection between
the water cycle and tornadoes?
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Hurricanes
 Hurricanes are formed from simple complexes of
thunderstorms.
 These thunderstorms can only grow to hurricane
strength with cooperation from both the ocean and
the atmosphere.
– The ocean water itself must be warmer than 26.5
degrees Celsius (81°F).
– The heat and moisture from this warm water is
ultimately the source of energy for hurricanes.
 Hurricanes will weaken rapidly when they travel
over land or colder ocean waters -- locations with
insufficient heat and/or moisture.
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Hurricanes
 Related to having warm ocean water, high
relative humidities in the lower and middle
troposphere are also required for hurricane
development.
 The high humidity reduces the amount of
evaporation in clouds and maximizes the
latent heat released because there is more
precipitation.
 The concentration of latent heat is critical to
driving the system.
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Hurricanes
 How Wind Shear Develops into Hurricanes:
http://ww2010.atmos.uiuc.edu/(Gh)/guides/
mtr/hurr/grow/home.rxml
 CISK:
http://ww2010.atmos.uiuc.edu/(Gh)/guides/
mtr/hurr/grow/cisk.rxml
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What is the connection between
the water cycle and hurricanes?
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Droughts
 Drought is when you have less rainfall than
you expected over an extended period of
time, usually several months or longer.
 If rain does occur it usually isn't enough for
the ground to absorb before it is evaporated
again.
 Most droughts tend to occur during summer,
as the weather is hot and water is quickly
evaporated.
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What is the connection between
the water cycle and droughts?
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What is the connection between
the water cycle and blizzards?
Floods?
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