WEATHER PART 2

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WEATHER PART 2
Water Vapor enters the atmosphere in 3 ways

1. Evaporation the change of phase from liquid to
   vapor
2. Transpiration plants releasing water vapor to
   the air (plant sweat)
3. Evapotranspiration - a combo of evaporation and
   transpiration

Factors that effect the rate at which water vapor
enters the air

1. Air Temperature as air temp. increases,
   evapotranspiration increases
2. Surface Area (how spread out something is) as
   surface area increases, evapotranpiration
   increases
3. Air Circulation (wind) the more air
   circulates, the more evapotranspiration
   occurs.
4. Moisture Content of the Air moist air already
   had a lot of water in it, so it does not want any
   more. Water will evaporate more easily off of
   surfaces when is air is dry.

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Atmospheric Moisture
Saturation when the air is holding all of the
water vapor that it can. There is no more room.
The air becomes saturated when it is cooled to
and reaches its Dewpoint Temperature.
Relative Humidity how much water vapor is in
the air compared to how much it can hold (which
is 100). If the Relative Humidity is 70, that
means it can still hold 30 more before it
reaches its Dewpoint Temperature and becomes
saturated.
Cloud a collection of tiny liquid water
droplets which remain in suspension (floating
because they are too light and little). Some very
high clouds are made of ice crystals (Cirrus
looks like feathers). Clouds are not made of
water vapor. They are either liquid or ice.
Each cloud droplet, raindrop, and snowflake form
around a tiny piece of dirt, dust, sand, pollen,
or salt. This tiny piece of dirt in the center
is called the condensation nuclei. Water needs
something to cling onto. Do not eat snow or
drink rain water. It is dirty.
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How a cloud forms

• Air rises, expands, and cools to its Dewpoint
  Temperature.
• When air reaches its Dewpoint Temperature, a few
  other things automatically happen
• a) the air is SATURATED
• b) the RELATIVE HUMIDITY is 100
• c) CONDENSATION OCCURRS
• Condensation makes a cloud. The water vapor
  turns to liquid, because heat was released.
  Clouds are liquid water droplets.

Precipitation does not come out of every cloud.
The cloud droplets are too light and too tiny to
fall. As condensation continues, the droplets
begin to grow in size. Once they are heavy and
big enough, they fall. That is precipitation.
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Types of Precipitation

1. Rain this happens when the air temperature
   reaches its dewpoint temperature (condensation),
   and that is above 0º C. This simply liquid water
   with a condensation nuclei in the center. Cloud
   droplets are also liquid water. When the cloud
   droplets reach a certain size, they start to fall
   as rain.
2. Snow this happens when the air temperature
   reaches its dewpoint temperature, and that is
   below freezing. This is not condensation, it is
   SUBLIMATION.
3. Sleet this is when the droplets begin to fall
   out of clouds as rain in the upper atmosphere,
   but the air temperature down here is below
   freezing, and the droplets turn into ice pellets
   before they reach the ground.
4. Hail this occurs from large thunderstorms. The
   droplets begin to fall as rain. These droplets
   are whisked back up very high into the storm
   clouds by the severe wind, and they freeze.
   Then, as they fall again and begin to melt, they
   are whisked back up into the storm and freeze
   again. Hail can have many layers.

Freezing Rain is when the air temperature is
higher than the ground temperature. Liquid rain
droplets freeze when they hit the ground.
Dew, Fog, and Frost are not forms of
precipitation!!!!
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A sling psychrometer measures the wet-bulb and
dry bulb temps.
Dry Bulb Temperature Air Temperature
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The Wet-Bulb Depression is the difference between
the wet-bulb and dry-bulb temperatures. You do
not have to subtract if you are given the
Wet-Bulb Depression.
The chart looks like and works like the Dewpoint
chart. Do not confuse them.
Air temp. and Relative Humidity are inverse. The
R.H. is high in the morning and night, and lowest
during mid-day.
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Air Masses there are two basic types of air
masses
High Pressure Anticyclone
Low Pressure - Cyclones

1. Winds blow counter-clockwise towards the center.
2. Convergent air mass
3. In the center, air is moist, cloudy, warmer.
4. Lower density air

1. Winds blow clockwise away from the center.
2. Divergent air mass
3. In the center, the air is dry, cool, clear, and
   windy.
4. Higher density air.

H
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Source Regions areas on the Earths surface
where air masses form and acquire their
characteristics.
The diagram above shows the source regions that
create air masses effecting the United States.
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Cold Front an anti-cyclone overtaking a
cyclone. The cold air in the anti-cyclone moves
in and pushes the warm cyclone out of the way.
Cold fronts have cooler, clear weather behind
them. Cold fronts travel the fastest.
Warm Front a cyclone overtaking an
anti-cyclone. The warm air in the cyclone
moves in and pushes the cooler air out of the
way. Warm fronts have warmer, cloudy, moister
air behind them.

• Occluded Fronts
• Results by warmer air being pushed above the
  surface of Earth
• by cooler air closing in from both sides.
• Associated with large areas of rainy, unsettled
  weather.
• Stationary Front
• Boundaries between air masses that are not
  moving.
• Winds are blowing in opposite directions along a
  boundary
• between warm and cold air masses.

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It always precipitates on the cold
front. High-Pressure Systems are behind a cold
front. Cold fronts have brief, heavy storms on
them.
It always precipitates ahead of the warm
front. Low-Pressure systems are behind a warm
front. Steady precipitation is ahead of these.
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Forecasting and reading weather maps
Synoptic Weather Map
As a class, we are going to write some weather
forecasts below
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Thunderstorm

• Thunderstorms can occur anywhere in the world and
  at any time of the day. All thunderstorms produce
  lightning and thunder. Some have the potential to
  produce damaging straight-line winds, large hail,
  heavy rain, flooding, and tornadoes.
• A thunderstorm is classified as severe when it
  contains one or more of the following phenomena
• Hail 3/4" or greater
• Winds gusting in excess of 50 knots (57.5 mph)
• A Tornado

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Lightning
A moving thunderstorm gathers a pool of
positively charged particles along the ground
that travel with the storm. As the differences in
charges continue to increase, positively charged
particles rise up taller objects such as trees,
houses, and telephone poles. The negatively
charged area in the storm will send out a charge
toward the ground called a stepped leader. It is
invisible to the human eye, and moves in steps in
less than a second toward the ground. When it
gets close to the ground, it is attracted by all
these positively charged objects, and a channel
develops. You see the electrical transfer in this
channel as lightning. There may be several return
strokes of electricity within the established
channel that you will see as flickering
lightning. Thunder The lightning channel heats
rapidly to 30,000 C. The rapid expansion of
heated air causes the thunder. Since light
travels faster than sound in the atmosphere, the
sound will be heard after the lightning. If you
see lightning and hear thunder at the same time,
that lightning is in your neighborhood!
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Tornado
Warm, humid conditions are very favorable for
thunderstorm development as strong updrafts feed
warm, moist air into thunderstorms. If the air is
very unstable, severe thunderstorms with damaging
winds, large hail, and sometimes tornadoes erupt.
A tornado is a violently rotating column of air
which is in contact with both a cumulonimbus (or,
in rare cases, a cumulus) cloud base and the
surface of the earth. Tornadoes come in many
sizes, but are typically in the form of a visible
condensation funnel, whose narrow end touches the
earth and is often encircled by a cloud of
debris. They are measured on the Fujita scale
(1-5), which is now changed to the EF scale
because of a few modifications.
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Category Description Level Of Damage
F-0 Gale Tornado40 - 72 MPH Chimneys damaged branches broken off trees shallow-rooted trees uprooted sign boards damaged.
F-1 Moderate Tornado73 - 112 MPH Roof surfaces peeled off mobile homes pushed off foundations or overturned moving autos pushed off roads.
F-2 Significant Tornado113 - 157 MPH Roofs torn off frame houses mobile homes demolished box cars pushed over large trees snapped or uprooted light-object projectiles generated.
F-3 Severe Tornado158 - 206 MPH Roofs and some walls torn off well-constructed houses trains overturned most trees in forest uprooted heavy cars lifted off the ground and thrown.
F-4 Devastating Tornado207 - 260 MPH Well-constructed houses leveled structures with weak foundations relocated cars thrown and large projectiles generated.
F-5 Incredible Tornado261 - 318 MPH Strong frame houses lifted off foundations and carried considerable distance to disintegrate automobile-sized projectiles hurtle through the air in excess of 100 yards
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Hurricane
Hurricanes start out as Tropical Depressions (mT,
Low-Pressure) in the Atlantic Ocean surrounding
the Equator, off the coast of Africa. Hurricane
season begins at the end of summer and runs into
late October. They form because the abundance of
summer heat evaporates large amounts of ocean
water putting it into the atmosphere. Once storm
winds reach 75 miles/hour, it is classified as a
hurricane. Hurricanes loose their energy after
they hit land, because the evaporation of ocean
water is their fuel. Hurricanes are measured by
The Saffir Simpson Scale of 1-5.
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1 Wind Speed 74 - 95 MPHStorm Surge 4 - 5 Feet Above Normal Primary damaged to unanchored mobile homes, shrubbery, and trees. Some coastal road flooding and minor pier damage. Little damage to building structures.
2 Wind Speed 96 - 110 MPHStorm Surge 6 - 8 Feet Above Normal Considerable damage to mobile homes, piers, and vegetation. Coastal and low-lying escape routes flood 2 - 4 hours before arrival of hurricane center. Buildings sustain roofing material, door, and window damage. Small craft in unprotected moorings break moorings.
3 Wind Speed 111 - 130 MPHStorm Surge 9 - 12 Feet Above Normal Mobile homes destroyed. Some structural damage to small homes and utility buildings. Flooding near coast destroys smaller structures larger structures damaged by floating debris. Terrain continuously lower than 5 feet. ASL may be flooded up to 6 miles inland.
4 Wind Speed 131 - 155 MPHStorm Surge 13 - 18 Feet Above Normal Extensive curtain wall failures with some complete roof structure faiture on small residences. Major erosion of beaches. Major damage to lower floors of structures near the shore. Terrain continuously lower than 10 feet. ASL may flood (and require mass evacuations) up to 6 miles inland.
5 Wind Speed Over 155 MPHStorm Surge Over 18 Feet Above Normal Complete road failure on many homes and industrial buildings. Some complete building failures. Major damage to lower floors of all structures located less than 15 feet ASL and within 500 yards of the shoreline. Massive evacuation of low ground residential areas may be required.