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Unit 6 Chapter 21 Weather
Section 1 Air Masses
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Equatorial air being warmer and lighter creates a low pressure
center. Colder air which is heavier migrates towards the lower
presser area.
How Air Moves
Air moves from ______________to ______________. At high
altitudes, air flows from the poles to the equator. This creates the
three wind belts which are in turn influenced by the Coriolis Effect.
Winds in the Northern Hemisphere ________________________
Winds in the Southern Hemisphere ________________________
Formation of Air Masses
An air mass is a large body of air in the lower troposphere.
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If it forms over a Polar region, it will be cool and dry. In the tropics,
it will be warm and moist. When it moves it takes the characteristics
with it. The Canadian cool air can travel out to Chicago and cool it.
The Gulf of Mexico will effect areas and make them warm and moist.
It can change as it moves into certain areas. When a cold polar
mass travels south, but can warm up as it continues to move south.
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Types of Air Masses
Air masses are classified according to the source area. The term
continental is given to dryer air because it forms over land. The term
maritime is given to moist air because it forms over water.
Continental Air Masses
Continental tropical – ________– forms over warm land
Warm and Dry
Continental polar – _________– forms over cold land
Cool and Dry
Continental Arctic –_______ – forms over very cold land
Cold and Dry – Winter only
Maritime Air Masses
Maritime tropical – ________ – forms over warm tropical seas
Warm and Moist
Maritime polar – _______ – forms over cold seas
Cool and Moist
Maritime Artic – ______ – forms over very cold seas
Cold and Moist – Extreme North or South only
North American Air Masses
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Tropical Air Masses
Continental Tropical air masses can form over the deserts of the
Southwestern United States. Only forms in the summer.
Maritime Tropical air masses can form over warm waters like the
Atlantic Ocean, Caribbean Sea & the Gulf of Mexico. This brings
mild, cloudy weather. They may also form over the Pacific Ocean;
however, they usually do not reach the coast.
Polar Air Masses
These air masses will form over Northern Canada, Northern Pacific
and the Northern Atlantic Oceans. These air masses will influence
the weather in North America.
North Atlantic Influences:
In the summer it can produce, cool weather with low clouds & fog.
In the winter it can produce cold, cloudy weather often with snow.
North Pacific Influences:
In the summer it can produce cool foggy weather.
In the winter it can produce cooler weather with rain or snow.
Section 2 Fronts
Air Fronts
They are the boundary between two different air masses. They
contain different conditions in each air mass which is why the
weather changes as front pass. They are most common at midlatitudes where southward-moving polar air masses and
northward-moving tropical air masses meet. Fronts usually
bring precipitation. They can have steep slopes. They can have
a 1 to 50 ratio which means that the temperature raises 1 km
vertical to 50 horizontal.
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Types of Fronts
A front is formed when two air masses collide.
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Cold Front – Will occur when colder air advances into and area with
warmer air. The cold air is denser making it stay toward the ground
pushing warm air above it.
Weather
Phenomenon
Prior to the Passing Contact with the
of the Front
Front
After the Passing of the
Front
Temperature
Warm
Cooling suddenly
Cold and getting colder
Atmospheric
Pressure
Decreasing steadily
Leveling off then
increasing
Increasing steadily
Winds
South to southeast
Variable and gusty
West to northwest
Precipitation
Showers
Heavy rain or snow,
Showers then clearing
hail sometimes
Clouds
Cirrus and
cirrostratus changing Cumulus and
later to cumulus and cumulonimbus
cumulonimbus
Cumulus
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Warm Front – Is when warm air is advancing into an area with colder
air. The cold air being dense will sink and the warm air will over ride
the cold air.
Weather
Phenomenon
Prior to the Passing of the Contact with the
Front
Front
Temperature Cool
Atmospheric
Decreasing steadily
Pressure
South to southeast
Winds
After the Passing of the Front
Warming
suddenly
Warmer then leveling off
Leveling off
Slight rise followed by a decrease
Variable
South to southwest
Light drizzle
None
Precipitation
Showers, snow, sleet or
drizzle
Clouds
Cirrus, cirrostratus,
Stratus,
altostratus, nimbostratus, sometimes
and then stratus
cumulonimbus
Stationary and Occluded Fronts
Clearing with scattered stratus,
sometimes scattered cumulonimbus
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No Frontal Movement. They stall where they are until one air mass
eventually gives up.
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________________– These will occur when fast moving cold air
mass pushes up the entire slow moving warm air mass. This forms
large areas of rain because there is a large area of rapid
condensation.
Polar Fronts and Midlatitude Cyclones
A polar front is the boundary where cold polar air meets the tropical
air mass of the midlatitudes.
Mid Latitude Cyclone (low)
At the end of WWI, Norwegian meteorologists studied these.
We still use the same principles they came up with but we have
updated and modified them.
They are areas of low pressure that are characterized by rotating
wind, which moves toward the rising air of the central, low-pressure
region.
Stages of a Midlatitude Cyclone
Develops in stages
1.
both warm and cold fronts are together (stationary)
2.
cold air pushes down from the north
3.
changes stationary front into a pair of warm and cold
fronts moving around a low pressure center
4.
as low moves towards the East it over takes the warm
occluded front.
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Weather conditions
Warm front – long periods of precipitation
Clearing – becomes warm and humid after precip
Cold front comes – brief heavy showers
Finally – clearing and cooler temperatures
The first stage can take anywhere from 12 to 24 hours to complete.
After that, the system can under go the final changes for
approximately 3 days. The system will stay in place until it is
replaced by another front.
It can cover as much as half of the continent.
Upper-Air Flow
This controls a surface low’s path and intensity. It will bend
back and forth to form ridges and troughs.
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Trough –
Sinking air increases the air pressure at the surface; if more
enters a high then leaves, it strengthens it. Eventually the high will
spiral into a surface low. The air will rise and rejoin the upper air
flow; if more air raises then sinks it will strengthen the low.
Meteorologists must monitor this to forecast whether a surface high
or low is likely to strengthen or weaken.
Weather associated with any low pressure system
A ________is another name for a ___________________.
The lowest pressure is in the center and the winds blow counter clockwise toward the center.
Remember winds are caused by differences in air pressure;
they always blow out from a HIGH PRESSURE and into a
LOW PRESSURE area.
Rising air adds more moisture and clouds to all weather fronts.
Typical weather changes associated with a low:
1.
Long periods of clouds (cirrus to cirrostratus, altostratus,
nimbostratus and stratus)
2.
Steady rain or snow
3.
Temperature changes as the front passes over
4.
General clearing until next front passes, warms up a bit
5.
More clouds and precipitation as another front passes;
scattered showers and possible thunderstorms.
6.
As cold front passes, the temp drops, winds shift and sky
clears
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Anticyclones
An ___________ is a ____________________. The highest
pressure is in the center and the winds blow clockwise out.
Weather associated with any high pressure system
1.
2.
3.
Little to no wind.
Days are hot, nights are cooler
If the system stays for awhile, it can bring droughts.
Severe Weather
This is classified by any storm that causes damage to property or life.
Thunderstorms
There are approximately 44,000 storms per year. They form as warm
air rises very quickly. If the condensation is rapid, you will have
quick heavy rain. There will always be thunder and lightening
associated with this storm. It may have strong winds, hail, and
tornadoes. Most will usually form in afternoon when surface
warming causes unstable air.
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Local (air-mass) thunderstorms – form as a land area is heated in a
warm moist air mass. Causes widely scattered short storms
Frontal Thunderstorms – as a cold front pushes into a warm moist air
mass the warm moist air pushes up quickly forming a row of
thunderstorms that last up to a few hours.
Squall Line
A Squall line is a thunderstorms that occurs in a line ahead of a
front. Strong winds precede the line and is caused by the rain
cooling the air
Super Cell
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Very large single cell thunderstorms with strong
updrafts.
Can cause a tornado
Causes the most damage by wind and hail
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Lightning is caused by the movement of electrical current.
Positive and negative charges create a spark. The spark travels
along the narrow path of charged ions and becomes a bolt.
Lightening will strike the highest point. It can heat the air to approx.
25,000o C. Flows best through metal, water and wet ground.
Can occur from:
cloud to ground
cloud to cloud
around its own cloud
and even ground to cloud (studies made)
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Thunder – the temperature of the lightening is hotter than the
surface of the sun which causes the air to expand quickly – causing
the booming sound.
Light travels at 3.0 x 108 m/s or 186,000 miles/sec
Sound travels at 3.0 x 103 m/s or 11 miles/sec
Therefore lightening is always seen before it is heard
Emergency Plan for Thunder/Lightening Storms
Lightening Do’s
- go inside
- stay in a car
- unplug appliances
- have battery powered
radio and phones handy
- keep calm
Don’ts
- go under a tree
- put hands outside of car
- play golf
- use the telephone
- take a bath or shower
- panic
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A hurricane is an intense tropical low-pressure area with
sustained winds of at least 75 miles per hour (120 km) or greater.
The storm contains strong winds and heavy rains, which cause
damages to property. Winds spiral toward the center or the eye.
The eye of the storm has almost no wind and no rain at all. They are
most common in the late summer and early fall in the NH.
To Form
1.
Supply of warm, moist air for a long period of time
2.
Water condenses, releasing large amounts of heat
3.
starts to rotate (coriolis effect)
4.
Does not form at the equator (usually between 5oand
20o Latitude)
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In the SH they are called typhoons or tropical cyclones. Atlantic
hurricanes start as lows as far away as West Africa; Pacific hurricanes
form to the west of Mexico and move westward; occasionally they
move north to California or west to Hawaii
Storm Surge –
When a hurricane piles up water along the shore and then
blows it inland; very dangerous during high tide. It can raise sea
level several meter high.
On the East Coast meteorologists announce the percent chance that
the eye will pass within 105km.
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119 – 153 km
Minimal
2
154 – 177 km
Moderate
3
178 – 209 km
Extensive
4
210 – 250 km
Extreme
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> 250 km
Catastrophic
Moves trees, unanchored mobile home,
some coastal flooding
Minor building damage, some trees blown
down
Some structural damage to small buildings,
mobile homes destroyed
Some roofs destroyed, evacuation as far a
10 km inland
Buildings destroyed, evacuation as far as 16
km in land
Emergency Plan for Hurricanes
- Evacuate the area if you are told to do so
- Find a sturdy room to stay in away from windows
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- Keep battery powered phone/radios handy
- Get out of a mobile home, find better shelter
- Store fresh water and canned goods incase there is no
power
- Don’t panic make sure you know the drill
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A tornado is a funnel shaped clouds that extends down from a
thunder cloud with winds from 100 – 300 m/h. There are more
in the US than anywhere else in the world; (Great Plains, South
Dakota to Texas – Tornado Alley) however, Europe does get
tornadoes. Cool air from the Rocky Mts meets warm wet air
from the Gulf of Mexico. Only about 1/3rd become tornadoes.
You may get one without a mesocyclone. They are driven by
upper level winds.
To form a Tornado
1.
You need a rotating updraft within a severe thunderstorm
(called a mesocyclone)
2.
The rotating cloud can become visible and form a wall cloud.
3.
10 – 20 minutes later, there may be a descending tornado.
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Shapes
Some are a vortex, or funnel-shaped cloud
Some are rope shaped
They are created when air in the center (which is low pressure) is
sucked into the funnel expanding and cooling the air. The water
vapor in the air condenses and forms a cloud.
Scale:
Invented by Tetsuya “Ted” Fujita
F0
F1
F2
F3
F4
F5
65 – 118 km
119 -181 km
182 – 253 km
254 – 332 km
333 – 419 km
420 – 513 km
Minor damage – break some windows
Downs trees, shifts mobile homes
Rips roofs off, destroys mobile homes
Lifts cars, partially destroys buildings
Levels sturdy buildings, tosses cars
Lifts and transports sturdy buildings
Water Spout –
- tornadoes over water; picks up water (heavy)
- slow wind speed
Emergency Plan for Tornadoes
- Find a sturdy room to stay in away from windows
- Keep battery powered phone/radios handy
- Get out of a mobile home, find better shelter
- Find a hole in the ground if you are outside
- Don’t panic make sure you know the drill
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A blizzard is a snowstorm with high winds and real cold temps. There
doesn’t have to be a massive amount of snow, but it does have to
have high winds.
a.
winds exceed 56 km
b.
temps are approx -7o C or lower
c.
Snow falling or blowing must reduce visibility
Section 3 Weather Instruments
Weather Forecasts-
Information is gathered from surface equipment, commercial
planes, satellites, weather balloons, etc. All data from around the
world, is transmitted (at Midnight and Noon) to a central bank of
computers at the National Meteorological Center in Camp Springs,
MD, where it is made into weather models based on past weather
patterns.
Local and Commercial weather services takes that information
and makes predictions based on the areas past and local weather
conditions.
Measuring Lower Atmospheric Conditions
During the day the atmospheric pressure can change drastically in a
short period. Meteorologists use the magnitude and speed of the
changes to predict future events.
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Thermometers are used to measure the temperature of the air. They
either have alcohol or mercury in the tubes to indicate changes.
Electrical thermometer use a current to measure the change
A thermistor or thermal register is the most accurate and quickest
indicator for temperature changes. They are used when you need to
monitor rapid temperature changes.
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A Barometer is used to measure the changes in air pressure.
Reasons for movement of air
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1. Hot air rises therefore it weighs less(molecules move farther apart)
2. Cool air sinks therefore it weighs more (molecules move closer together)
3. Relative humidity changes (more water- less weight).
Air is 78%- Nitrogen Weighs 28
21%- Oxygen
Weighs 32
1%- all other gases with some water
Water weighs 18
When the air is more humid there is more water in the air, which replaces the
heavier nitrogen and oxygen. This makes the air weigh less and lowers the
air pressure.
High (H) pressure- generally cool and dry (fair)
Pressure- anything above 1013.2 mb
Clockwise rotation
Low (L) Pressure Anything below1013.2 mb
Pressure generally warm and humid (precip)
Counter-clockwise rotation
Wind Speed
Anemometer
- Measures wind speed 10 meters above ground.
- Can be hand held or ones mounted on poles.
- Used by all airports
- Measures both speed and direction (vector quantity)
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Other
A.
B.
C.
Ways:
White Caps occur in winds approx 20-30km/hr
Umbrellas break with winds approx 40km/hr
Trees uprooted with winds approx 90km/hr
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Wind Direction
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A wind vane is an instrument that has an arrowhead on one
end and a broadtail on the other. The tail resists the wind (tail
opposite direction of wind). Always point the direction that the
WIND CAME FROM.
Measuring Upper Atmospheric Conditions
____________________________
Using weather stations to assist especially in areas or times that
other equipment could not be used. Most weather stations are at
airports. People are also used to phone in any unusual conditions
which increase the chances of citing these events.
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This is an instrument package that is carried into the atmosphere on
a balloon. It will measure Relative Humidity, Air Pressure and
Temperature. The balloon is tracked to record speed and altitude
and intensity of precipitation also.
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To use the radar, meteorologists emit pulses of microwaves and wait
for them to bounce back when they hit precip. The time it takes for
the wave to return is how far away the storm is. Not always very
accurate since it does not detect the true shape of some storms.
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This shows winds moving toward or away from the instrument.
Radar waves reflected by particles moving toward the radar are
shorter than waves moving away. These are transferred to a color
screen, which uses colors to determine strength and speeds. For
tornadoes they can get about 20minutes warning.
Weather Satellites –
Satellites are able to take readings from orbit that fills in the
missing information. It can also take pictures form space showing
cloud movement. This is done every 30 minutes.
Two Types:
Visible Images are black and white pictures of the earth. White is
sunlight reflected (brighter means thick clouds) gray is land and black
is water. These are used to estimate severity and stages of
hurricanes. The disadvantage of this is that it is not available at night
Infrared uses temperature so it can be used day or night. Bright
areas are cold temps, dark areas are warmer. Can be used to
determine how tall clouds are. Can estimate wind speed, direction
and altitude of wind, and water vapor.
Computer Models
Computers collect atmospheric pressure, humidity, temperature, wind
speed and direction and precipitation. They use all the conditions in
an area to predict current and future weather based on the current
conditions and past weather movements.
Section 4 Forecasting the Weather
This was not easy to do until the invention of the telegraph in 1844.
This allowed Meteorologists to share the information and help piece
together a forecast.
Local weather stations send information hourly to the National
Weather Station. Westhampton Beach is a NWS and so it Upton
(Brookhaven National Lab)
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Map of all these stations are made every 3 hours by the
National Center for Environmental Protection (NCEP)
Putting the maps together makes it a time lapses picture
Global Weather Monitoring
Weather stations report conditions several times per hour. They
monitor barometric pressure and how it has changed, the speed and
direction of the wind, precipitation, temperature and humidity. They
also record the amount and height of cloud cover along with the
general weather condition. This information is sent and gathered at a
collection center that disperses the information world wide.
The World Meteorological Organization (WMO) is an organization that
helps train and promotes people to watch the weather and report the
conditions.
Weather Maps
Information is collected and transferred on to maps so Meteorologists
can use the information to understand current weather conditions in
order to make some form of forecast.
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Weather Symbols
A Station Model is used to plot the information on a weather map. It
is a cluster of meteorological symbols that represent weather
conditions.
Things to remember:
Below 500 add a 10 to the front and decimal point between the last two
numbers.
Above 500 add a 9 to the front of the number and a decimal point between the
last two numbers.
The barometric trend is in code too. There should be a decimal between the two
numbers. This represents what has happened to the pressure over a period of 3
hours.
Wind is named for the direction is comes from
Temperature and dew point temeprature is listed in F not C
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Plotting Temperature and Pressure
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Plotting Fronts and Precipitation
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Some weather maps will color code the precipitation according to the
type. Light green usually represents a light rain, while gray to white
will represent snow. See weather map above.
Weather Forecasts
Most meteorologists know they can not predict the weather.
They take all the information and put it together. They also
take into consideration the history, topography, geography
and urbanization of an area. The most accurate forecasts
are the short range ones, like 2 – 3 days.
Weather Data
Meteorologists use various radars and computers to enhance the
information. The different models represented by the computers
allow them to compare what the other areas see. Since different
people and equipment collect and put together the information, there
may be slight differences to the data; this is why meteorologists use
different sources to help make the forecasts
Temperature, wind direction, wind speed, cloudiness and
precipitation are the variables that are forecasted with the most
accuracy.
Types of Forecasts
Daily – ________________________________________________
Extended- _____________________________________________.
Medium Range – ________________________________________
Long Range – ___________________________________________
Severe Weather Watches and Warnings
Meteorologists can track a weather condition from the beginning to the end
which will help decide what action to take.
Watch______________________________________
Warning
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Controlling the Weather
Some scientists try to cloud seed but they are not as successful as
they want to be. In Russia some scientists have had success seeding
rain clouds so that they did not produce hail.
Hurricane Control
Project Stormfury was established in order to try to stop or
slow down hurricanes. This project which took place from
1962-1983 seeded 4 hurricanes with mixed results.
Lightning Control
Scientists try to seed the clouds to disperse the lightning, but they
have very little luck. Meteorologists have trouble predicting when
and where lightning will strike so this often fails. They use artificially
made lightning to help with the control of this.