Transcript Unit 6 Chapter 21 Weather Section 1 Air Masses Air masses are

Unit 6 Chapter 21 Weather
Section 1 Air Masses
Air masses are caused by differences in air pressure which is caused by unequal
heating of the Earth’s surfaces. 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 high pressure to lower pressure. 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 turn toward the right.
Winds in the Southern Hemisphere turn toward the left.
Formation of Air Masses
An air mass is a large body of air in the lower troposphere. It gets its
characteristics by forming over specific world areas (source regions). 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.
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 – cT – forms over warm land
Warm and Dry
Continental polar – cP– forms over cold land
Cool and Dry
Continental Arctic – cA – forms over very cold land
Cold and Dry – Winter only
Maritime Air Masses
Maritime tropical – mT – forms over warm tropical seas
Warm and Moist
Maritime polar – mP – forms over cold seas
Cool and Moist
Maritime Artic – mA – forms over very cold seas
Cold and Moist – Extreme North or South only
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 mid-latitudes where southwardmoving 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.
*** AT ALL FRONTS WARM AIR RISES,
COOLS FROM EXPANSION,
CONDENSATION OCCURS, CLOUDS
FORM, PRECIPITATION MAY RESULT***
Types of Fronts
A front is formed when two air masses collide.
Cold Fronts
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 of the
Front
Contact with the 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, hail
sometimes
Showers then clearing
Clouds
Cirrus and cirrostratus
changing later to
cumulus and
cumulonimbus
Cumulus and
cumulonimbus
Cumulus
Warm Fronts
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
Front
Contact with the
Front
After the Passing of the Front
Temperature
Cool
Warming suddenly
Warmer then leveling off
Atmospheric Pressure
Decreasing steadily
Leveling off
Slight rise followed by a decrease
Winds
South to southeast
Variable
South to southwest
Precipitation
Showers, snow, sleet or
drizzle
Light drizzle
None
Clouds
Cirrus, cirrostratus,
altostratus,
nimbostratus, and then
stratus
Stratus, sometimes
cumulonimbus
Clearing with scattered stratus, sometimes
scattered cumulonimbus
Stationary and Occluded Fronts
Stationary front
No Frontal Movement. They stall where they are until one air mass eventually
gives up.
Occluded Fronts – 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.
Stages of a Midlatitude Cyclone
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Develops in stages
both warm and cold fronts are together (stationary)
cold air pushes down from the north
changes stationary front into a pair of warm and cold fronts moving around a
low pressure center
as low moves towards the East it over takes the warm occluded front.
Weather conditions
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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.
Upper-Air Flow
This controls a surface low’s path and intensity. It will bend back and forth to
form ridges and troughs.
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 Cyclone is another name for a Low Pressure System.
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
Anticyclones
An Anticyclone is a High Pressure System. The highest pressure is in the center and the
winds blow clockwise out.
Weather associated with any high pressure system
1.Little to no wind.
2.Days are hot, nights are cooler
3.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.
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
-Very large single cell thunderstorms with strong updrafts.
-Can cause a tornado
-Causes the most damage by wind and hail
Lightning
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)
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/sec or 186,000
miles/sec
Sound travels at 3.0 x 103 m/sec or 11 miles/sec
Therefore lightening is always seen before it is heard
Emergency Plan for
Thunder/Lightening Storms
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Do
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Stay Calm
Go into a building
Unplug appliances that are
not surge protected
Stay in your car, all
appendages in
Have emergency kit
available
Have a plan
Don’t Do
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Panic
Go under a tree
Stick you hands out of a
car window
Play Golf
Swim in a pool or body of
water
Be stupid
Hurricanes
• A hurricane is an intense tropical low-pressure area with
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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)
Hurricane Ivan 2004
Typhoon hitting Taiwan 2005
• 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.
1
119 – 153 km
Minimal
Moves trees, unanchored mobile home, some coastal
flooding
2
154 – 177 km
Moderate
Minor building damage, some trees blown down
3
178 – 209 km
Extensive
Some structural damage to small buildings, mobile homes
destroyed
4
210 – 250 km
Extreme
Some roofs destroyed, evacuation as far a 10 km inland
5
> 250 km
Catastrophic
Buildings destroyed, evacuation as far as 16 km in land
http://www.nhc.noaa.gov/surge/animations/hurricane_stormsurge.swf
Emergency Plan for Hurricanes
Evacuate the area if you are told to do so
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
Store fresh water and canned goods incase
there is no power
Don’t panic make sure you know the drill
Tornadoes
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.
The Tetsuya Fajita Scale
F0
65 – 118 km
Minor damage – break some windows
F1
119 -181 km
Downs trees, shifts mobile homes
F2
182 – 253 km
Rips roofs off, destroys mobile homes
F3
254 – 332 km
Lifts cars, partially destroys buildings
F4
333 – 419 km
Levels sturdy buildings, tosses cars
F5
420 – 513 km
Lifts and transports sturdy buildings
Blizzard
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.
winds exceed 56 km
temps are approx -7o C or lower
Snow falling or blowing must reduce
visibility
Section 3 Weather Instruments
Weather ForecastsInformation 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.
Air Temperature
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.
Air Pressure
A Barometer is used to measure the changes in air
pressure.
Reasons for movement of air
Hot air rises therefore it weighs less (molecules move farther apart)
Cool air sinks therefore it weighs more (molecules move closer together)
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)
*** A wind is named for the direction from which it comes****
Other Ways to check wind speed:
White Caps occur in winds approx 2030km/hr
Umbrellas break with winds approx
40km/hr
Trees uprooted with winds approx 90km/hr
Wind Direction
Wind VanesA 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
Surface Observation
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.
Radiosonde
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.
Radar
To use the radar, meteorologists emit pulses of microwaves and wait for
Regular Radar –
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.
Doppler radar (Doppler Effect) –
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 20 minute 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.
Visible Images
They 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
This 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)
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.
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
Plotting Temperature and
Pressure
Isobars are lines that connect areas of
equal pressure. They can show High
and Low Pressure areas on a weather
map.
Isotherms are lines of equal
temperature. Some times
they are also represented by
color too.
Plotting Temperature and Pressure
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 – the most accurate when they predict for approximately a 48
period.
Extended- usually some accuracy, but predicts for a 3 – 5 day.
Medium Range – not very accurate for a 3 -7 day period
Long Range – the least accurate, but predicts for a 7 - 10 day period
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-
24 – 36 hours away
Warning
24 or less hours away
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.