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"WEATHER”

TOPIC 7

(page 107)

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weather -

• condition of the atmosphere at any given location for a short period of time.

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weather variables;• temperature, air pressure, wind,

moisture conditions, cloud cover, precipitation, and storms.

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Most of the weather changes occur in the troposphere.

The troposphere is the part of the atmosphere immediately above Earth's surface.

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Mount Everest 29035 feet (5.5 miles)

Max. Airspace altitude from 40,000 feet (7.6 miles)

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Weather is caused by variations in insolation.

Variations in insolation cause heat energy to be unevenly distributed in the atmosphere.

Heat energy tends to move toward a condition of more uniform distribution.

The movement of heat energy results in the constant changes in the atmosphere that are a major cause of weather.

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7.1) ATMOSPHERIC TEMPERATURE

Temperature conversions; Celsius, Fahrenheit, Kelvin scales (see Earth Science Reference Tables).

Examples;

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Air temperature is usually measured using a thermometer.

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Temperature is modeled on maps and charts by the use of lines called isotherms.

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Heating of the Atmosphere.

The sun is the original source of almost all the heat in the atmosphere.

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5 ways in which the atmosphere is heated

1 -Absorption of insolation by gases and aerosols.

2 - Conduction between Earth's surface and the atmosphere.

3 - Absorption of long-wave radiation from earth by green house gases.

4 - Condensation releases latent heat into the atmosphere.

5 - Friction from the Coriolis effect.

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Convectional Transfer of Heat in the Atmosphere.

Heat energy is transferred within the atmosphere by convection.

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• Differences in air pressure cause air to move in circular patterns called convection currents.

• Wind is the part of the convection current that is parallel to Earth's surface.

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Heating of Air by Compression & Cooling of

Air by Expansion.When a gas expands… its temperature decreases.

example ; spray can, car tire.

When a gas is compressed…. its temperature increases.

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Within the troposphere…..

As air risesatmospheric pressure decreases…the air expands and its temperature decreases.

As air descends…atmospheric pressure increases…the air is compressed and its temperature increases.

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Rising air becomes cooler

Sinking air becomes warmer

mountain(side view)

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Under average conditions air temperature in the

troposphere decreases with increasing

altitude.

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See Earth Science Reference Tables; "Selected Properties of Earth's Atmosphere"

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7.2) ATMOSPHERIC PRESSURE & DENSITY

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The denser the atmosphere…

the greater its the weight, and

the greater its air pressure.

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atmospheric pressure - • a.k.a.;

barometric pressure or air pressure.

• the pressure due to the weight of the overlying atmosphere.

diagram

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Measurement of and Changes in Air Pressure

• The instrument that is used to measure air pressure is a…barometer.

Mercury barometer

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Air pressure is measure in either

1)Millibars

2)Inches

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To convert from millibars to inches of mercury use the Earth Science Reference Tables, page 13.

Examples.

1) 976.0 mb = 2) 1001.0 mb = 3) 30.00 inches = 4) 29.09 inches =

28.82 inches29.56 inches

1016.0 mb

985.0 mb

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Lines drawn on a weather map that connect locations that have the same barometric pressure are called…

isobars

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The Effect of Temperature on Air Pressure.

As the temperature of the air increases, the air expands and its density and pressure decrease.

•example; Hot Air Balloon

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The Effect of Water Vapor on Air Pressure.

The greater the amount of water vapor in the air,

the lower the air density and pressure.

molecular weightwater (H20)

1(2) + 16 = 18

molecular weightair

Nitrogen (N2) Oxygen (O2)14 (2) = 28 16 (2) = 32

one molecule of water weighs less than one molecule of air.

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The Effect of Altitude on Atmospheric Pressure.

As altitude or elevation increases, atmospheric density and pressure

decrease.

diagram

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less pressure

more pressure

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SUMMARY;

As either altitude, temperature or moisture content increases,

air pressure decreases.

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7.3) WIND (p.111)

windthe horizontal movement of air

parallel to Earth's surface.

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Wind Speed

Winds are caused by differences in air pressure.

air moves from High pressure towards Low pressure.

Differences in air pressure are caused by…

1)differences in temperature.2)differences in water vapor content.

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air pressure gradient • the difference in air pressure for a

specific distance.

• The closer together the isobars on a weather map are, the greater (steeper) the pressure gradient.

• The greater the pressure gradient, the faster the wind.

• diagram:

steep gradientstrong wind

low gradientweak winds

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anemometer

• an instrument that measures wind speed.

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Land Breeze vs. Sea Breeze (diagrams)

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Land breeze

LAND WATER

coolwarm

airsinks

airrises

LOWHIGH

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Sea breeze

LAND WATER

coolwarm

airsinks

airrises

LOWHIGH

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Wind Direction.

•Wind moves from areas of high pressure to areas of low pressure.

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Coriolis effect

(caused by Earth rotation)

deflects winds to the right in the Northern Hemisphere.

deflects winds to the left in the Southern Hemisphere.

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A wind is named for the direction from which it

comes.

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wind vane (weather vane)• measures the direction of

the wind.

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7-4) CIRCULATION OF AIR IN THE TROPOSPHERE.

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Convection Cells.

Unequal distribution of insolation on Earth results in…

unequal heating and

differences in air pressure.

Cooler air, being denser, sinks toward Earth.

Warmer air, which is less dense, rises.

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Air rises at 0o latitude (the equator) and 60o North and South Latitudes.

Air sinks near 30o and 90o North and South Latitudes.

Regions where air comes together are called regionsof convergence.

Regions where air spreadsout are regions of divergence.

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0o

90oN

90oS

60oS

30oS

30oN

60oN

H

H

H

H

L

L

L

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jet stream

Bands of easterly moving air at the top of the troposphere.

Winds can blow 200 + mph.

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Planetary Wind, Pressure & Moisture Belts

See ESRT.

Remember……air moves from high to low

pressure.

wind follows a curved path due to Coriolis Effect.

wind is named for where the wind comes from.

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high

high

high

high

low

low

low

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prevailing winds

Within the planetary wind belts,

the winds move generally in a specific direction much of the time.

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Seasonal Shifting of the Wind and Pressure Belts.

Wind and pressure belts shift….

northward during our summer (June 21).

southward during our winter (Dec. 21).

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Weather Movement in the Contiguous United States

The US is affected by planetary winds that blow from the SW to the NE - "south-westerlies".

weather generally moves from southwest to northeast.

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Formation of Surface Ocean Currents

• see ESRT; page 4 "Surface Ocean Currents".

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Surface ocean currents are caused by wind blowing over the oceans and transferring energy to the water.

•Direction of currents is affected by direction of the planetary winds.

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7.5) ATMOSPHERIC MOISTURE.

• Gaseous water in the atmosphere is called…

water vapor.

• 70% of Earth’s surface is covered by water.

• Water enters the atmosphere by…evaporationsublimationtranspiration

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evaporation• is the process by which liquid

changes to a gas.

transpiration• is the process by which plants

release water vapor as part of their life functions.

evapotranspiration = evaporation and transpiration

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Energy of Evaporation and Transpiration of Water

Almost all the energy for evaporation and transpiration comes from …..

insolation.

When evaporation or transpiration occurs….the more energetic molecules leave the

liquid.

the average kinetic energy for the remaining molecules is less.

temperature becomes less.

THEREFORE, “Evaporation is a cooling process”

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Process of Evaporation

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4 Factors that Affect the Rate of Evaporation

1)Amount of energy Available. the higher the temperature, the faster the

evaporation.

2)Surface Area the more surface area, the faster the

evaporation.

3)Degree of Saturation the drier the air, the faster the evaporation.

4)Wind Speed the greater the wind speed, the faster the

evaporation.

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Humidity, Temperature, and Dew Point

humidity – the amount of water vapor in the atmosphere.

absolute humidity - the amount (mass) of water vapor in each unit volume of air.

The warmer the air, the more water vapor the air can hold.

relative humidity - the amount of water vapor in the air compared to the amount of water vapor the air can hold (given as a percentage).

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Relative Humidity and Temperature

If more water vapor is added to the air …..

relative humidity increases,absolute humidity increases.

If temperature increases…..relative humidity decreases,absolute humidity remains the same.

If temperature decreases,relative humidity increases,absolute humidity remains the same.

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Dew Point

dew point - the temperature at which the air is filled with water vapor.

the temperature at which humidity = 100%

It is the temperature at which water vapor will condense.

When the air temperature reaches the dew point, …clouds and/or precipitation are produced.

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As the amount of water vapor in the air increases, the dew point also rises.

80o

40o

air temp.

dew point

differenceindicateshumidity

air temp.dew point

80o

75o

(dry)

moisture added to

airair becomesmore humid

wetair,

higherdew

point

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humidity is not listed on the station model.

compare temperature to dewpoint.

if temperature = dewpoint, humidity is 100%

the greater the difference, the drier the air.

HUMIDITY

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Measuring Relative Humidity.

psychrometer – an instrument used to measure

relative humidity.

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Wet bulb / Dry bulbSince evaporation is a cooling process….

The amount of cooling depends on the rate of evaporation.

If the air is saturated, no evaporation…..wet bulb and dry bulb temp will be the same.

The drier the air, the more evaporation….the cooler the wet bulb.

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Determining Relative Humidity and Dew Point

Practice Questions.

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Use the diagram on page 12 of the Earth Science Reference Tables and the data below, to calculate the relative humidity.

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Dry Bulb Wet Bulb Difference RH %

1) 28 16

2) 20 12

3) 12 7

4) 2 1

5) -10 -10

1

0 100

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48 5

36 8

2612

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Use the diagram on page 12 of the Earth Science Reference Tables and the data below, to calculate the Dew Point.

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Dry Bulb Wet Bulb Difference Dew Pt.

1) 28 16

2) 20 12

3) 12 7

4) 2 1

5) -10 -10

12 7

8 4

5 1

1 -1

0 -10

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Precipitationprecipitation – the falling of liquid or solid water

from clouds toward the surface of Earth.

rain gauge – is used to measure the amount of liquid

precipitation.

Snow depth is measured with a ruler.

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Cloud Formation

Need condensation nuclei and water vapor.

condensation nuclei (aerosols; dust,pollen)

provides a surface for water to condense upon.

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Steps for cloud formation

Warm air rises.

As air rises, pressure decreases, the air expands, and the temperature of the air decreases.

When the air temperature = the dew point temperature, the air becomes saturated and water condenses on the condensation nuclei.

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cloud – a collection of liquid water

droplets and/or ice crystal suspended in the atmosphere.

If a cloud is on, or just above, Earth's surface, it is called fog.

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7-6) AIR MASSES AND FRONTS

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air mass -

a large body of air in the troposphere with similar characteristics of pressure, moisture, and temperature.

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Characteristics of Air Masses

An air mass forms when a large mass of air remains stationary over a part of Earth's surface.

the air mass acquires the characteristics of that surface.

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If the air mass forms…..

Over water = , symbol = ( )

Over land = , symbol = ( )

Near the equator = , symbol = ( )

Near the Pole , symbol = ( )

humid

dry

hot

cold

m

c

T

P

maritime

continental

tropical

polar

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See symbols on Earth Science Reference Tables p.

,13

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Air Mass Worksheet

Label each air mass with…

1 – whether it is continental or maritime.

2 – whether it is tropical or polar (arctic).

3 – its characteristics; humid or dry, warm or cold.

4 – its origin (where does come from).

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c

c

c

mm

m m

PA

P

P

T

T

T

humid, colddry, very cold

dry, cold

humid, cold

humid, warm

dry, warm

humid, warm

North PacificNorthern Canada

Central Canada

North Atlantic

Equatorial Pacific

Mexico

Gulf of Mexico

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Lows and Highs

The troposphere is divided into two portions according to…..

1) air pressure2) circulation of the

winds

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Low (cyclone)

low pressure

winds blow toward its center.

counterclockwise circulation in Northern Hemisphere.

usually associated with stormy weather, cloudy skies, and much precipitation.

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Highs (anticyclones)high pressure.

winds blow away from its center.

clockwise rotation in Northern Hemisphere.

usually associated with clear skies and little to no precipitation.

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Fronts

front - the boundary (or interface) where two air masses of different characteristics meet.

most storms in the United States are usually associated with fronts.

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4 Types of Fronts

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1 - Cold Frontboundary between an advancing cold

air mass and a warmer air mass.

the cold air pushes forward like a wedge.

diagram

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2 - Warm Front

boundary between an advancing warm air mass and a retreating wedge of cooler air.

less dense warm air is forced to rise over the cold air mass.

diagram

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3 - Occluded Front

occurs when a cold front overtakes a warm front, lifting the warm air mass off the ground.

diagram

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4 - Stationary Frontoccurs when two adjacent air masses

remain in the same positions.

diagram symbol

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Symbols for Fronts

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Label fronts.

Hint; A-B is an occluded front.

Place L for location of Low Pressure system.

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L

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On the satellite image, draw the proper symbol to represent the most probable front on line XY.

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Which type of front is located between Buffalo and Detroit?

(1) stationary

(2) warm(3) occluded (4) cold

(1) (2) (3) (4)

Which cross section best represents the fronts and air movements in the lower atmosphere along line XY?

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Tracks of Air Masses and Fronts

most tracks follow a southwest to northeast or westerly to easterly route in the United States.

If a low-pressure system follows a typical storm track across New York State, it will move toward the...

(1) southeast (3) southwest

(2) northeast (4) northwest

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7-7) Storms and Severe Weather

p.125Storm

a violent or severe disturbance of the atmosphere.

usually associated with; high winds, precipitation, fronts, and low air pressure.

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Mid-latitude or Cyclonic Storms

formed when air masses move along polar fronts.

polar frontsmid-latitude boundary between colder air near the poles, and warmer air near the equator.

mid-latitude storms develop when a warm air mass is lifted producing an occluded front.

may produce thunderstorms, hail and tornadoes.

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Hurricanesbegin as low pressure centers over warm

tropical ocean water.

usually occur in late summer or late fall.

the lower the air pressure, the stronger the wind speed.

once sustained winds reach 74 mph it is classified as a hurricane.

the center of the hurricane is called the “eye”.

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the hurricane loses energy when it is no longer over warm water (moves over land or over colder water)

hurricanes originally move from East to West across the Atlantic.

however, once they begin to lose energy (or move further into the prevailing wind belt – SW) the hurricane will hook towards the NE.

most deaths are caused by drowning; storm surge and rainfall lead to massive flooding.

storm surge – low pressure and strong winds, from the hurricane, cause sea level to rise.

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Base your answers on the satellite image below, which shows a Northern Hemisphere hurricane.

What is the usual surface wind pattern around the eye of Northern Hemisphere hurricanes?

(1) clockwise and outward (2) counterclockwise and outward(3) clockwise and inward(4) counterclockwise and inward

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Which air mass is normally associated with the formation of hurricanes?

(1) continental tropical (2) maritime tropical(3) continental polar(4) maritime polar

Clouds form in the hurricane because the air is

(1) sinking, expanding, and cooling (2) sinking, compressing, and warming (3) rising, expanding, and cooling(4) rising, compressing, and warming

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When the eye of this hurricane reaches 43° N latitude, this hurricane will most likely be pushed by planetary winds toward the

(1) northwest (3) southwest(2) northeast (4) southeast

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Thunderstorms

heavy rainstorms accompanied by thunder and lightning.

occur when warm air is uplifted along fronts.

only thunderstorms produce hail.

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Tornadoesa rapidly rotating, extremely low-pressure

funnel that hangs down from thunderstorm clouds.

usually last only a few minutes.

maximum calculated wind speed = 318 mph.

form along fronts where there is a big difference in temperature.

safety; go to the strongest structure, the strongest room, at the lowest possible level and cover yourself.

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Base your answers on the map,which shows a portion of the United States where 148 tornadoes occurred during a 24-hour period in April 1974. The paths of the tornadoes areshown.

Explain why all the tornadoes moved toward the northeast.

Describe the air movement most likely found within these tornadoes.

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A school receives a tornado warning. Describe one emergency action that a teacher and the students in a classroom should immediately take to protect themselves from injury.

Most of these tornadoes occurred with thunderstorms along cold fronts. Identify the water cycle process that forms clouds along cold fronts.

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Blizzards

a storm with winds of 35 mph or greater and large amounts of falling or blowing snow.

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Lake Effect

Need; open water, wind and freezing temperature.

As air passes over the lake it picks up moisture.

Air is forced to rise as it passes over land.

As the air rises, it cools, and precipitation occurs.

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Which New York State location is most likely to experience the heaviest winter snowfall when the surface winds are blowing from the west or northwest?

(1) New York City (2) Binghamton (3) Oswego(4) Plattsburgh

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Emergency Preparedness

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7-8) Weather Prediction and Probability

p.130

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Relationship between weather variables

1) As temperature increases air pressure decreases.

2) As the temperature gets closer to the dew point, the chance of precipitation increases.

3) As the difference between air temperature and the dew point temperature becomes smaller, the relative humidity (air saturation) increases.

4) As the air pressure gradient increases, the wind speeds increase.

5) Rapidly falling air pressure = the approach of a storm and increased chance of precipitation.

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Weather Maps and Station Models

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Which station model correctly represents the weather conditions in an area that is experiencing winds from the northeast at 25 knots and has had a steady drop in barometric pressure of 2.7 millibars during the last three hours?

(1) (2) (3) (4)

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The diagram below shows conditions that commonly cause fog to form over land in coastal areas.

A weather station at the lighthouse records a temperature of 36°F and an air pressure of 1016.4 mb. Using the proper weather map symbols, place the following information in the correct positions on the weather station model.

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Present weatherDewpointAir pressureWind directionWind speed

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Present weatherDewpointAir pressureWind directionWind speed

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Base your answers on the station model below, which shows the weather conditions at Rochester, New York, at 4 p.m. on a particular day in June.

What was the actual barometric pressure, according to the station model, to the

nearest tenth of a millibar?

The winds shown by this station model were blowing from which compass direction and at what wind speed?