Topic 7 Weather

Atmospheric Temperatures

Scale:

Instrument:

Isotherms:

Celsius, Kelvin or Fahrenheit in U.S.

Thermometer

Lines on map that show equal temp.

Heating of the Atmosphere

Source: The Sun

Relationship between insolation and temp:Direct

Ways in Which the Atmosphere is Heated

Conduction:

Direct Absorption:

Greenhouse Effect:

Condensation and Sublimation:

Coriolis Effect:

Air warms from contact with earth

Aerosols are heated by sun

Absorption of long wave infrared light

Both release energy

Rotation of earth causes friction between air and surface

Convectional Transfer of Heat in the Atmosphere

Convection:

Convection Currents:

Air movement due to density differences

Circular patterns of air movement

Heating and Cooling of Air by Expansion and Compression

Expansion: Causes air to coolWhen does air expand: When it risesCompression: Causes air to warmWhen does air compress:When it sinks

Atmospheric Pressure and Density

Atmospheric Pressure: Pressure from weight of overlying air

Measurement and Changes in Air PressureBarometer: Instrument to measure pressureMercury Barometer: Standard but dangerous

Aneroid Barometer: Common and portable

Units: Inches of mercury, millibars or Atm

Isobar:Line on a map that shows equal pressure

Standard pressure at sea level: 14.7lbs./in2, 29.92in of Hg, 1013.2 mb

Conversion: 1020.0mb= 30.12 inches

28.70 inches= 972.0mb

Effect of Temperature on PressureIf air temperature rises what happens to:

1) Density of air? decreases

2) Air pressure? decreases

Effect of Water Vapor on Air PressureHigh water content: Less dense air, lower

pressure

Low water content: More dense air, higher pressure

Masses:

H2O= 18

N2= 28

O2= 32

Water vapor replaces N2 and O2 causing density to go down!!!

Effect of Altitude on Atmospheric pressure

As altitude increases what happens to density?

Air pressure?

Decreases

Decreases

Wind:Movement of air from one place to anotherWind Speed

What causes wind: Differences in air pressure (flows from High to Low)

High Wind Speed: High air pressure gradient (isobars close together)

Low Wind Speed: Low pressure gradient (isobars are far apart)

Air Pressure Gradient: Change in A.P. / Distance

Instrument: Anemometer

Units: Miles per hour or knots

Conversion to mph: 1knot = 1.15 mph

Local WindsLand Breeze: Night time breeze from the landSea Breeze: Day time breeze from the sea

Wind Direction

What direction does wind move? High to Low

What role does the Coriolis Effect play?

Winds are deflected to the right in the northern hemisphere.

How are winds named?From where they come from

Formation of Waves on Surface Waters

What causes waves? Friction between wind and water

Factors of wave height: Strength of wind and duration of wind

1) Duration: How long the wind has been in contact with water

2) Strength: Velocity of the wind

General Circulation of Air in the Troposphere

Convection Cells

What causes global wind patterns? Unequal heating of earth’s surface

Planetary wind belts (pg. 14 ESRT): Study this page

Jet Stream: High speed, high altitude westerly wind

Uses: Planes use it when flying east

Effects: Influences weather in the U.S.

Seasonal Shifting of Wind and Pressure Belts

What causes global winds? Convection due to unequal heating of earth

What could cause global winds to shift? Tilt of earth’s axis

Effects of shifts: Weather patterns change in certain seasons

Monsoons: Wet season of South East Asia

Importance

Weather Movements in the U.S.Direction: From the southwest to the northeast

Name: Prevailing Westerlies

1)____Which map shows the most likely path this low-pressure center will follow during the next 12 hours?

Formation of Surface Ocean Currents

What causes ocean currents? Global winds

Coriolis Effect: Deflects currents to right in N. Hemisphere, left in Southern

Do they shift? Currents spin clockwise in the N. Hemisphere, counter clockwise in Southern

Atmospheric MoistureStates of moisture in the atmosphere: Solid, Liquid and Gas

How does water vapor enter the atmosphere?

1)

2)

Evaporation (change from liquid to gas)

Transpiration (vapor released by plants during photosynthesis)

Energy of Evaporation and Transpiration

Heat of Vaporization: 2260J/g

Where does the energy come from? The Sun

What happens to the temp. of a liquid when molecules evaporate? Decreases

Factors Affecting Evaporation1) Energy Available: More energy causes a a higher rate of

evaporation

4) Wind Speed:

3) Degree of Saturation:

2) Surface Area:

High wind speed causes greater evaporation

Humid air does not allow evaporation

More surface area causes higher rate of evaporation

Humidity Temperature and Dew Point

Humidity: Water content in the atmosphere

Absolute Humidity: The total mass of water in a parcel of air

Moisture Capacity:

Relative Humidity:

Maximum water air can hold at a certain temp.

Ratio of moisture in the air compared to moisture capacity

Practice1) A.H.=

M.C.=

R.H.=

3) A.H.=

M.C.=

R.H.=

2) A.H.=

M.C.=

R.H.=

5g

10g

10g

10g

2g

10g

50% 20%100%

Relative humidity and Temperature

Warm air: Can hold a lot of water

Cold air: Can’t hold much water

What happens to R.H. if temp increases:

What happens to the absolute humidity?

What happens to R.H. and A.H. if water is added?

No change

R.H.=Up and A.H.=Up

Decreases:

Dew PointDew point: Temperature at which the air becomes saturated

How is the dew point reached: Add moisture or cool the air

What is the relative humidity at the dew point: 100%

What happens to the dew point if the absolute humidity is increased?

The dew point temperature would fall

Explanation

Measuring Relative HumidityInstrument: Sling Psychrometer

Dry-bulb: Normal thermometer

Wet-Bulb: Thermometer with a wet wick at the end

Procedure

1) Locate dry-bulb page 12 ESRT

2) Subtract wet from dry

3) Locate difference on top of chart

4) Where row and column meet is the R.H.

Determining Dew Point Temp.

1) Locate dry-bulb page 12 ESRT2) Subtract wet from dry

3) Locate difference on top of chart

4) Where row and column meet is the D.P.

PrecipitationInstrument: Rain Gauge

Rain: Falling liquid droplets >.2mm

Drizzle:

Snow:

Sleet:

Freezing Rain:

Falling liquid droplets from .2mm to .5mm

Rain or drizzle that freezes on contact with earth surface

Falling ice crystals formed from combining ice crystals

Ice pellets formed by freezing of rain as it falls

Layers of ice and snow from repeated up and down movementsHail:

Cloud FormationHow does it happen? Air cools below the dew point

Where does it happen? The troposphere

What determines ice or liquid? The temperature

Cloud cover: Fraction or percent of the sky covered by clouds

Fog: Clouds that form at ground level

Condensation Nuclei: Dust or aerosols which water vapor condenses

seeding

Calculating Cloud Base Altitude

Air Masses and Fronts

mP mP

cTmT

cA

mT

cP

Air Masses: Large body of air with similar characteristics throughout

Winds:

Low Pressure and High Pressure SystemsCyclones (lows): Low pressure weather system

Weather Associated: Nasty weather with wind and precipitation

COUNTERCLOCKWISE, INWARD, and UP

Anticyclone (highs): High pressure weather systems

Weather associated: Cool nice weather, clear skiesWinds: CLOCKWISE, OUTWARD, and DOWN

Fronts Boundary between two different air massesFronts:

Cold Front

Cold Front: Leading edge of a cold air mass

Weather: Stormy weather followed by cool dry air

Warm Front

Warm Front: Leading edge of a warm air mass

Weather: Long period of rain follow by warm weather

Occluded Front

Occluded Front: Warm air mass pinched between two cold air masses

Weather: Periods of stormy weather

Stationary Front

Stationary Front: Two air masses remain stationary

Weather: Long periods of precipitation

Storms and Severe WeatherStorm: Severe disturbance in the atmosphere

HurricanesWhere does the energy come from? Evaporating water from the oceans

Wind Speed: 74 mph or higher

What causes Hurricanes to weaken? Going over land or cold water (NO evap)

Where are they most dangerous? Gulf of Mexico and S.E. coast of US

Effects: Flooding, high winds, rain (MOST DANGEROUS PART IS Storm SERGE)

ThunderstormsCauses: Collision of warm and cold air masses

Weather Associated: Hard rain, thunder, lightning, hail, wind

Hazards: Flooding, lightning (cause more deaths annually than all other storms)

TornadoesWind Speed: Speeds from 40mph to >350mph

Diameter: From a few feet to > a half mile

Tornado alley: Midwest (Texas – S. Dakota and Minnesota)

Hazards: Strong winds, flying debris, heavy rains

Wind Direction: Counterclockwise, in and upFormation

BlizzardsCriteria: 35 mph wind

Hazards: White outs, high winds, cold temperatures

Nor’easter: Powerful snow producing storm

March 3rd 1993

How can you prepare?

Weather Map Station ModelsSynoptic Weather Map: Several weather

variables on one map

Station Models: Map symbol with weather information