WEATHERWEATHER

UNIT 7TOPIC 7 in Review Book

• Weather is a description of the atmospheric variables at a particular location during a specific, short, period of time.

• Most of the weather changes occur in the troposphere.

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

• The sun is the source of the majority of the heat energy in the atmosphere.

I. Ways in which the atmosphere is heatedI. Ways in which the atmosphere is heated

1. Conduction: moves heat from the surface into the atmosphere

2. Direct absorption of insolation by gases and aerosols

3. Absorption of infrared radiation (from the surface) by clouds, water vapor and carbon dioxide.

4. Condensation and sublimation release energy during the formation of clouds, fog, dew and frost.

Distribution of heat energyDistribution of heat energy

• Heat energy is transferred throughout the troposphere by convection…

• the movement of a fluid due to differences in density caused by changes in temperature.

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II. Atmospheric VariablesII. Atmospheric Variables

• are properties of the air that change from time to time.

Atmospheric variables include:

Atmospheric variables include:

1. Temperature2. Air pressure3. Wind4. Moisture5. Cloud cover6. Precipitation

Instruments usedInstruments used

1. Thermometer2. Barometer3. Anemometer (speed)4. Sling psychrometer5. Visual estimations6. Rain/snow gauge

A. Weather RelationshipsA. Weather Relationships

• The interaction and interrelationship between the atmospheric variables result in weather patterns.

• Changes in one variable can result in changes in other variables.

1. Temperature variations:determined by daily and seasonal changes in insolation.

• Temperature can change adiabatically due to changes in the altitude of the air.

• Ascending air expands and cools

• Descending air compresses and warms

Atmospheric factors that influence temperature

Atmospheric factors that influence temperature

A. Cloud cover:•During the day:

–as cloud cover increases–insolation received decreases

–temperature decreases

• At night–as cloud cover increases–absorption of infrared radiation increases

–increasing temperatures

B. Aerosols•As particulates in the atmosphere increase–insolation received by the earth decreases (due to scattering)

–temperatures decrease

C. Volcanic eruptions result in large amounts of gas and dust distributed throughout the atmosphere…resulting in decreased worldwide temperatures.

D. Asteroids and large meteoroids disperse large volumes of debris upon impact which would be distributed through the atmosphere and decrease global temperatures.

• Evidence in the rock record indicates that the consequences of an asteroid impact precipitated the extinction of the dinosaurs.

2. Pressure variations:Atmospheric pressure is also called barometric pressure.It is the pressure due to the weight of the atmosphere pushing down on any given area.

Factors that influence pressure

Factors that influence pressure

A. Temperature:•An increase in the density results in an increase in the amount of pressure exerted by the air

•Consequently, cold air exerts a higher pressure than warm air.

B. Moisture content• Water vapor in air causes

the air to “weigh” less…• consequently, moist air

exerts less pressure than dry air.

C. Altitude:• As elevation above the

surface of the earth increases, the air becomes less dense…

• consequently exerting less pressure

As temperature increasesAir pressure decreasesAs moisture content increasesAir pressure decreasesAs elevation increasesAir pressure decreases

3. Wind3. Wind

• the horizontal movement of air parallel to the surface of the earth.

• Winds are identified by their direction of origin…

• where they came from NOT where they are going!

a. Cause of winda. Cause of wind

• Winds are caused by differences in air pressure.

• The steeper the pressure gradient, the faster the winds.

• Winds move from areas of high pressure to areas of low pressure.

• The direction is modified by the rotation of the earth (the Coriolis Effect)

• Winds curve to the right in the northern hemisphere and to the left in the southern hemisphere

WINDS BLOWFROMHIGH

TOLOW

• Wind blowing over bodies of water creates friction at the air-water interface. The friction transfers the energy from the wind to the water produces waves.

Water, Pressure and WindWater, Pressure and Wind

• Water is a poor absorber and has a high specific heat therefore it heats slowly during the day and cools slowly at night. The opposite is true of land surfaces.

• Because of this, during the day there are warmer temperatures with a lower pressure over the land and cooler temperatures with a higher pressure over the water creating on-shore breezes

• At night, the water cools slowly so the air over the water stays warm while the air over the land gets cooler. This creates lower temperatures with a higher pressure over the land resulting in off-shore breezes

Zones of convergence:Zones of convergence:

–Regions in which air comes together creating vertical, rising currents

–Associated with low pressure

–Winds blow horizontally towards these regions

Zones of divergence:Zones of divergence:

–Regions in which air spreads out from vertical, sinking air currents

–Associated with high pressure

–Winds blow horizontally away from these regions.

Jet streamsJet streams

• Jet streams are bands of easterly moving air at the top of the troposphere.

• They have an influence on the formation and direction of weather patterns in mid-latitude regions such as the US

Planetary wind beltsPlanetary wind belts

• Created in the troposphere• Shift north and south according to the seasons

• Associated with pressure and moisture conditions

• Called “prevailing winds”

• Most of the US is affected by planetary winds that blow from the southwest to the northeast…called prevailing southwesterly winds.

• Weather changes generally follow this direction

4. Moisture4. Moisture

• Water vapor: gaseous water in the atmosphere.

• Moisture enters the atmosphere by evapotranspiration

Factors that influence the rate of evaporation

Factors that influence the rate of evaporation

A. Amount of energy:– increases in energy– increase temperature– increases rate of

evaporation

B. Surface areathe more spread out the water is, the faster the rate of evaporation

– increase in surface area increases rate of

evaporation

C. Saturation:the higher the water vapor content, the closer the air is to being saturated (holding all of the moisture it can hold) and the slower the rate of evaporation

D. Wind speed:increases in wind speedincreases the rate of evaporation

HUMIDITYHUMIDITY

• A term that refers to the water vapor content of the atmosphere.

• The maximum humidity is called the moisture capacity and increases as temperature increases.

• Warm air can hold more moisture than cold air!

RELATIVE HUMIDITYRELATIVE HUMIDITY

• A comparative value between the maximum moisture capacity and the amount of moisture currently held in the atmosphere; expressed as a percentage out of 100.

• It is the percent of saturation…the closer the amount of water vapor in the air is to the maximum amount possible, the higher the relative humidity.

• If temperature increases and moisture content remains the same, humidity will decrease.

• If temperature decreases and moisture content remains the same, humidity will increase

• If temperature remains constant and moisture content increases, humidity will increase.

• If temperature remains constant and moisture content decreases, humidity will decrease.

DEWPOINT TEMPERATUREDEWPOINT TEMPERATURE

• is the temperature at which the air is holding all of the moisture it is capable of holding.

• the temperature to which the air must be cooled to be saturated.

• If the air temperature drops below the dewpoint temperature, the water vapor in the air will condense and clouds will form

• As the moisture content of the air increases, the dewpoint temperature increases as well.

Measuring Dewpoint and Relative Humidity

Measuring Dewpoint and Relative Humidity

• A sling psychrometer is used to measure both dewpoint temperature and relative humidity.

• It contains two thermometers:

• One is an ordinary one (which measures air temperature)

• The other has a “sock” on the bulb that gets moistened

• The original thermometer is called the “dry bulb thermometer”

• The “socked” thermometer is called the “wet bulb thermometer”

• When the psychrometer is whirled through the air, the temperature of the wet bulb thermometer decreases because of the cooling effect of evaporation.

Steps:Steps:1. Read the dry-bulb

temperature2. Read the wet-bulb

temperature3. Subtract the two values4. Using the ESRT: find the box

where the dry-bulb meets the difference.

• Using the dew point table tells you the dew point temperature

• Using the relative humidity table tells you the relative humidity.

• Condensation occurs when the air temperature is at the dew point temperature

• The altitude at which the air temperature equals the dew point temperature is the altitude at which clouds form

5. Precipitation 6. Clouds5. Precipitation 6. Clouds

• the falling of liquid or solid water from clouds towards the surface of the earth.

FORMS OF PRECIPITATIONFORMS OF PRECIPITATION

1. Rain2. Drizzle3. Snow4. Sleet5. Freezing rain6. Hail

• Rain is liquid precipitation with drops larger than 0.5 mm in diameter

• Drizzle is liquid precipitation with drops smaller than 0.5 mm in diameter

• Snow is precipitation that is composed of one or more solid, six-sided ice crystals that form in the clouds directly by sublimation of water vapor to ice

• Sleet is precipitation composed of transparent ice pellets, less than 5 mm in size, that forms by the freezing of falling rain

• Freezing rain is rain that freezes as it hits the surface of the earth

• Hail is a round solid form of precipitation composed of concentric layers of ice and snow…hail only forms from cumulonimbus clouds with vertical currents strong enough to send the mass up and down several times

• Precipitation occurs when the temperature of the air cools to the dew point temperature and the water vapor condenses to liquid or solid water that is too heavy to be supported by the vertical currents within the cloud

Condensation and cloudsCondensation and clouds

• Condensation at the earth’s surface produces fog

• Sublimation at the earth’s surface produces frost

• Low density condensation in the lower atmosphere produces haze

• Condensation in the atmosphere produces clouds

• Condensation and sublimation release tremendous amounts of energy into the atmosphere

• In order for condensation to occur, a surface is needed.

• This surface is called a condensation nuclei and can be natural (dust, pollen, ash) or a pollutant

• When precipitation occurs, the condensation nuclei falls with it...because of this, precipitation is

• an atmospheric cleanser!

Determining cloud base elevation

Determining cloud base elevation

• On the graph included, the dashed lines represent the decrease in dew point with an increase in altitude

• the slashed diagonal lines represent the decrease in temperature with an increase in altitude.

• For example: if the temperature is 24º C and the dew point temperature is 20ºC, the base of the cloud is found at 0.5 km.

TYPES OF CLOUDSTYPES OF CLOUDS

• Nimbostratus: low rain clouds• Cumulonimbus: tall thunderclouds

• Cumulus: “fluffy” fair weather clouds

• Cirrus: high “wispy” fair weather clouds

III. WEATHER MAPS AND FORECASTING

III. WEATHER MAPS AND FORECASTING

A. Air Masseshuge bodies of air in the troposphere having similar pressure, moisture, wind and temperature characteristics throughout.

• The geographic region in which an air mass is formed is its source region.

• The longer an air mass remains over its source region, the larger and more like the region it becomes.

Source RegionsSource Regions

• Maritime: air mass that forms over water…m…wet

• Continental: air mass that forms over land…c…dry

• Polar: air mass that forms over high latitudes…P…cold

• Arctic: air mass that forms over the polar regions …A…really cold

• Tropical: air mass that forms in lower latitudes…T…warm

B. Pressure & Air MassesB. Pressure & Air Masses

• Cyclone: a low-pressure air mass

• Winds move counterclockwise in towards the center and rises vertically

• forming clouds and precipitation

• Anticyclone: a high-pressure air mass

• Winds descend and move clockwise away from the center

• Resulting in cool, clear weather

C. FrontsC. Fronts• The boundary between two air masses

• Frontal interfaces are where two air masses meet

• Weather is unstable and associated with clouds, strong winds and precipitation

• Fronts are named for the temperature of the air mass behind it.

• See ESRT page 13 for front symbols

1. Warm Fronts1. Warm Fronts

• Occur when warm air meets and rises over cold air

• Precipitation precedes the warm front

• Warm air dominates until another front moves in

2. Cold Fronts2. Cold Fronts

• Occur when cold air meets and pushes up warm air.

• Cold fronts move faster than warm fronts

• Clouds and precipitation come with the cold front

• Cumulonimbus clouds arrive quickly with short and violent periods of precipitation.

• When the front passes, the weather changes quickly

• the air becomes cooler and drier

3. Stationary Fronts3. Stationary Fronts

• Occur when a warm air-mass and a cold air-mass are side by side with neither air mass moving

• Results in weather similar to a warm front

4. Occluded Fronts4. Occluded Fronts

• Occur when a warm air mass is between two colder air masses. The faster moving cold air mass overtakes the warm air mass, lifting it up off the ground resulting in heavy precipitation

D. StormsD. Storms

1. Thunderstorms:fueled by the latent heat of condensationresult in strong upward and downward currents of air

Lightning results from charge differences between the top of the cloud and either the bottom of the same cloud, another cloud or the ground.

• Thunder results when the heat from the lightning flash causes the air to expand explosively. The rumbling results when the flash is very long causing a difference in times of heating

Storm safetyStorm safety

• Remain inside and away from glass structures

• Avoid the use of telephones and appliances

• If outside, remain in low-lying areas. Avoid trees!

2. Tornadoes2. Tornadoes

• The center has a very low air pressure

• Local storms of short duration with violent wind speeds characterized by a rotating funnel of air that extends down from a cumulonimbus cloud

• They have a relatively small diameter and last for a short period of time. Their paths are sporadic making them very difficult to predict.

• Tornadoes form when cP air masses meet mT air masses.

• This tends to occur most frequently in the central US region

• They are usually accompanied by heavy rain, lightning and hail

• Waterspouts are tornadoes over water

Tornado SafetyTornado Safety

• Go to the basement or an interior room on the lowest floor of the building you are in.

• Protect yourself from flying debris

• Stay away from exterior walls or glassy areas

• Do not remain in cars or mobile homes…leave them and go to a substantial structure

• In lieu of a structure, lie flat in the nearest ditch or depression and use your hands to cover your head

3. Hurricanes3. Hurricanes

• Intense tropical low pressure system that has formed over an ocean

• Winds spiral in towards the center

• They are fueled by the heat energy stored in water vapor

• When this heat energy is released it warms the air causing it to rise…reducing the pressure allowing more air to converge inward

• They develop most often in late summer when ocean water is warmest (27ºC or higher)

• They begin as tropical storms and are not classified as hurricanes until they have wind speeds greater than 115 km/hr

• Hurricanes lose strength when–They move onto land or over cooler water and do not receive any more of the warm, moist air that fuels them

Hurricane SafetyHurricane Safety

• Listen to the radio to stay informed

• Evacuate if directed to• Stock up on food, water, flashlights, batteries and other necessities for power outages

• Clean and fill bathtubs with water for cleaning and toilets

• Stay inside and away from windows, skylights and glass doors

• Find a safe area in your home…an interior, reinforced room like a closet or bathroom on the lowest floor

• Afterwards, be careful of downed power lines and damaged trees

• For all weather-related danger…use common sense, anticipate probable outcomes and prepare for them…stock up on non-perishable food items and bottled water, have flashlights and batteries available.

Emergency preparedness includes

Emergency preparedness includes

1. Prior planning by individuals, families, communities and government agencies

2. Designing evacuation routes out of the area or to safer areas within a building

3. Having emergency supplies available such as food, fresh water, lighting, heat and back up electrical power.

E. Synoptic Weather MapsE. Synoptic Weather Maps• Short summaries of weather variables for a short period of time

• Pertinent information is plotted around a central circle indicating the conditions of a specific area. This is called a station model

Station ModelsStation Models

• All variables remain in fixed locations as indicated in the ESRT on page 13

• The only variable that changes position around the circle is wind direction

Abbreviating air pressureAbbreviating air pressure

• Drop the first number (9 or 10)

• Drop the decimal point• Drop the units• 998.6 mb = 986• 974.8 mb = 748• 1012.3 mb = 123• 1000.1 mb = 001

“Reconstituting” air pressure

“Reconstituting” air pressure

• If abbreviation is–greater than 500 add a 9–less than 500 add a 10

• Add the decimal point before the last digit• Add units

• 786 = 978.6 mb• 984 = 998.4 mb• 023 = 1002.3 mb• 010 = 1001.0 mb

Shading indicates amount of

cloud cover

Air pressure

Barometric trend

Amount of precipitation

Air temperature ºF

Present weather & visibility

Dew Point Temperature ºF

“stick” points to where wind comes from. “feathers” indicatewind speed

THE END!`