Meteo 3: Chapter 3Meteo 3: Chapter 3

Seasons, Controllers of Seasons, Controllers of Temperature and FrontsTemperature and Fronts

Read Pages 77-90, 92-100Read Pages 77-90, 92-100

ReviewReview

Energy transfer from the sun to the earthEnergy transfer from the sun to the earth Energy transfer in the atmosphere – Energy transfer in the atmosphere –

radiation/conduction/convectionradiation/conduction/convection What can we can apply this to?What can we can apply this to?

The Reason for the SeasonsThe Reason for the Seasons

The plane of the Earth’s orbit and the plane of the Earth’s The plane of the Earth’s orbit and the plane of the Earth’s equator make a 23.5º angleequator make a 23.5º angle– Earth revolves around sun once every 365.25 daysEarth revolves around sun once every 365.25 days– Orientation of Earth with respect to sun’s rays constantly changing Orientation of Earth with respect to sun’s rays constantly changing

throughout yearthroughout year Summer Solstice (N.H.)- Sun over 23.5º N (Tropic of Cancer)- Summer Solstice (N.H.)- Sun over 23.5º N (Tropic of Cancer)-

around June 21- More daylight hours than any other dayaround June 21- More daylight hours than any other day Winter Solstice (N.H.)- Sun over 23.5 º S (Tropic of Capricorn)- Winter Solstice (N.H.)- Sun over 23.5 º S (Tropic of Capricorn)-

around Dec. 21- Fewest daylight hoursaround Dec. 21- Fewest daylight hours Opposite seasons in Southern HemisphereOpposite seasons in Southern Hemisphere On equinoxes (vernal and autumnal), sun directly over equatorOn equinoxes (vernal and autumnal), sun directly over equator

Earth’s Tilt and the SeasonsEarth’s Tilt and the Seasons

For a more whizz-bang animated explanation, let’s see For a more whizz-bang animated explanation, let’s see the CDthe CD

Determinants of Temperature ClimatologyDeterminants of Temperature Climatology

LatitudeLatitude

Proximity to Bodies of WaterProximity to Bodies of Water

ElevationElevation

LatitudeLatitude

High latitudes colder than tropicsHigh latitudes colder than tropics– Solar radiation strikes more directly strikes over Solar radiation strikes more directly strikes over

tropicstropics– Less variation in temperature in the tropics as Less variation in temperature in the tropics as

compared to mid/high latitudescompared to mid/high latitudes– Driving force of most weatherDriving force of most weather

Warm air transported northward and cold air Warm air transported northward and cold air transported southward, to eliminate latitudinal transported southward, to eliminate latitudinal temperature contraststemperature contrasts

Horizontal Distribution of TemperatureHorizontal Distribution of Temperature

What season does this show?What season does this show?

Proximity to Bodies of WaterProximity to Bodies of Water

Water’s heat capacity is ~3X that of land, Water’s heat capacity is ~3X that of land, so it is slow to warm and coolso it is slow to warm and cool

Winds also mix the topmost layer of water, Winds also mix the topmost layer of water, causing upwellingcausing upwelling

Result: Changes in air temperature are less Result: Changes in air temperature are less over water than land, because air is heated over water than land, because air is heated by surface belowby surface below

Temperature and WaterTemperature and Water

Locations near Locations near water have small water have small seasonalities- seasonalities- change in change in temperature temperature between summer between summer and winterand winter

Temperature and WaterTemperature and Water

Moderating effect is Moderating effect is much greater at much greater at locations downwind locations downwind of water, as of water, as prevailing winds prevailing winds blow air modified by blow air modified by water inlandwater inland

Astronomical versus Meteorological SeasonsAstronomical versus Meteorological Seasons

Time lag between astronomical and Time lag between astronomical and meteorological definitions meteorological definitions

Meteorologists classify seasons based on average Meteorologists classify seasons based on average temperature. These are as follows ( in N.H.):temperature. These are as follows ( in N.H.):– ““Meteorological Winter”- December - FebruaryMeteorological Winter”- December - February– ““Meteorological Spring”- March - MayMeteorological Spring”- March - May– ““Meteorological Summer”- June - AugustMeteorological Summer”- June - August– ““Meteorological Fall”- September - NovemberMeteorological Fall”- September - November

More on Water’s RoleMore on Water’s Role

Also explains why warmest and coldest Also explains why warmest and coldest times of year occur about 1 month after times of year occur about 1 month after solsticessolstices

Ocean currents also regulate temperatureOcean currents also regulate temperature– Driven by prevailing windsDriven by prevailing winds– Try to erase temperature contrast between tropics & Try to erase temperature contrast between tropics &

polespoles– Northward flowing currents transport warm water north, Northward flowing currents transport warm water north,

southward flowing currents transport cold water southsouthward flowing currents transport cold water south

Elephant PoloElephant Polo

You’re only as You’re only as good as your good as your elephantelephantBeware of Beware of elephant polo elephant polo spiesspies

Gulf StreamGulf Stream

Gulf Stream Gulf Stream most important most important for U.S. East for U.S. East Coast weatherCoast weatherFlows south to Flows south to north – warm north – warm ocean currentocean current

Ocean CurrentsOcean Currents

Elevation and TemperatureElevation and Temperature

Higher elevations are usually colderHigher elevations are usually colder Lapse rate-Lapse rate- rate of temperature decrease with rate of temperature decrease with

heightheight– Avg. environmental lapse rate in Avg. environmental lapse rate in troposphere troposphere (lowest (lowest

10 km of atmosphere) = 6.5ºC/km10 km of atmosphere) = 6.5ºC/km– Because atmosphere is heated from ground up!Because atmosphere is heated from ground up!

Elevation on TemperatureElevation on Temperature

What mountain is What mountain is this?this?Why are mountains Why are mountains cooler than lower cooler than lower terrain during the day, terrain during the day, even though they’re even though they’re closer to the sun?closer to the sun?

Controllers of Daily TemperatureControllers of Daily Temperature

Some dependence on geographical factors, but Some dependence on geographical factors, but local weather conditions control diurnal local weather conditions control diurnal temperature range. First, the geographic factors:temperature range. First, the geographic factors:– Proximity to water bodies: Smaller diurnal ranges near Proximity to water bodies: Smaller diurnal ranges near

large water bodieslarge water bodies– Latitude: Large range in low-mid latitudes because sun’s Latitude: Large range in low-mid latitudes because sun’s

angle varies greatly compared to high latitudesangle varies greatly compared to high latitudes– Elevation: Hills usually chillier during day, but valleys Elevation: Hills usually chillier during day, but valleys

sometimes colder than hilltops at nightsometimes colder than hilltops at night– Urbanization: Cities warmer than suburbs, esp. at nightUrbanization: Cities warmer than suburbs, esp. at night

Weather’s Impact on Daily TemperaturesWeather’s Impact on Daily Temperatures

WindWind: Can blow in warmer or colder air masses : Can blow in warmer or colder air masses (temperature advection)(temperature advection)

CloudsClouds: Lower daytime temperatures, but higher : Lower daytime temperatures, but higher nighttime temperaturesnighttime temperatures

Water VaporWater Vapor: Humid areas have higher overnight : Humid areas have higher overnight lows, dry areas rapidly cool at nightlows, dry areas rapidly cool at night

PrecipitationPrecipitation: Lower air temperature via : Lower air temperature via evaporational coolingevaporational cooling

Snow CoverSnow Cover: Lower temperature, esp. at night: Lower temperature, esp. at night

How to measure temperatureHow to measure temperature

Thermometer: Thermometer: Instrument used to measure Instrument used to measure temperaturetemperature– Housed in a white enclosed shelter with open vents on Housed in a white enclosed shelter with open vents on

sides to allow free flow of outside air to contact sides to allow free flow of outside air to contact instruments (Stevenson Screen)instruments (Stevenson Screen)

– Instruments about 5 feet off ground Instruments about 5 feet off ground – Protects instruments from direct sunlight => reason why Protects instruments from direct sunlight => reason why

bank thermometers are erroneously high is because they bank thermometers are erroneously high is because they are exposed to direct sunare exposed to direct sun

– Lowest recorded temp: -129ºF (Vostok, Antarctica)Lowest recorded temp: -129ºF (Vostok, Antarctica)– Highest recorded temp: 136ºF (Libyan Desert)Highest recorded temp: 136ºF (Libyan Desert)

Stevenson ScreenStevenson Screen

Air MassesAir Masses

Air mass-Air mass- large volume of air with uniform large volume of air with uniform temperature and moisture propertiestemperature and moisture properties– Form as air acquires properties of its underlying surfaceForm as air acquires properties of its underlying surface– Span several thousand kilometers in horizontalSpan several thousand kilometers in horizontal– Described with two letter abbreviations (cP, cT, cA, mP, Described with two letter abbreviations (cP, cT, cA, mP,

mT)mT) c = continentalc = continental m = maritimem = maritime P = polarP = polar T = tropicalT = tropical A = arcticA = arctic

Air Mass Source RegionsAir Mass Source Regions

FrontsFronts

Cold and warm air masses collide, narrow Cold and warm air masses collide, narrow boundary between them is a boundary between them is a frontfront. .

Type of front is determined by movement of cold Type of front is determined by movement of cold air b/c cold air is more denseair b/c cold air is more dense– Cold front => cold air advancesCold front => cold air advances– Warm front => cold air retreatsWarm front => cold air retreats– Stationary front => cold air moves little or not at allStationary front => cold air moves little or not at all

Large temperature gradients at front (front located Large temperature gradients at front (front located on warm side of gradient) ….frontal passage on warm side of gradient) ….frontal passage comes with wind shift & large temperature change comes with wind shift & large temperature change

FrontsFronts

Cross-sections of FrontsCross-sections of Fronts

Cross-sections of FrontsCross-sections of Fronts

Cross-sections of FrontsCross-sections of Fronts