Meteo 3: Chapter 3

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Meteo 3 Chapter 3

• Seasons, Controllers of Temperature and Fronts
• Read Pages 77-90, 92-100

2
Review

• Energy transfer from the sun to the earth
• Energy transfer in the atmosphere
  radiation/conduction/convection
• What can we can apply this to?

3
The Reason for the Seasons

• The plane of the Earths orbit and the plane of
  the Earths equator make a 23.5º angle
• Earth revolves around sun once every 365.25 days
• Orientation of Earth with respect to suns rays
  constantly changing throughout year
• Summer Solstice (N.H.)- Sun over 23.5º N (Tropic
  of Cancer)- around June 21- More daylight hours
  than any other day
• Winter Solstice (N.H.)- Sun over 23.5 º S (Tropic
  of Capricorn)- around Dec. 21- Fewest daylight
  hours
• Opposite seasons in Southern Hemisphere
• On equinoxes (vernal and autumnal), sun directly
  over equator

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Earths Tilt and the Seasons

• For a more whizz-bang animated explanation, lets
  see the CD

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Astronomical versus Meteorological Seasons

• Time lag between astronomical and meteorological
  definitions
• Meteorologists classify seasons based on average
  temperature. These are as follows ( in N.H.)
• Meteorological Winter- December - February
• Meteorological Spring- March - May
• Meteorological Summer- June - August
• Meteorological Fall- September - November

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Determinants of Temperature Climatology

• Latitude
• Proximity to Bodies of Water
• Elevation

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Latitude

• High latitudes colder than tropics
• Solar radiation strikes more directly strikes
  over tropics
• Driving force of most weather
• Goal is to transport warm air northward and cold
  air southward, eliminating latitudinal
  temperature contrasts

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Horizontal Distribution of Temperature

• What season does this show?

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Proximity to Bodies of Water

• Waters heat capacity is 3X that of land, so it
  is slow to warm and cool
• Winds also mix the topmost layer of water,
  causing upwelling
• Result Changes in air temperature are less over
  water than land, because air is heated by surface
  below

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Temperature and Water

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

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Temperature and Water

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

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More on Waters Role

• Also explains why warmest and coldest times of
  year occur about 1 month after solstices
• Ocean currents also regulate temperature
• Driven by prevailing winds
• Try to erase temperature contrast between tropics
  poles
• Northward flowing currents transport warm water
  north, southward flowing currents transport cold
  water south

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Gulf Stream

• Gulf Stream most important for U.S. East Coast
  weather
• Flows south to north warm ocean current

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Ocean Currents
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Elevation and Temperature

• Higher elevations are usually colder
• Lapse rate- rate of temperature decrease with
  height
• Avg. environmental lapse rate in troposphere
  (lowest 10 km of atmosphere) 6.5ºC/km
• Because atmosphere is heated from ground up!

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Elevation on Temperature

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

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Controllers of Daily Temperature

• Some dependence on geographical factors, but
  local weather conditions control diurnal
  temperature range. First, the geographic
  factors
• Proximity to water bodies Smaller diurnal ranges
  near large water bodies
• Latitude Large range in low-mid latitudes
  because suns angle varies greatly compared to
  high latitudes
• Elevation Hills usually chillier during day, but
  valleys sometimes colder than hilltops at night
• Urbanization Cities warmer than suburbs, esp. at
  night

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Weathers Impact on Daily Temperatures

• Wind Can blow in warmer or colder air masses
  (temperature advection)
• Clouds Lower daytime temperatures, but higher
  nighttime temperatures
• Water Vapor Humid areas have higher overnight
  lows, dry areas rapidly cool at night
• Precipitation Lower air temperature via
  evaporational cooling
• Snow Cover Lower temperature, esp. at night

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How to measure temperature

• Thermometer Instrument used to measure
  temperature
• Housed in a white enclosed shelter with open
  vents on sides to allow free flow of outside air
  to contact instruments (Stevenson Screen)
• Instruments about 5 feet off ground
• Protects instruments from direct sunlight gt
  reason why bank thermometers are erroneously high
  is because they are exposed to direct sun
• Lowest recorded temp -129ºF (Vostok, Antarctica)
• Highest recorded temp 136ºF (Libyan Desert)

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Stevenson Screen
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Air Masses

• Air mass- large volume of air with uniform
  temperature and moisture properties
• Form as air acquires properties of its underlying
  surface
• Span several thousand kilometers in horizontal
• Described with two letter abbreviations (cP, cT,
  cA, mP, mT)
• c continental
• m maritime
• P polar
• T tropical
• A arctic

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Air Mass Source Regions
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Fronts

• Cold and warm air masses collide, narrow boundary
  between them is a front.
• Type of front is determined by movement of cold
  air b/c cold air is more dense
• Cold front gt cold air advances
• Warm front gt cold air retreats
• Stationary front gt cold air moves little or not
  at all
• Large temperature gradients at front (front
  located on warm side of gradient) .frontal
  passage comes with wind shift large temperature
  change

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Fronts
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Cross-sections of Fronts
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Cross-sections of Fronts
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Cross-sections of Fronts