ES 1110: Chapter 3

1
ES 1110 Chapter 3

• Temperature

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Temperature

• Recall that temperature is the average kinetic
  energy of the molecules in that substance
• A change in temperature requires
• The net energy budget
• The specific heat
• Whether or not phase changes occur

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Surface Temperature

• Thermometers are placed in the shade
• Thermometer is at a height of 1.5 meters
• Therefore, surface temperature is the temperature
  of the air near the ground, not the temperature
  of the ground surface
• The average global surface temperature is 59 F
  (15 C)
• The extremes in surface temperature
• Highest 136.4 F (58 C) El Azizia, Libya
• Lowest -129 F (-89 C) Vostok, Antarctica

4
Surface Energy Budget

• An energy balance exists if energy gains equal
  energy losses
• The Earth-atmosphere system, averaged over a
  year, is in energy balance
• As a result, the average global surface
  temperature does not change
• Over short periods of time, or in localized
  regions, there usually is an energy imbalance
• Energy gains gt energy losses temperature rises
• Energy losses gt energy gains temperature falls

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Global Energy Budget

• Figure 3.2, Page 59

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Surface Temperatures

• The surface air temperature is determined by
  energy exchanges with the surface
• Turbulence is irregular air motions that result
  as heat and moisture from the surface mix upward
• Conduction, convection, radiation, sensible and
  latent heat transfers, and turbulence all act at
  the same time to transfer heat
• There is no simple equation to express the
  relationship between temperature and surface
  conditions

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Temperature Cycles

• Diurnal means daily
• The typical diurnal temperature cycle is warmest
  in the afternoon and coolest near dawn
• The typical seasonal (or annual) temperature
  cycle is warmest in the summer and colder in the
  winter

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Temperature Variables

• The diurnal temperature range is the difference
  between the maximum and minimum temperatures of
  any given day
• The daily mean temperature is simply the average
  between the high and low temperature for that day
• The monthly mean temperature is calculated by
  averaging all the daily mean temperatures for
  each day of the month
• The annual temperature range is the difference
  between the warmest and coldest monthly mean
  temperatures
• The annual average temperature is the average of
  the monthly mean temperatures

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Temperature Variables

• Figure 3.3, Page 60

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Influences of Temperature

• Latitude
• Surface Type
• Elevation and Aspect
• Effects of Large Bodies of Water
• Cloud Cover

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Latitude

• The latitude of a location dictates the angle of
  insolation during an entire year
• Insolation INcoming SOLar radiATION
• The lower the latitude, the higher the Sun is in
  the sky year round
• The intensity of the Suns rays and number of
  daylight hours depend on latitude
• Because the Suns location changes dramatically
  in the subtropics during the day, greater diurnal
  temperature variations occur
• The maximum temperature of a location lags the
  time of maximum solar input (summer solstice)
• After the summer solstice, energy gains still
  exceed energy losses which results in
  temperatures still increasing

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Effect of Latitude

• Figure 3.4, Page 61

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Insolation of New York vs. Miami

• Figure 3.5, Page 61

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Surface Type

• The surface of the Earth absorbs approximately
  50 of the insolation received at the top of the
  atmosphere
• The atmosphere is heated by the surface
• Surface type plays an important role in
  determining the surface air temperature
• Dry sand poor conductor of heat and low
  specific heat, so the top heats up rapidly
• Desert locations get very hot and have a large
  diurnal and annual temperature range
• Vegetation modifies the diurnal annual
  temperature range in two ways
• Plants consume some solar energy for
  photosynthesis
• Transpiration by plants also consumes solar
  energy
• Evaporation uses energy that would otherwise heat
  the surface

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Effect of Surface Type

• Figure 3.6, Page 62

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Elevation and Aspect

• Higher elevations are colder than lower
  elevations for a few reasons
• The air is less dense as you go up (fewer
  molecules to absorb energy)
• Terrestrial radiation can more easily escape to
  outer space with fewer molecules
• Higher winds aloft mix energy throughout
• Aspect is the direction that a mountain slope
  faces
• North-facing slopes receive less solar radiation
  than south-facing slopes
• South-facing slopes are therefore warmer
• More solar radiation results in increased
  evaporation, reduced soil moisture, and sparse
  vegetation on the south-facing slope

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Effect of Elevation

• Figure 3.7, Page 62

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Effect of Aspect

• Figure 3.8, Page 63

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Effects of Large Bodies of Water

• Diurnal and annual temperature ranges are less
  for locations near large bodies of water
• Factors that contribute to the difference between
  continental and maritime locations
• The specific heat of water is about 3 times
  greater than land (heats up and cools down more
  slowly)
• Evaporation of water consumes energy
• Mixing and transparency of water allows the solar
  radiation to be distributed throughout a large
  depth
• Proximity of warm and cold ocean currents can
  also affect the temperature

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Effect of Nearby Bodies of Water

• Figure 3.10, Page 64

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Effect of Ocean Current Temperature

• Figure 3.11, Page 65

22
Cloud Cover

• Clouds reflect and absorb solar energy
• They reduce the amount of solar radiation
  reaching the surface, and cause daytime cooling
• The thicker the cloud, the more pronounced the
  cooling
• Clouds also cause nighttime warming due to the
  emission of longwave radiation to the ground
• Cloudy regions are warmer than clear regions at
  night

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Effect of Clouds on Energy Budget

• Figure 3.12, Page 66

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Effect of Cloud Cover

• Figure 3.13, Page 66

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Interannual Temperature Variations

• Normal Temperatures 30-year average
• Interannual temperature variations are
  temperature changes from one year to the next
• Anomalies Departures from the normal value
• The interannual temperature pattern has been a
  persistent upward trend since the 1990s
• Volcanic eruptions, oceanic temperature phenomena
  (El Niño La Niña) can cause anomalies in this
  pattern

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Global Annual Temperatures over 120 Years

• Figure 3.14, Page 67

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Diurnal Temperature Cycle

• Averaged over many years, a regular pattern of
  temperature change can be seen over the course of
  a day
• Temperature changes are driven by the difference
  in insolation vs. outgoing energy losses
• Sunrise ground warms the atmosphere
• Air temperature increases because insolation
  offsets outgoing emission
• Noon insolation values peak
• After noon insolation still offsets outgoing
  emission, so temperature continues to increase
• About 4 p.m. energy losses begin to offset
  insolation, highest temperature of the day
• Sunset loss of insolation
• Energy losses exceed gains all night long, so
  temperatures fall until sunrise
• Variations from this pattern arise with changes
  in latitude, surface type, elevation and aspect,
  relationship to large bodies of water, and cloud
  cover

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Diurnal Variation in Temperature

• Figure 3.16, Page 71

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Temperature Variations with Height

• Lapse rate the change in temperature with
  increasing altitude
• The average lapse rate in the troposphere is 6.5
  C per kilometer
• Lapse rates are assumed to be negative (cooling
  with height)
• Environmental lapse rate the specific change in
  temperature with altitude at any particular time
  and location
• Environmental lapse rates can change hour-to-hour
  and day-to-day
• Environmental lapse rates are measured by weather
  balloon

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Stability

• The temperature difference between the
  environment and an air parcel determines the
  stability of the atmosphere
• If a parcel is lifted and is warmer than the
  environment at that level, it will be buoyant and
  continue to rise on its own
• If a parcel is lifted and is colder than the
  environment at that level, it will be negatively
  buoyant and will sink back down to its original
  level
• If the two temperatures are identical, the parcel
  will remain at the new elevation

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Types of Stability

• Absolutely Unstable The environment has a lapse
  rate greater than dry adiabatic
• In an absolutely unstable environment, a parcel
  will always be warmer (no matter if it is lifted
  dry or moist adiabatically)
• Absolutely unstable environments only exist very
  near the ground (mirages form because of this)
• Absolutely Stable The environment has a lapse
  rate less than moist adiabatic
• In an absolutely stable environment, the parcel
  will always be colder than the environment (no
  matter if it is lifted dry or moist adiabatically)

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Inversions

• Lapse rates are always assumed to be negative
  (cooling with height)
• Inversion temperature increases with height
• Inversions are an extreme case of a stable
  atmosphere
• Inversions act as a lid, suppressing vertical
  air motions
• High air pollution incidents are common with
  inversions
• The stratosphere and thermosphere are two layers
  of the atmosphere with inversions
• Inversions can happen in the troposphere whenever
  warm air is above cold air
• Topography (valleys) commonly develop inversions

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Nocturnal Inversions

• At the surface, 3 p.m. usually has the highest
  temperature
• By 8 p.m., the Earths surface has cooled because
  energy losses gt gains
• Air in contact with the cool ground loses heat
  and cools as well
• An inversion develops until around mid-morning

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Typical Lapse Rate 3 p.m.

• Figure 3.18A, Page 77

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Typical Lapse Rate 8 p.m.

• Figure 3.18B, Page 77

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Typical Lapse Rate 5 a.m.

• Figure 3.18C, Page 77

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Typical Lapse Rate 10 a.m.

• Figure 3.18D, Page 77

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Nocturnal Inversion Factors

• To develop a nocturnal inversion, the following
  is helpful
• Lack of clouds (allow easy escape of terrestrial
  radiation)
• Lack of winds (winds provide mixing to disrupt an
  inversion)
• Winter nights (longer period of darkness), but
  inversions can happen with any season
• Condition of ground (snow allows surface to cool
  off rapidly)

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Cold Air Draining in a Valley

• Figure 3.20, Page 79

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Wind-Chill Temperature

• The cooling power of the wind is measured by the
  wind-chill factor
• Calm winds thin layer of air insulates us
• High winds insulating layer is blown away
  making it feel colder to us
• The wind-chill describes the increased loss of
  heat by the movement of air
• The wind chill is relevant to humans and other
  animals (not cars, buildings, etc.)

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Wind-Chill Equivalent Temperature

• Expressed in degrees
• Translates the bodys heat losses under the
  current temperature and wind conditions into air
  temperature that would produce equivalent heat
  losses
• Has been recently updated to be more accurate
• Cold temperatures plus wind cause danger to
  exposed flesh

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Temperature and Agriculture

• Cold air outbreaks can damage crops and be costly
  to farmers
• Nocturnal inversions can result in crop damage
• Surface temperatures are measured at 1.5 meters,
  not next to the ground where small crops are
  growing
• Ways to prevent vegetation damage from nocturnal
  inversions
• Covering with plastic sheets (prevents heat
  escape)
• Large heaters (supply heat and mixes air in
  inversion)
• Mechanical mixing (large fans)
• Freezing water on the plants
• Plant damage occurs at -2 C, not 0 C
• Latent heat release can prevent the temperature
  from dropping down to -2 C