METEOROLOGY

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METEOROLOGY

• GEL-1370

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Chapter Three

• Air Temperature

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Daily Temperature Variations

• We are going to learn answers to the following
  questions
• Why warmest time of the day is normally in the
  afternoon
• Why the coldest time is usually in the early
  morning

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Temperature variations

• Each sunny day air goes through a daily cycle of
  warming and cooling
• Around noon, earth receives the most intense
  solar rays maximum temperature is reached in
  the late afternoon even though incoming solar
  radiation decreases in intensity after noon,
  incoming solar radiation gt outgoing heat energy
  from the surface --- energy surplus for 2-4 hours
  after noon
• Highest reading of temperature occurs at
• 3-500 PM if cloud free, summer afternoon
• If afternoon cloud exists, max temp occurs 1-2 hr
  earlier
• Cloudy days Overall daytime temp lower clouds
  reflect a great deal of incoming sunlight

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Sunny, calm day air temperature profile
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Temperature variations-contd.

• Adjacent to large water bodies Warmest part of
  the day occurs at noon or before
• Wet soil Only small heat is used to heat and
  major portion goes in evaporating the water
• Clear skies (low precipitation), low humidity,
  meager vegetation --- leads to hot air
• Two places in the same latitude have different
  temps. eg., Atlanta (humid) s max July average
  temp.30.5C Phoenix (AZ) .40.5C
• Late afternoon, early morning Air starts
  cooling, as air begins to loose more energy than
  they receive
• Radiation cooling Both air and ground cool by
  radiating infrared radiation

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Radiation Inversion

• Radiation Inversion Measured increase in air
  temp just above the ground formed mainly
  through radiational cooling of the surface Most
  inversions occur on clear, calm nights (also
  called nocturnal inversions)
• Windless night is essential for a strong
  radiation inversion breeze can mix the colder
  air at the surface with the warmer air above
• A long night contributes to a strong
  inversion-longer the night, longer the time of
  radiational cooling
• Radiation inversion is more likely with a clear
  sky and dry air
• How cold the night becomes depends on length of
  night, moisture content of the air, cloudiness
  and wind

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Air temp is controlled by incoming energy
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Radiation temp inversion- increase in air temp
with increasing height
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Daily Temperature variations

• Valley bottoms are colder than surrounding
  hillsides
• How cold season affects crops
• Lower branches of a tree are the most susceptible
  to damage
• Orchard Heaters are used to set-up convection
  currents
• Wind Machines to the cold air at the ground with
  the warmer air above
• Trees can be protected by irrigation water has
  a high heat capacity
• Wet soil has a higher thermal conductivity than
  dry soil heat from subsurface is conducted
  upward
• Freeze Air becomes colder with height water
  spray on the branches becomes ice crystal and
  latent heat released keeps the temperature at 0C

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Temperature profile in a hill-valley region
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Orchard Heaters circulate air by convection
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Wind machines mix cooler surface air with warmer
air above
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Temperature Controls

• The Main Temperature controls are
• Latitude (Temperature decrease poleward from the
  tropics and subtropics in both Jan and July
  greater variation in solar radiation in winter
  between low and high latitudes than summer)
• Land Water Distribution (for a given latitude,
  the temp are much lower in the middle of
  continents than near the ocean water has higher
  specific heat capacity)
• Ocean Currents Air temp at margin of continents
  are affected by ocean currents
• Elevation Air temperature varies with height
• Specific Heat Heat needed to raise the temp of 1
  g of substance by 1C (5 times more heat is
  needed for water than soil or rock)

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Average air temp near sea level in Jan.
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Average air temp near sea level in July
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Air Temperature Variations

• Diurnal range of temp. Difference between the
  daily maximum and minimum temperature (largest
  range in deserts-cloud-free, little water vapor
  to radiate IR back to the surface)
• Mean Daily Temperature Average of highest and
  lowest temperature for a 24-hour period
• Normal Mean Temperature Average of the mean
  daily temperatures for a particular date averaged
  for a 30-yr period
• Annual Range of Temperature Difference between
  the average temp of the warmest and the coldest
  months in equator, this range is small (3C in
  Quito, Ecuador 62 C Yakutsk in northeastern
  Siberia near the Arctic Circle)

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Air Temperature Variations contd.

• Mean Annual Temperature The average of the 12
  monthly average temperatures or sum of the daily
  means divided by 365
• Example A southwestern City has a Normal
  temperature of 68F (during the last 30 yrs, the
  values ranged from 47 to 89 F) What is a
  typical high temperature? Any high temp between
  47 to 89 F (two standard deviations on either
  side of 68F ) would be considered typical for
  this day.
• Unusual when the maximum value exceeds this range
• For Rainfall, the same reasoning is valid

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Decrease of Range from earths surface
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High temp on March 15 for 30 yrs in a city in
southwestern US (dashed line Normal Temp.)
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Comparison of temp data for 2 cities with the
same mean annual temperature
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Use of Temperature Data

• Heating degree-day A measure of the energy needs
  usage of furnace below 65F Mean temperature
  - 65F (one day MT 75 F --- 75-6510 heating
  degree-day)
• Cooling degree-day A measure of the energy
  needed to cool indoor air to a comfortable level
  65 F - Mean temperature - 65F (one day MT 45
  F --- 65-4520 cooling degree-day)
• Uses of these two parameters Allows builder to
  plan the size type of equipment needed for
  adequate airconditioning/heating to predict
  power demand during peak energy periods
  practical indication of energy needs in a year

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Mean annual total heating degree-days in
thousands of F (base 65F)
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Mean annual total cooling degree-days in
thousands of F (base 65F)
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Use of temperature data contd.

• Growing degree-day A day on which the mean daily
  temperature is one degree above the base
  temperature (minimum temperature required for
  growth) An index to determine when a crop will
  be ready for harvesting eg., for Rice, base temp
  is 60F in one sunny day in TX, the mean temp is
  70 F rice would accumulate 10 growing
  degree-days (70-60 F)
• Crop (variety/location) Base Temp Growing
  degree-days
• (F) (to
  Maturity)
• Beans (Snap/S. Carolina) 50 1200-1300
• Wheat (Indiana) 40 2100-2400
• Rice (Vegold/Arkansas) 60 1700-2100
• Peas (Early/Indiana) 40 1100-1200
• Cotton (Delta Smooth/Ark.) 60 1900-2500
• Corn (Sweet/Indiana) 50 2200-2800

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Air temperature use and human comfort

• Moisture content and other factors do affect the
  growing degree-days, yet useful to forecast the
  dates of crop maturity
• Human Comfort Human bodys perception depends on
  atmospheric conditions --- due to how we exchange
  heat energy with the environment
• Heat Exchange from the body Heat produced and
  adsorbed (radiation absorption gain by
  conduction/convection) Heat lost to the
  surroundings (emit IR energy loses by
  conduction/convection)
• Sensible Temp. The temp we perceive on a cold
  day, thin layer of warm air molecules forms close
  to the skin, protecting it from the surrounding
  cooler air and rapid transfer of heat

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Human comfort contd.

• Wind-Chill Factor Blowing Wind sweeps away the
  insulating layer heat is constantly removed
  from the body Faster the wind speed, greater
  the heat loss and the colder we feel

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Wind Chill Equivalent Temperature (F)

• Air Temp in 1st Row and Wind Speed (mi/hr) in 1st
  Column

30 15 5 0 -5 -15 -30 -40
5 27 11 0 -5 -10 -21 -36 -47
15 9 -11 -25 -31 -38 -51 -72 -85
25 1 -22 -36 -44 -51 -66 -88 -103
35 -4 -27 -43 -52 -58 -74 -97 -113
45 -6 -30 -46 -54 -62 -78 -102 -117
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Human comfort contd.

• Frostbite Freezing of skin - due to high wind
  below-freezing air makes the skin freeze and
  discolor
• Cold, rainy day often feels colder than a dry
  one because skin conducts heat away from the body
  better than air does (Fairbanks, AK vs Coastal
  Texas for the same temperature)
• In cold weather, heat is easily dissipated
  through the skin to counter this loss, the
  peripheral blood vessels of the body constrict,
  cutting off the flow of blood to the outer layers
  of the skin in hot weather, to radiate more
  heat, blood vessels enlarge

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Measuring Air Temperature

• Liquid-in glass thermometer When air temp
  increases, liquid in the bulb expands, and rises
  the tube when temp decreases, liquid contracts,
  and moves down the tube length of the liquid in
  the tube represents temp.
• Maximum and Minimum thermometer To measure daily
  maximum and minimum temperatures Maximum Ther.
  When temp begins to drop, the small constriction
  prevents the Hg from flowing back into the bulb
  stationary column--- max temp.

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Minimum Temperature

• Minimum Thermometer Measures the lowest
  temperature reached during a given period. Most
  minimum temp thermometers use alcohol as a liquid
  (freezing point -130C, Hg
• -39 C) When air temp drops decreasing,
  liquid and index marker stop moving down the
  bore when the air warms, the alcohol expands but
  moves freely up, but index marker does not move
  minimum temp is read by observing the upper end
  of the marker

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Other Thermometers

• Highly accurate temp measurements made with
  electrical thermometer Thermister, Electrical
  resistance thermometer Wheatstone bridge
  (Resistance a temp.)
• Radiometers Infrared sensors measure emitted
  radiation- by measuring the intensity and
  wavelength, satellites can estimate the air temp.
• Bimetallic Thermometer Temperature sensitive
  part of the Thermograph an instrument that
  measures and records temperature

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Thermograph with a bimetallic Thermometer