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                                    Chapter Three

                                      Air Temperature
                            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
                                  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 > outgoing heat energy from the surface --- energy surplus for 2-4
    hours after noon
•   Highest reading of temperature occurs at:
     – 3-5:00 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
                             Sunny, calm day – air temperature profile
                           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.~30.5°C; Phoenix (AZ): .~40.5°C
•   Late afternoon, early morning: Air starts cooling, as air begins to loose more energy than they
•   Radiation cooling: Both air and ground cool by radiating infrared radiation
                                       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
                             Air temp is controlled by incoming energy
             Radiation temp inversion- increase in air temp with increasing height
                                  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 0°C
                        Temperature profile in a hill-valley region
                        Orchard Heaters circulate air by convection
                  Wind machines mix cooler surface air with warmer air above
                                   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 1°C (5 times more heat is
        needed for water than soil or rock)
                           Average air temp near sea level in Jan.
                           Average air temp near sea level in July
                                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 (3°C in Quito, Ecuador; 62 °C Yakutsk in
    northeastern Siberia near the Arctic Circle)
                             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 68°F (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 68°F ) would be considered typical for
    this day.
•   Unusual when the maximum value exceeds this range
•   For Rainfall, the same reasoning is valid
                                 Decrease of Range from earth’s surface
    High temp on March 15 for 30 yrs in a city in southwestern US (dashed line: Normal
Comparison of temp data for 2 cities with the same mean annual temperature
                                        Use of Temperature Data
•   Heating degree-day: A measure of the energy needs – usage of furnace below 65°F; = Mean
    temperature - 65°F (one day MT: 75 °F --- 75-65=10 heating degree-day)
•   Cooling degree-day: A measure of the energy needed to cool indoor air to a comfortable level
    = 65 °F - Mean temperature - 65°F (one day MT: 45 °F --- 65-45=20 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

     Mean annual total heating degree-days in thousands of °F (base: 65°F)
     Mean annual total cooling degree-days in thousands of °F (base: 65°F)
                       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 60°F; 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
                                 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 body’s 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
•   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
                                             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
                              Wind Chill Equivalent Temperature (°F)
•                      st
    Air Temp in 1 Row and Wind Speed (mi/hr) in 1st Column
                                               Human comfort – contd.
•   Frostbite: Freezing of skin - due to high wind below-freezing air makes the skin freeze and
•   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
                                   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.
                                 Minimum Temperature
• Minimum Thermometer: Measures the lowest temperature reached during a given period. Most
     minimum temp thermometers use alcohol as a liquid (freezing point -130°C, 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
                                   Other Thermometers
•   Highly accurate temp measurements made with electrical thermometer – Thermister, Electrical
    resistance thermometer – Wheatstone bridge (Resistance 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
              Thermograph with a bimetallic Thermometer

                                  Chapter –3- Summary

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