meteorology gel-1370 chapter three chapter three air temperature
TRANSCRIPT
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METEOROLOGY
GEL-1370
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Chapter ThreeChapter Three
Air TemperatureAir Temperature
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Daily Temperature VariationsWe 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 > 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
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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.~30.5°C; Phoenix (AZ): .~40.5°C
• 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 0°C
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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 1°C (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 (3°C 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 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
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Decrease of Range from earth’s 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 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
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Mean annual total heating degree-days in thousands of °F (base: 65°F)
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Mean annual total cooling degree-days in thousands of °F (base: 65°F)
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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 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
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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 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 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 -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
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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
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Thermograph with a bimetallic Thermometer