METEOROLOGY
GEL-1370
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
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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.
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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
Radiation Inversion
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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
Air temp is controlled by incoming energy
Radiation temp inversion- increase in air temp with increasing height
Daily Temperature variations
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Valley bottoms are colder than surrounding hillsides
How cold season affects crops:
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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
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The Main Temperature controls are;
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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)
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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
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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.
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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
Temp.)
Comparison of temp data for 2 cities with the same mean annual temperature
Use of Temperature Data
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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.
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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)
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Crop (variety/location)
Base Temp
Growing degree-days
(°F)
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Beans (Snap/S. Carolina)
Wheat (Indiana)
Rice (Vegold/Arkansas)
Peas (Early/Indiana)
Cotton (Delta Smooth/Ark.)
(to Maturity)
50
40
60
1200-1300
2100-2400
1700-2100
40
60
• Corn (Sweet/Indiana) 50
1100-1200
1900-2500
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
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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
Human comfort – contd.
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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)
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Air Temp in 1 Row and Wind Speed (mi/hr) in 1st Column
Human comfort – contd.
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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
Measuring Air Temperature
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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.
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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
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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