3: Chapter Temperature For the Record: Air Temperature Data
Air temperature is measured at thousands of locations worldwide using a variety of thermometers. Daily Mean: Daily Range: Monthly Mean: Annual Mean: Annual Range:
In order to examine the distribution of temp. on maps, we use:
lines indicating equal (iso) temperature (therms) (Fig 3.2).
Why Temperatures Vary: The Controls of Temperature
Latitude (which determines the amount of solar radiation received) is the major controller of temperature (as seen in Figure 2.9). There are however other controls: -
Land and Water
Land surfaces heat more rapidly and to a higher temperature (daytime) and cool more rapidly and to a lower temp. (nighttime) than do water surfaces (Fig. 3.3), because: 1) 2) 3) The Specific Heat [def: as the amount of heat needed to raise the temperature of 1 gram of a substance 1oC] of water is 3 times greater that the specific heat of land; as a result:
Figs 3.4, 3.5;Table 3.1 illustrate that: Oceans act as regulators of temperature:
keeping coastal areas: -
when compared to mid-continent locations.
Ocean Currents
Greatly influence temps. because of their ability to transport warm water towards the poles and cold water towards the equator. (Figs. 3.7, 3.8) Examples: The Gulf Stream, with temp. between 75-85o keeps:
The California Current, with temp. between 45o55okeeps:
Altitude
Recall from Chapter 1 that temperature generally decreases wrt height in the troposphere, Additional example: Even though Hawaii is in the middle of the Pacific Ocean and is found at low latitudes (20oN), snow often falls on Mauna Loa because its elevation is ~ 13,000 feet.
Geographic Position
A location's proximity to certain geographic features can also control temperatures: -
Cloud Cover
Clouds, because they limit both incoming solar (reducing afternoon maxima) and outgoing terrestrial (reducing early morning minima):
World Distribution of Temperatures
Figs. 3.15, 3.16 and 3.17 illustrate (excluding elevation and with considerable smoothing) the cumulative influence of the controls. - isotherms indicate a decrease toward the poles - tropical latitudes experience very little annual variation - mid-latitude, continental areas experience very large annual variation -
NC Distribution of Temperatures
NC Distribution of Temperatures
Cycles of Air Temperature
Daily (Diurnal) Temperature Variations
Fig 3.18 illustrates both typical and atypical daily temperature variation. On typical days: After sunrise: solar radiation heats the earth's surface which warms the first few inches of the air through conduction leading to convection and hence :
Additional heating occurs as the atmosphere captures Earth's long radiation After sunset:: both the surface and the atmosphere lose heat (cool) by emitting long radiation thru a process called:
Lag of the Maximum Although incoming solar radiation is most intense at noon, the maximum temperatures to tend occur considerably later: This lag is due to the fact that: -
As a result: Temp. will continue to rise, reaching a maximum when the: outgoing terrestrial radiation = the incoming solar radiation. The temperature will begin to fall in the afternoon once: outgoing terrestrial > incoming solar, reaching a min. well past midnight: -
Magnitude of Daily Temperature Changes
Influenced by several factors including: -
Annual Temperature Variations
The same reasoning can be used to examine the seasonal lag in temperature which generally results in: -
Annual Lag of Temperature
Annual Lag of Temperature for Raleigh
Temperature Measurement
There are numerous types of thermometers (meter or measure of therms), operating on different physical principles, each with their own advantages and disadvantages. -
Mechanical Thermometers
Most substances contract when cooled and expand when heated. This attribute is used by: Liquid-in-glass thermometers (Fig. 3.21) Bimetal Strip -
Electrical Thermometers
The flow of electricity is temperature dependent Thermisters (thermal resisters) (Fig. 3.24) -
Radiometers
Sensors that measure infrared radiation, which as we saw earlier is dependent on temperature (Stephan-Boltzmann Law) -
Temperature Scales
The lowest temperature possible (theoretically) is that at which all atoms/molecules stop moving:
At AZ we start a temperature scale called the:
Kelvin is convenient for scientific computations because it is never negative; however, non-scientist generally use either: At AZ, the temperature is equal to:
The figure below (and Fig 3.26) compare the scales which can be derived from the conversion equations provided.
Notes:
One K degree
=
Applications of Temperature Data Heating Degree Days (HDD) and Cooling Degree Days (CDD)
(Table 3.2) Indices developed by HVAC engineers to estimate seasonal heating and cooling needs. Based on the assumption that people will turn on their furnaces (ac) when the Daily Mean Temperature drops below (rises above) 65oF HDD = CDD = Examples: Daily Max T = 38oF Daily Min T = 28oF Mean T = 33oF HDD = Daily Max T = 92oF Daily Min T = 74oF Mean T = 83oF CDD = Raleigh averages:
Growing Degree Days (GDD)
Not to be outdone, farmers have developed GDD units to guide for planting/maturation/harvest of crops. -
Temperature and Comfort
Heat Stress Index (Discussed in Chapter 4) Wind Chill Temperature (WCT) (Box 3.5) Developed (updated in 2001) by scientists using a combination of: Determine air's ability to remove heat from human bodies (Figs. 3.H). WCT = 35.74 + 0.6215 (T) 35.75(V0.16) + 0.4275(T)(V0.16) Temperature = 10oF Wind Speed = 10 mph Wind Chill Temperature = This means that your skin would lose the same amount of heat as if the air temperature were -4o F, with no wind, resulting in: Vocabulary Review: Know all of the terms. Review Questions: Be able to answer all of the questions except: 13.
Example: