Chapter 3 - Temperature

In Chapter 2, we saw how the atmosphere is heated, and examined the role of the Sun-Earth relationship in causing temperature variations from season to season, and latitude to latitude.

Air Temperature Data

1. Temperatures are measured routinely at locations all over the globe
2. The daily mean temperature is the average of all readings taken throughout a day (or perhaps only the highest and lowest)
3. Monthly mean temperature is the average of the daily mean temperatures
4. Annual mean temperature is the average of the 12 monthly mean temperatures
5. Daily temperature range = maximum - minimum temperatures for the day
6. Annual temperature range = maximum - minimum monthly mean temperatures for the year
7. The variation of temperature over a large area is illustrated by drawing contours of isotherms (equal temperatures)
8. Isotherm maps also show temperature gradients (how temperature changes with horizontal distance)

Box 3-1 North America's Hottest & Coldest Places

1. Every state has a maximum temperature record of 38C (100F) or higher.
2. Hawaii's temperature are moderated by the surrounding sea
3. Death Valley is so hot because (a) Mountains cut off cooling sea breezes (b) the skies are usually clear (no shade) (c) no water to cool by evaporation (d) air subsiding into the valley is warmed by compression as it descends.
4. Lowest temperatures are observed (a) in Winter, (b) away from the moderating effects of the sea, (c) at high latitudes (and high altitudes)

Box 3-2 Heat Waves (A deadly event)

1. A heat wave is a prolonged period of abnormally hot and usually humid weather.
2. A series of 3 days with T > 95F is a heat wave in Boston, but not in Houston.
3. See Figure 3-C for weather-related fatalities.

Why does Temperature Vary?

The controls of temperature are:

1. Latitude (angle of Sun) - Chapter 2
2. Differential heating of land and water (they heat up/cool down differently)
3. Ocean Currents
4. Altitude
5. Geographic position
6. Cloud cover & albedo

Land & Water

Land heats up more rapidly than water, and reaches a higher temperature. Why?

Land cools more rapidly than water, and to a lower temperature. Why?

1. Water is mobile - convection causes mixing. More water has to be heated with the same amount of heat, so temperature rise is smaller
2. Only the surface of soil & rocks is heated - underlying parts are heated by conduction (soil and rock are poor conductors)
3. Water is transparent, so direct heating can occur to depths of several meters
4. Specific Heat - the amount of heat required to raise the temperature of one gram of a substance by one degree C
5. SH of water is three times greater than that of land/rock. Therefore needs more heat to give same temperature rise
6. Water evaporates (at the surface) and therefore cools
7. Ocean currents also affect the temperature of coastal regions
8. Ocean currents are set in motion by winds
9. Ocean water movement accounts for 1/4 of the transfer of heat from tropical to polar regions (winds do 3/4)

Altitude

1. Temperature decreases with increasing altitude (6.5 degrees C /km)
2. Lower density of atmosphere also leads to rapid daytime heating and rapid nighttime cooling

Geographic Position

1. Windward coasts are affected by moderating winds off the sea; Leeward coasts are not
2. Mountain ranges can act as barriers

Cloud Cover and Albedo

1. Clouds both reflect and trap radiation
2. Surface albedo controls how much incident radiation is absorbed

World Distribution of Temperature

1. Isotherms trend east-west
2. Temperature is mostly controlled by the Sun (Sun angle, latitude)
3. Land shows more extremes in temperature than water does
4. Isotherms also show effect of ocean currents

Global Annual Temperature Ranges

1. Annual ranges smaller near the equator, and increase towards the poles.
2. Outside the tropics, the ranges are greater for land than for sea.
3. Range increases as move away from coast.

Latitude & Temperature Range

1. Latitude is the most important solar control (Figs 3-15 and 3-16)
2. Seasonal difference of temperature is greater for locations further from the equator because the Suns' energy spreads over a larger area (Winnipeg vs. San Antonio)
3. Greatest seasonal variation is at Yakutsk, central Siberia. Why?

Cycles of Air Temperature

1. Daily temperature variations are mostly due to the Earth's rotation (Sun's apparent motion across the sky)
2. Minimum temperature occurs just before sunrise
3. Maximum temperature occurs mid-to-late afternoon, not at noon - lag of the maximum
4. Although the intensity of solar radiation drops after noon, it still exceeds the outgoing energy from the Earth, so a heat surplus builds up
5. Also, the Earth gives more heat to the atmosphere than it can radiate away, so the air gets hotter

Magnitude of Daily Temperature Changes

1. Greatest when the variation of the Sun angle is greatest (low & mid latitudes)
2. Temperature range for a windward coast is usually small because the air is warmed by the sea, which experiences only a small temperature change during the day
3. Temperature changes during a cloudy day are usually less than during a cloudless day

Box 3-4 The Urban Heat Island

Why are cities warmer than the surrounding countryside?

1. Surfaces are very different. Buildings, concrete & asphalt absorb & store greater quantities of solar radiation than farm land. At night, city surfaces gradually release stored heat, keeping the temperature higher than surrounding countryside
2. Rain water runs off rapidly - therefore no cooling by evaporation
3. Waste heat is generated by many city activities - home heating, A/C, Factories
4. Blanket of pollution over city absorbs surface radiation, and re-emit some back to the ground

Temperature Measurement

1. Mechanical thermometers - change of physical property - liquid-in-glass, maximum, minimum; also bimetallic strips
2. Electrical thermometers - electrical resistance changes with temperature

Temperature Scales

1. Fahrenheit - water boils at 212F, freezes at 32F (steam point & ice point).
2. Celsius - water boils at 100C, freezes at 0C.
3. Absolute or Kelvin - absolute zero is the temperature at which all molecular motion ceases. Kelvin = 1C.
   Second required point for temperature scale assumes that the triple point of water occurs at 273.16K (ice, water, water vapor all present)
4. C = K - 273
5. C = (5/9)(F - 32)

Applications of Temperature Data

1. Degree day = one degree x one day
2. Heating-degrees days. Reference daily mean temperature is 65F. A daily mean temperature of 64F corresponds to one heating-degree day
3. Cooling-degrees days. Reference daily mean temperature is 65F. A daily mean temperature of 66F corresponds to one cooling-degree day
4. Growing-degree days. Same idea. Base temperature is different for different crops.

Temperature and Comfort

1. Body cools by evaporating water from the skin (sweating/perspiration)
2. Cooling is less efficient if the air already contains a lot of water
3. Cooling is more efficient if the moist air around the body is swept away (fans)
4. Heat index is a measure of the combined effect of temperature and humidity
5. Wind chill - winds carry heat away from the body. Wind chill depends on temperature and speed of wind.

SO - what impact does wind chill have on exposed water pipes or your car's radiator?